ALBERT

Notes: Abstract The Pongkor gold-silver epithermal deposit with reserves of at least 98 tonnes of gold and 1026 tonnes of silver, average grades 16.4 g/t Au and 171.2 g/t Ag is one of the most recent and largest gold and silver discoveries in Indonesia, proven within a short period (1988–1991). 40Ar/39Ar dating on adularia samples give an age of 2.05 ± 0.05 Ma. The deposit is of the low-sulfidation epithermal type and consists of four main mineralized quartz veins located close to the internal rim of a volcano-tectonic depression (caldera). This resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks. This volcanic unit unconformably overlies Miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks. The mineralized bodies are thick (average 4.2 m), steeply dipping, quartz-carbonate-adularia veins with a very low sulfide content (〈0.5 wt.%). Their genesis is related to an extensional episode within a tectonic corridor showing NW-SE and NNE-SSW conjugate strike-slip faults, the major vein being located on the inner rim of the caldera. The vein fill reveals four successive stages of deposition marked by a specific facies: (1) carbonate-quartz breccia with dominant quartz and calcite and minor kutnahorite, rhodochrosite, and rhodonite (CQ facies), (2) a network of banded quartz and former carbonate transformed into manganese oxides through supergene alteration (MOQ facies), (3) banded opaline milky quartz (BOQ facies), and (4) grey, locally banded, sulfide-rich quartz breccia cutting all the other types (GSQ facies). Adularia was deposited at the same time as the quartz. The mineralogy and internal structures of the veins (crustiform banding, vugs, collapse breccia) clearly indicate a dilational context, which is common in low-sulfidation epithermal systems. Gold and silver grades, as well as sulfide mineral abundances, increase steadily through stages 1 to 4, locally reaching 1 kg/t in the GSQ facies. The sulfides are dominated by pyrite, accompanied by common acanthite-aguilarite, polybasite-pearceite and electrum in which the gold content ranges from 48 to 74 wt.%. Sphalerite, galena, chalcopyrite and hessite are fairly rare, although present within the CQ facies. The fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 382 °C, indicating boiling of a hydrothermal fluid with an initial temperature of around 205 °C; no marked difference is seen in the GSQ facies, which has the highest gold content. Salinities are low, generally below 1 wt.% eq. NaCl. Lead isotope compositions of the associated volcanic rocks and the mineralization are very similar, 206Pb/204Pb between 18.706 and 18.814␣and between 18.744 and 18.801 respectively, demonstrating a genetic link between the Pliocene volcanism and the auriferous hydrothermal activity. The isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust that melted and remobilized during the Pliocene volcanic and hydrothermal events. These conclusions seem applicable to the entire Bayah Dome. The existence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins. Late intense weathering of the ore deposit, to depths of 250 m below the surface, has given rise to manganese oxide layers, limonite zones, and silver micronuggets within the veins, as well as to gold enrichment.

Notes: Abstract Carbonate-limonite veins formed in steeply dipping fractures in the upper few hundred metres of basement greywacke in the actively rising Southern Alps of New Zealand. The veins are found commonly in extensional fractures near to, but not in, major faults associated with mountain uplift, and/or sinistral faults which bound mountain ranges. Some of the veins contain sulphides and minor gold deposited as part of incrementally formed fracture fillings. Oxygen isotope ratios of calcite range widely between +6 and +24‰, and calcite δ13CPDB=−5.5 to −11.5‰. The veins formed from isotopically exchanged crustal fluid with a probable meteoric water component. The shallow vein network is the near-surface expression of a tectonically induced hydrothermal system which has deposited gold-bearing veins with a mesothermal style over several vertical kilometres. This vein network has formed in a dilatational zone of the oblique collisional orogen where near-vertical fractures tap deep-sourced fluids. Similar processes acting at the southern end of the Southern Alps in the Miocene resulted in locally rich mesothermal quartz-gold veins.

Notes: Abstract Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO2-H2O inclusions, which have a wide range in CO2 content and coexist with lesser primary CO2-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO2 in stage I inclusions and 10–20 mol% CO2 in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th total than, their coexistent CO2-H2O inclusions. The coexisting CO2-rich, CO2-H2O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO2-H2O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO2-H2O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins.

Notes: Abstract Gold mineralization in the Kolar schist belt of the Dharwar craton occurs dominantly in the form of a sulfide-poor Au-quartz lode (the Champion lode exposed in the Mysore and other mines) and sulfide-rich auriferous lodes (from the Nundydroog mine). Fluid inclusion microthermometric experiments were conducted on primary inclusions in quartz intimately associated with Au-mineralization. Homogenization studies on aqueous-biphase (L + V), aqueous polyphase (L + V+ halite) and aqueous-carbonic (LCO2± VCO2 + Laq) inclusions from the Champion lode furnish a temperature range of 120 to 420 °C. Freezing of aqueous biphase inclusions and dissolution of halite in the aqueous polyphase inclusions provide salinity of 5 to 50 wt.% NaCl equivalent. Fluid inclusion thermobarometry from the total homogenization of aqueous-carbonic inclusions and from intersecting isochores of coeval pure-carbonic (LCO2± VCO2) and pure-aqueous inclusions constrain the P-T path of evolution of the fluid in the Champion lode. Gold precipitation was likely to have been brought about in response to a sharp fall in pressure with attendant unmixing of liquid-CO2 from the parent H2O-CO2 fluid of possible metamorphic origin. This would imply transportation of gold by some pressure-sensitive complex such as the Au-carbonyl. Fluid characteristics are different in the sulfide-rich auriferous lodes, as indicated by the virtual absence of the CO2-bearing and the halite-bearing inclusions. The fluid evolution path, as evident from the crude positive colinearity of temperature and salinity, is due to mixing of a low (≤200 °C) temperature-low saline (≤7 wt.% NaCl equivalent) fluid with a high temperature (≥400 °C)-high saline (≥50 wt.% NaCl equivalent) fluid. The lack of CO2 and association of Au with sulfides indicate a different mode of gold transport, as chloride or bisulfide complexing, deposition of which was possibly brought about by fluid mixing.

Notes: Abstract The Hercynian mercury mineralization of Las Cuevas is hosted by a highly folded and sheared sequence of basalts, intrusive breccias, slates, psamitic rocks and quartzites. The mineral paragenesis is simple and consists of cinnabar, native mercury and pyrite. Hydrothermal alteration can be divided into `proximal' and `distal' with respect to the mineralized bodies. The proximal alteration (≤1.3 m wide) consists of quartz-pyrophyllite-kaolinite, quartz-pyrophyllite-(kaolinite)-(illite), and quartz-illite-(pyrophyllite)-(kaolinite). The distal alteration (∼100 m wide) consists of (quartz)-illite-chlorite-(pyrophyllite), or rectorite-(chlorite). These assemblages overprint an earlier, regional alteration consisting of quartz-chlorite-albite-carbonates (±ankerite, ±siderite, ±magnesite, ±calcite). The mercury deposit of Las Cuevas can be regarded as an unusual combination of mercury deposition and advanced argillic alteration within a relatively deep environment (≥1.8 km).

Notes: Abstract The Rocabruna and Coll de Pal barite deposits, located in the eastern Pyrenees of Spain, fill karstic cavities within carbonate rocks of Cambrian and Devonian age, respectively. The deposits contain barite, chalcopyrite, tetrahedrite, pyrite and minor sphalerite and galena with saddle dolomite and quartz as gangue. Fluid inclusion data from Coll de Pal quartz and dolomite indicate that the mineralizing fluid was a polysaline CaCl2-rich brine, with temperatures between 125 and 150 °C. C and O isotopic compositions of carbonates in both deposits are consistent with a progressive increase in temperature during deposition. The 34S values of barite, which range from 14.2 to 15.9‰ in Coll de Pal, and from 13.9 to 19.3‰ in Rocabruna, together with 87Sr/86Sr ratios ranging from 0.7118 to 0.7168 in Rocabruna, and from 0.7115 to 0.7136 in Coll de Pal, indicate two different fluid sources. We propose that these deposits formed as a result of mixing between a Ba-rich, sulfate-poor hot fluid, and sulfate-rich solutions of surficial origin. The different Sr isotope ratios in the deposits indicate that the hot Ba-rich fluids involved in each deposit were equilibrated with different rock types (carbonates and shales), in agreement with the geology of the two areas.

Notes: Abstract The Rogaland anorthosite province (S. Norway) contains numerous Fe-Ti oxide deposits, including the second most important ilmenite deposit in the world, the Tellnes deposit. The largest deposits are located in the Åna-Sira anorthosite massif. Others occur in the Håland-Helleren anorthosite massif, particularly along the deformed contact with the Egersund-Ogna massif, where they were previously considered formed by metasomatic processes. All deposits are now regarded as magmatic. The structure, mineralogy and geochemistry of 11 selected Fe-Ti deposits (Tellnes, Storgangen, Blåfjell, Laksedal, Kydlandsvatn, Kagnuden, Rødemyr, Hestnes, Eigerøy, Svånes, and Jerneld) are discussed in light of recent models proposed for the origin of Rogaland anorthosites and related rocks. Massif-type anorthosites result from the diapiric uprise of a plagioclase crystal mush which crystallized along a large P–T interval. Except for Tellnes, which is related to a post-deformation dyke, the Fe-Ti deposits in anorthosite massifs have been deformed by this movement during and after their crystallization. The differentiation process of the jotunitic parental magma has built up cumulates in the Bjerkreim-Sokndal layered intrusion and liquids in the Tellnes dyke and other jotunitic intrusions. Ilmenite is a liquidus mineral immediately after plagioclase in the sequence of crystallization of these jotunites, its interstitial character in the rocks resulting from subsolidus recrystallization. Ilmenite can thus accumulate early in the evolution of jotunitic magmas. This feature, together with high contents in Cr, V, Mg and Ni, links the Jerneld, Blåfjell and Svånes deposits (type␣1) to the early evolution of a jotunitic magma. In the Bjerkreim-Sokndal intrusion, magnetite can appear with ilmenite at the very beginning of the sequence of crystallization, but normally crystallizes after orthopyroxene and before clinopyroxene and apatite. The early appearance of magnetite is a characteristic feature of type 2 deposits (Tellnes, Storgangen, Kydlandsvatn, Rødemyr I) and suggests conditions similar to the early magnetite cumulates in the Bjerkreim-Sokndal intrusion. Evidence of layering further favours gravity-controlled sorting processes to concentrate the oxides. Large-scale subsolidus segregation of the oxides due to high-temperature deformation can further concentrate these minerals in silicate-absent meter-sized masses. Type 3 deposits (Rødemyr II, Kagnuden, Hestnes and Eigerøy) could be derived from the more evolved stages of differentiation, as indicated by high REE in apatite, high Ti and Zn in magnetite and relatively low Cr, V, Mg, Ni contents in both oxides. The Cr content in both oxide minerals is however higher than in the equivalent cumulates of the Bjerkreim-Sokndal intrusion. Although immiscibility as the mechanism of enrichment leading to silicate-absent oxide-apatite veins, as in Hestnes and Eigerøy, cannot be precluded, there is no direct evidence in the veins, nor has any structural or geochemical evidence of immiscibility ever been found in jotunite dykes and Fe-Ti-P-rich rocks. Further investigations on the influence of subsolidus exchange of elements between the two oxides are needed to improve the use of trace elements as differentiation indexes.

Notes: Abstract Epigenetic, vein-type graphite mineralization originates by deposition from C—O—H fluids in high-temperature environments. Consequently, fluid-deposited graphite is uniformly highly crystalline in volumetrically large occurrences. This work examines the factors controlling graphite crystallinity in fluid-deposited occurrences with reference to some case studies from southern Spain where vein-type graphite is associated with a variety of host rocks. Possible causes influencing high crystallinity of graphite in these occurrences include: (1) large graphite occurrences are generated from large volumes of fluids that maintain their temperatures for long periods of time, which is easier at higher temperatures; (2) high temperature conditions are required for a fluid to precipitate a major part of its dissolved carbon during a small temperature decrease; (3) carbon is incorporated into C—O—H fluids mainly through devolatilization reactions which also require high temperatures; (4) highly crystalline graphite generated at high-T/high-P conditions is less susceptible to resorption as P decreases or by subsequent fluid flow; (5) graphite precipitation involves high activation energy that can be overcome only if the temperature is high enough. These causes can be extrapolated to most vein-type graphite deposits worldwide.

Notes: Abstract Three types of base metal deposits occur in the Devonian of the Central Pyrenees accompanying and post-dating the Hercynian orogeny. Since spatial and temporal relationships between mineralization can be recognized in the Yenefrito area, they have been incorporated into a metallogenetic model. Sedex-like ore, composed of sphalerite with minor galena, pyrite, arsenopyrite and magnetite, is enclosed within a carbonate-siltstone succession containing volcanic sills. Crust-type mineralization is linked to an unconformity landscape and made up by quartz and minor sphalerite, pyrite and galena. Finally, vein deposit cross-cutting these deposit types, contains galena, sphalerite and pyrite. Sedex-like and crust-type deposits are linked to Hercynian extensional tectonics which favoured the development of an unstable depositional environment mainly controlled by synsedimentary faults, being sedex mineralizations developed prior to crust-type. Both mineralization types suffered diagenesis and deformation during the Hercynian. The vein-type was generated by extensional movements related to early Alpine rifting.

Notes: Abstract The East Uralian Zone south of Cheljabinsk consists of a Silurian to Early Carboniferous volcanosedimentary sequence on Proterozoic basement, intruded by postorogenic Permian granitic rocks. The 7 by 1–2 km Bereznjakovskoje gold trend is hosted by dacitic to andesitic volcanic-subvolcanic units of Late Devonian age. Epithermal gold mineralization is of the sulfide-rich low-sulfidation (adularia-sericite) type with the metal spectrum of Au-Ag-As-Sb-Cu-Zn-Pb and a characteristic Te component. Silicic and sericitic alteration overprints barren propylitic pyrite-rich assemblages in stockworks and shear zones. Precious-metal mineralization is related to a fahlore-telluride stage overprinting earlier pyrite. Late stage redistribution and weathering liberates gold from telluride mineral phases with fixation of native gold (high fineness) on vugs and grain boundaries, and in intergrowth aggregates with supergene jarosite/limonite. Epithermal mineralization formed below the boiling level and is possibly related to a porphyry system at depth.

Notes: Abstract Magmatic Ni-Cu sulfide deposits form as the result of segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma, and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce an ore deposit. The appropriate physical environment is required so that the sulfide liquid mixes with enough magma to become adequately enriched in chalcophile metals, and then is concentrated in a restricted locality so that the resulting concentration is of ore grade. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 × 2 × 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, trans-crustal fault and immediately below the centre of a 3.5 km-thick volcanic basin. Studies of the overlying basalts have shown that lavas forming a 500 m-thick sequence within these have lost 75% of their Cu and Ni and more than 90% of their PGE. Overlying basalts show a gradual recovery in their chalcophile element concentrations to reach “normal” values 500 m above the top of the highly depleted zone. The ore-bearing Noril'sk-type intrusions correlate with those basalts above the depleted zone that contain “normal” levels of chalcophile elements. The high proportion of sulfide (2–10 wt.%) associated with the Noril'sk-type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+8–+12 ∂34S) are explicable if the ore-bearing bodies are exit conduits from high level intrusions, along which magma has flowed en route to extrude at surface. The first magma to enter these intrusions reacted with much evaporitic sulfur, at a low “R” value and thus gave rise to sulfides with low metal tenors. Successive flow of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows above the highly depleted flows. The Voisey's Bay deposit lies partly within a 30–100 m-thick sheet of troctolite, interpreted as a feeder for the 1.334 Ga Voisey's Bay intrusion, and partly at the base of this intrusion, where the feeder adjoins it. Studies of olivine compositions indicate that an early pulse of magma through the feeder and into the intrusion was Ni depleted but that subsequent pulses were much less depleted. Trace element, Re-Os and S and O isotope data, and mineralogical studies indicate that the magma pulses interacted with country gneiss, probably principally in a deeper level intrusion, extracting SiO2, Na2O, K2O and possibly sulfur form the gneiss, which accounts for the magma becoming sulfide saturated. The Jinchuan deposit of north central China occurs within a 6 km-long dyke-like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt magma. Flow of magma into the intrusion has resulted in olivine and sulfide being retained where the keel was widening out into the intrusion. The West Australian komatiite-related deposits occur in thermal erosional troughs which have developed due to the channelisation of magma flow and the resulting thermal erosion of underlying sediments and basalt by the hot komatiite magma. The sediments are sulfide-rich, and may have contributed substantially to the sulfide of the ores. The mineralisation in the Duluth complex occurs in troctolitic intrusions along the western margin of the complex as a result of magma interacting with and extracting sulfur from the underlying graphite- and sulfide-bearing sediments. No magma flow channels have been identified so far, and the lack of magma flow subsequent to the development of sulfide immiscibility is regarded as the reason why these deposits are not of economic grade. When most major Ni-Cu sulfide deposits are compared, they prove to have a number of features in common; olivine-rich magma, proximity to a major crustal fault, sulfide-bearing country rocks, chalcophile element depletion in related intrusive or extrusive rocks, field and/or geochemical evidence of interaction between the magma and the country rocks, and the presence of or proximity to a magma conduit. The features are thought to explain the three key requirements (sulfide immiscibilty, adequate mixing between sulfides and magma, and localisation of the sulfides) discussed and have important implications with respect to exploration.

Notes: Abstract Lead isotope ratios of galena from the carbonate-hosted massive sulphide deposits of Kabwe (Pb-Zn) and Tsumeb (Pb-Zn-Cu) in Zambia and Namibia, respectively, have been measured and found to be homogeneous and characteristic of upper crustal source rocks. Kabwe galena has average isotope ratios of 206/204Pb = 17.997 ± 0.007, 207/204Pb = 15.713 ± 0.010 and 208/204Pb = 38.410 ± 0.033. Tsumeb galena has slightly higher 206/204Pb (18.112 ± 0.035) and slightly lower 207/204Pb (15.674 ± 0.016) and 208/204Pb (38.276 ± 0.073) ratios than Kabwe galena. The isotopic differences are attributed to local differences in the age and composition of the respective source rocks for Kabwe and Tsumeb. The homogeneity of the ore lead in the two epigenetic deposits suggests lead sources of uniform isotopic composition or, alternatively, thorough mixing of lead derived from sources with relatively similar isotopic compositions. Both deposits have relatively high 238U/204Pb ratios of 10.31 and 10.09 for Kabwe and Tsumeb galenas, respectively. These isotope ratios are considered to be typical of the upper continental crust in the Damaran-Lufilian orogenic belt, as also indicated by basement rocks and Cu-Co sulphides in stratiform Katangan metasediments which have a mean μ-value of 10.25 ± 0.12 in the Copperbelt region of Zambia and the Democratic Republic of Congo (formerly Zaire). The 232Th/204Pb isotope ratios of 43.08 and 40.42 for Kabwe and Tsumeb suggest Th-enriched source regions with 232Th/235U (κ-values) of 4.18 and 4.01, respectively. Model isotopic ages determined for the Kabwe (680 Ma) and Tsumeb (530 Ma) deposits indicate that the timing of the mineralisation was probably related to phases of orogenic activity associated with the Pan-African Lufilian and Damaran orogenies, respectively. Galena from the carbonate-hosted Kipushi Cu-Pb-Zn massive sulphide deposit in the Congo also has homogeneous lead isotope ratios, but its isotopic composition is comparable to that of the average global lead evolution curve for conformable massive sulphide deposits. The μ (9.84) and κ (3.69) values indicate a significant mantle component, and the isotopic age of the Kipushi deposit (456 Ma) suggests that the emplacement of the mineralisation was related to a post-tectonic phase of igneous activity in the Lufilian belt. The isotope ratios (206/204Pb, 207/204Pb, 208/204Pb) of the three deposits are markedly different from the heterogeneous lead ratios of the Katangan Cu-Co stratiform mineralisation of the Copperbelt as well as those of the volcanogenic Nampundwe massive pyrite deposit in the Zambezi belt which typically define radiogenic linear trends on lead-lead plots. The host-rock dolomite of the Kabwe deposit also has homogeneous lead isotope ratios identical to the ore galena. This observation indicates contamination of the Kabwe Dolomite Formation with ore lead during mineralisation.

Notes: Abstract The investigation of stable and radiogenic isotopes and of platinum-group (PGE) and rare earth elements (REE) in chromitites and associated ultramafic rocks of the Kempirsai Massif, southern Urals, gives strong evidence for a multistage formation of giant ophiolitic-podiform chromite deposits present in the southeastern part of the massif. The Kempirsai ophiolite massif is divided by a shear zone into two parts: in the northwestern area, small bodies of Al-rich chromite formed from basaltic melts between 420 to 400 Ma, according to Sm-Nd mineral isochrons of harzburgite, pyroxenite, websterite and gabbro. Harzburgites and pyroxenites in this area are enriched in light REE and have ɛNd(400) 〉 +6 and ɛSr(400) ∼ +5. Chromitites have scattered PGE distributions (Pd/Ir, 0.4–7.0), being partly enriched in Pd and Pt. γOs(400) of one chromitite is −4.4. The southeastern part of the Kempirsai Massif, well-known for its world-class deposits of podiform low-Al magnesiochromite, is characterized by harzburgite and dunite enriched in light REE with very low ɛNd(400) (+4.3 to –17.1) and positive ɛSr(400) (〉+10) values. Chromitites are strongly enriched in Ir, Os and Ru and depleted in Pd and Pt. γOs(400) of three chromitites is uniform and approaches C1 and DMM compositions. In veins and pods postdating crystallization of massive chromite, pargasitic amphibole formed in equilibrium with fluid-inclusion-bearing chromite at temperatures close to 1000 °C. These amphiboles give 40Ar/39Ar stepwise heating ages of 365 to 385 Ma and are characterized by low ɛNd(400) (+0.6 to −4.6) and general enrichment in REE. The cooling ages correspond to a 379.3 ± 1.6 Ma Rb-Sr mineral isochron produced from amphibole and phlogopite of a pyroxenite vein in the western part of the massif. From these data it is concluded that parts of the Kempirsai Massif have been pervasively metasomatized by large amounts of fluids and melts derived from a subducted slab composed of oceanic crust and sediments. Subduction occurred at least 15–35 Ma after a melting event that produced a typical ophiolitic sequence in the Paleozoic Sakmara Zone. We conclude that large chromite orebodies formed from second-stage high-Mg melts that interacted with depleted mantle and fluids on their way upward in a suprasubduction zone regime, and in a fore-arc position to the Magnitogorsk island arc.

Notes: Abstract The Campanian-Maastrichtian Agbaja Ironstone Formation of the Nupe basin, Nigeria, forms a major part of the about 2 billion tons of iron ore reserves of the Middle Niger Embayment. The ironstone deposits were previously reported to be similar to the Minette-type ironstones because of their depositional patterns, composition and inferred origin. Four rock-types are recognized within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, mud-ironstone and bog iron ore. In the ironstones, kaolinite of both the groundmass and the ooids/pisoids is of lateritic origin, whereas the associated quartz, mica and heavy minerals are of detrital origin. Ooids and pisoids were formed by mechanical accretion of platy kaolinite crystals by rolling on the sea floor in a near-shore environment, and were subsequently transported and deposited together with a fine-grained kaolinitic groundmass. Pyrite (mainly framboidal) and siderite (both exclusively occurring as pseudomorphs of goethite and/or hematite) are diagenetic whereas goethite is post-diagenetic in origin, resulting from the ferruginization of the kaolinitic precursor. Crandallite-gorxeicite-goyazite, bolivarite and boehmite are also post-diagenetic in origin. Hematite was formed from the dehydration of goethite, whereas gibbsite (restricted to the upper part of the deposit) is of recent and in situ lateritic origin. The presence of newly formed authigenic pyrite and siderite (now replaced by hematite and goethite) are indicators of a reducing environment during diagenesis. The absence of diagenetic chamositic clay minerals, evidently caused by a low Mg concentration, suggests that fully marine conditions were not established during sedimentation. This is supported by the lack of fossils, brecciated shell materials and bioturbation features in the deposit. Reworking and redeposition of the primary constituents are inferred from broken pisoids, nuclei of pisoidal/ooidal fragments in pisoids and high iron concentrations present in the pisoids and ooids compared to that of the groundmass. These observations indicate that the Agbaja ironstone deposits of the Lokoja study area exhibit some environmental and mineralogical characteristics that are markedly different from other known deposits of Minette-type, where primary chamositic clay minerals generally form the protore for the ironstones. The recognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age (Late Cretaceous) in Nigeria, Sudan and Egypt have implications for the paleoenvironmental interpretations of Phanerozoic ironstone deposits.

Notes: Abstract The manganese deposit of Nsuta, in the Ashanti Belt of Southern Ghana, is sandwiched between Birimian metasedimentary rocks. The metasedimentary rocks contain interbedded carbonate-rich layers, which exhibit a characteristic banded appearance near the contact with the orebody. The orebody is a carbonate-type manganese-formation and in terms of origin is considered here as a Mn-analogue of the volcanogenic-exhalative Algoma type iron-formation. The protolith of the orebody (chemical sediment including Fe-bearing rhodochrosite and alabandite) is envisioned to have been formed in a marine basin with relatively high CO2 activity and Eh-pH conditions were extremely low (Eh 1 to −0.6 Volt and pH 8 to 11) during Birimian times (2170–2180 Ma). These conditions occurred immediately below the shelf break in a shallow-marine environment. Subsequent submarine weathering (halmyrolysis) followed later by metamorphism of Eburnian age (2100 Ma) led to the formation of Mg-Ca-Fe-bearing rhodochrosite, the dominant mineral in the orebody. Other minerals of the orebody are: sulfides (e.g. two generations of alabandite sphalerite, pyrite, millerite, niccolite, gersdorffite, and molybdenite), oxides and hydroxides (vanadium-bearing jacobsite, galaxite; brucite, Mn2+-todorokite), Mn-silicates and an unknown boron mineral. Pyrochroite, possibly preceded by manganosite, occurs as a retrograde mineral. This mineral assemblage forms the protore of the Nsuta deposit. Opaque Mn4+-todorokite replacing Mn2+-todorokite, manganite, manganomelane, pyrolusite and nsutite which formed at the expense of rhodochrosite, are of supergene origin and represent the economic part of the deposit. The orebody is interleaved between the associated pelitic-psammitic metasedimentary rocks suggesting that its protoliths was deposited over a time interval during the sedimentation of the latter. Both units underwent subsequent processes (submarine weathering and metamorphism) together. The compositional differences between the orebody with high Mn and CO2 and low Si and Al contents relative to the metasedimentary rocks are explained by a model involving the continuous sedimentation of continent-derived materials (protolith of the metasedimentary rocks). During this time a pulsatory phase of submarine volcanism and consequent precipitation of materials of essentially volcanogenic-exhalative origin occurred (protolith of the orebody). From the exhalations, the carbonate minerals in both the manganese-rich sediments and the metasedimentary host-rocks (in the latter in the form of layers and disseminations leading to relatively high concentrations of Mn, Ca and CO2) were precipitated.

Notes: Abstract Os-rich laurite and erlichmanite are the dominant PGM inclusions in chromitites of the Othrys ophiolite complex. Close association of the PGM sulfides with enstatite, Na-rich pargasite, clinopyroxene, phlogopite, and Cu-Ni sulfides indicates crystallization at high temperature, in the presence of an alkali-rich fluid phase, under relatively-high sulfur fugacity. Because of the predominance of Ru-Os-Ir phases, the PGM assemblage of Othrys is similar to that of chromitites located in the mantle unit of other ophiolite complexes of the Balkan peninsula (Vourinos, Skyros Island, Rhodope) although it is distinguished because of the Os-rich composition of laurite and the presence of erlichmanite. Comparison among literature data for these complexes indicates that the Os/Ru ratios vary consistently in laurite and bulk chromitite with respect to the chondritic Os/Ru value. This suggests that fractionation between the two elements occurred, being apparently registered in the composition of laurite inclusions. Among other factors, fluctuation of sulfur fugacity during fractionation, as well as variation of the Os/Ru ratio in the parent melts of the chromitites might be invoked to explain the Ru-Os decoupling.

Notes: Abstract Potholes represent areas where the normally planar PGE-rich Merensky Reef of the upper Critical Zone of the Bushveld Complex transgresses its footwall, such geometric relationships being unusual in layered intrusions. The recognition of vertical dykes of Merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull-apart sites resulting from tensional deformation due to the loading effects of major new magma additions. In contrast, crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement. The Merensky Reef and its footwall have distinct initial Sr-isotope ratios (R 0 〉 0.7066 and 〈0.7066, respectively), which may be used to constrain these processes related to pothole formation. Merensky Reef in potholes (R 0 = 0.7069−0.7078) shows no isotopic evidence of assimilation of, or reaction with, footwall material. Discrete, discordant replacement bodies of anorthosite extend from the footwall lithologies to cross-cut the Merensky Reef and its hanging wall. The initial Sr-isotope ratio in these replaced rocks is totally reset to footwall values (R 0 = 0.7066), and immediately adjacent stratiform lithologies are slightly modified towards footwall values. In contrast, Neptunian pyroxenitic (Merensky) dykes cross-cutting the footwall lithologies, with a large surface area to volume ratio, and low Sr content, do not display footwall-like Sr-isotope initial-ratios (R 0 = 0.7077), and thus show no evidence for assimilation of or reaction with footwall material. Furthermore, pegmatoidal replacement pyroxenite (“replacement pegmatoid”), at the base of the Merensky Reef within potholes, has a high initial-ratio (R 0 〉 0.7071), and so models of pervasive metasomatism by footwall material are not applicable. This isotopic evidence indicates that there was no active interaction of footwall material with the overlying magma during, or after, the formation of Merensky Reef potholes, a basic tenet of existing pothole formation hypotheses involving footwall mass-transfer. In contrast, the isotopic data are entirely consistent with an extensional model for pothole formation, with the more radiogenic Merensky magma migrating laterally to fill extensional zones in the footwall layers.

Notes: Abstract An under-recognized and economically important class of intrusion-related gold deposits, which occur within magmatic provinces best known for tungsten and/or tin mineralization, is described with reference to seven major deposits (Fort Knox, Mokrsko, Salave, Vasilkovskoe, Timbarra, Kidston and Kori Kollo). These gold deposits contain a metal suite that includes some combination of bismuth, tungsten, arsenic, tin, molybdenum, tellurium and antimony, and contrasts with that found in the more widely-developed gold-rich porphyry copper and related deposits. The gold deposits associated with tungsten and/or tin provinces are located in cratonic margins, in a landward or back-arc position relative to continental margin arcs (where recognized), or within continental collisional settings. The deposits are related genetically to felsic domes, stocks or plutons of intermediate oxidation state, both magnetite- and ilmenite-series magmas are represented. The intrusion-hosted gold deposits are most commonly of sheeted vein/veinlet type, although greisen-like, disseminated and breccia deposits are also described. Gold may also be concentrated more distally (1–3 km) with respect to the intrusions, where deposits may be of skarn, disseminated replacement or vein types. K-feldspar, albite and/or sericitic alteration assemblages, commonly including carbonate, accompany the gold mineralization. In sheeted vein deposits, alteration is normally restricted to narrow envelopes around veins, whereas more pervasive alteration occurs in greisen-like, disseminated and shallow (〈3 km) deposits. The gold mineralization is commonly present with low total sulphide contents (〈3%), mainly pyrite and lesser arsenopyrite. In several deposits, bismuth minerals are closely associated with gold, and bismuth-gold and tellurium-gold correlations exist. Most deposits contain tungsten, tin, molybdenum and antimony, although generally these do not correlate with gold; tungsten and molybdenum concentrations may increase with depth or may occur in separate zones. Base metals generally are present in minor amounts (e.g. 〈100 ppm Cu). The distinct spatial association with felsic intrusions, combined with the consistent metal signature, suggests a magmatic-hydrothermal origin. Fluid inclusions studies indicate the presence of high-salinity fluids in some deposits, and low-salinity and carbonic fluids in most deposits, similar to the composition of fluids in intrusion-related tungsten deposits. Variations in mineralization style largely reflect depth of formation and location relative to the intrusive centre. Several deposits in this class contain 〉100 tonnes (3 million oz) of gold, thereby highlighting the gold potential of intrusion-related deposits beyond the more traditionally explored gold and copper provinces in arc terranes.

Notes: Abstract Southern Cross was one of the earliest gold mining centres in Western Australia. Over 142 tonnes of gold have been produced from the district, and, on a gold per hectare basis, the Southern Cross greenstone belt in the southwestern Yilgarn Craton is the most productive of Western Australia's Archaean greenstone belts. The SW Yilgarn Craton is characterised by high-grade (amphibolite- to granulite-facies) metamorphism, extensive granitoid magmatism and older greenstone volcanism ages, compared to the well-known greenschist-facies metamorphism and younger (2.7 Ga) eruption ages which dominate in the Eastern Goldfields Province. The Pb-isotope compositions of deep-seated granitoids in the SW Archaean Yilgarn Craton, which were emplaced coeval with a craton-wide major orogenic lode-gold mineralization event at about 2.64–2.63 Ga, have been determined for 96 whole-rock and 24 K-feldspar samples. The Pb isotope data of the granitoids are consistent with a crustal origin for their genesis, probably by reworking (partial melting) of older continental crust. The Pb isotope composition of greenstones, which are the main host rocks for gold mineralisation, and pyrites from the komatiite-hosted syngenetic Ni deposits in the amphibolite-facies Forrestania greenstone belt, have also been determined, with initial Pb-isotope ratios higher than that for the Eastern Goldfields Province. The Pb isotopic character of the orogenic lode-gold deposits in the region is intermediate between coeval granitoid and greenstone Pb, indicating that the ore fluids contained metals from both reservoirs. The Pb in the ore fluid of the most deeply formed deposit, Griffin's Find, overlaps the isotopic composition of coeval granitoids, indicating the deep-seated granitoid magmatism was the primary source for Pb in the ore fluids.

Notes: Abstract Niuxinshan is a typical example of the numerous mesothermal gold deposits formed during Mesozoic tectono-magmatic reactivation of the Archean North China Craton in eastern Hebei province. Gold occurs in quartz-sulfide lodes in Archean amphibolites and also in greisen zones in the Mesozoic Niuxinshan granite stock. Four mineralization stages can be recognized from early to late: (1) quartz-K-feldspar, (2) quartz-pyrite, (3) quartz-polysulfide, and (4) quartz-carbonate. Gold mineralization mainly occurs in stages 2 and 3. Fluid inclusions in quartz and fluorite from greisen zones in the Niuxinshan granite, and inclusions in vein quartz and sphalerite from stages 1 to 3 in the amphibolites, have been studied by microthermometry. Three compositional types of inclusions are recognized: type 1 (Tp1) are H2O-CO2-bearing inclusions and include primary (Tp1-P) and secondary (Tp1-S) inclusions. These are found in quartz and fluorite from the greisen zones as well as in vein quartz and sphalerite from stages 1 to 3. The Tp1-P inclusions are considered to represent the gold-bearing hydrothermal fluids. Type 2 (Tp2-S) are secondary H2O-CO2 + solid phase inclusions in fluorite from the greisen zones. Type 3 (Tp3-S) are secondary aqueous inclusions with a solid phase which coexist with the Tp2-S in fluorite from the greisen zones. The Tp1-P inclusions show variable VCO2 (commonly 0.3 to 0.6) and XCO2 values (mainly 0.1 to 0.4). The salinities of inclusions cluster around 3 to 11 wt.% NaCl equivalent and their homogenization temperatures to the liquid phase (Th(L)) fall dominantly in the range of 260 to 360 °C. The compositional variations of inclusions in stage 1 probably result from exsolution of magmatic fluids at various stages; immiscibility or boiling of the fluids can be ruled out. The compositional variations of inclusions in the greisen zones and in vein stages 2 and 3 are attributed to cooling, mixing (dilution), and necking-down of the fluids. The Tp1-S and Tp2-S inclusions show salinities of 3 to 6 wt.% NaCl equivalent and XCO2 values of 0.04 to 0.17. Th(L) clusters at 240 to 260 °C. The Tp3-S inclusions have salinities of 3 to 6 wt.% NaCl equivalent and Th(L) of 170 to 240 °C. Isochoric reconstructions, combined with oxygen and sulfur isotope geothermometry of mineral pairs, give trapping P-T conditions for the gold-bearing fluids. The greisen zones formed at 310 to 460 °C and 1.3 to 3.7 kbar; stage 1 veins at 300 to 430 °C and 1.2 to 3.7 kbar; stage 2 veins at 290 to 380 °C and 1 to 3 kbar; stage 3 veins at 250 to 350 °C and 1 to 3 kbar. H2O-CO2 fluids with low to moderate salinities and moderate to high densities (0.66 to 1.01 g/cm3) dominated at early mineralization stages, and evolved towards H2O-richer and CO2- and less saline fluids through time. The retrograde P-T evolution probably resulted from regional uplift and cooling of gold-bearing hydrothermal fluids. The gold bisulfide complex was dominant in the fluids during mineralization and gold deposition was mainly induced by decreases of temperature and pressure, as well as destabilization of the bisulfide complex during sulfidization of wall rocks.

Notes: Abstract Greenschist facies schist which hosts the Macraes Mine in East Otago, New Zealand has been pervasively altered by post-metamorphic (lower greenschist facies) fluids over a 120 m thick section perpendicular to foliation. Metamorphic titanite has been replaced by rutile, and epidote has been replaced by a variety of metamorphic minerals including siderite, chlorite, muscovite and calcite. The early stages of this alteration occurred during development of a ductile cleavage associated with kilometre scale recumbent folding. The cleavage was widely overprinted by a subparallel set of spaced (mm scale) microshears which are locally enriched in rutile and hydrothermal graphite. Strain was then concentrated into narrow (m scale) zones where more intensely deformed portions of the rock are crossed and highly disrupted by closely spaced (100 μm scale) microshears. The highly strained rocks show a combination of mylonitic and cataclastic microstructures, including crystal-plastic grain size reduction and recrystallization of micas to form a new foliation. Muscovite has grown at the expense of albite in the mylonitic cataclasites. Hydrothermal alteration was accompanied by addition of pyrite, arsenopyrite and gold without development of quartz veins. Gold precipitated with sulphides during reduction of the fluid by hydrothermal graphite. The whole altered rock sequence was later cut sporadically by mesothermal quartz veins which contain gold, scheelite, rutile, pyrite and arsenopyrite. This deposit displays a continuum of post-metamorphic processes and hydrothermal fluid flow which occurred during uplift of the schist belt.

Notes: Abstract Phanerozoic metallogenesis in Europe displays divergent characteristics which may be related to the variable nature of the three great European Phanerozoic orogens: Caledonian, Variscan and Alpine. These reflect different geodynamic processes. The Caledonian orogen resulted from the interaction of essentially oceanic with continental lithosphere, whilst the Variscan and Alpine orogens evolved mainly from continent-continent collisions with the involvement of a series of smaller oceanic basins. Each major stage of the orogenic processes is characterised by a typical metallogeny. The occurrence of subduction-related processes in the Caledonian orogeny gave rise to extensive VHMS deposition with characteristic Zn:Cu ratio signatures. The relative lack of abundance of metal-rich, Andean-type porphyry-type mineralisation remains unexplained, unless present erosional levels have prevented the preservation of such deposits. Continent-continent collisions do not appear to result in extensive mineralisation unless elevated heat flows result, possibly as a result of lithospheric delamination at the peak collisional stage. The development of late stage and peri-orogenic sedimentary basins are characterised by extensive Pb-Zn-Ba-F mineralisation as expressions of basinal fluid flow of regional dimensions.

Notes: Abstract The epithermal Au-Ag Shkol'noe deposit is located in the Kandjol ore field, Kurama Mountains. This region is a part of the east-west trending Late Hercynian Bel'tau-Kurama volcanic belt, an Andean-style collisional margin. The deposit comprises a number of quartz-carbonate veins hosted by the syn-subductional Middle Carboniferous Karamazar granodiorites. The Au-Ag mineralization is considered to be the result of the earliest hydrothermal event in the region. The Rb-Sr isochron age 296.3 ± 1.3 Ma and an initial 87Sr/86Sr0=0.7071 ± 2 ratio were obtained for an adularia-sericite-quartz-calcite sample from Au-Ag mineralization. The 87Sr/86Sr ratio range from 0.70645 ± 10 to 0.70741 ± 10 was obtained for the calcites from the earlier and later mineral assemblages. The Rb-Sr age is interpreted as a real geological age of the Au-Ag mineralization. It corresponds to the initial stage of the Late Carboniferous – Early Permian collision following the main syn-subduction stage of Bel'tau-Kurama volcanic belt evolution. The comparison of the Rb-Sr age with previously obtained 40Ar-39Ar and K-Ar data for adularia from the Au-Ag mineralization implies that gangue minerals of the Shkol'noe deposit bears the fingerprint of at least three events in its history. They are (1) Au-Ag mineralization at 296.3 ± 1.3 Ma; and (2) two subsequent thermal pulses at 277 ± 4 and 263–267 ± 8 Ma. The minimum time scale for the hydrothermal activity within the Shkol'noe deposit is thus approximately 30 million years. A general uniformity of the strontium source during the hydrothermal processes within the Au-Ag Shkol'noe deposit (87Sr/86Sr0=0.70645 ± 10 to 0.70741 ± 10) is suggested as well as within the Bel'tau-Kurama belt (87Sr/86Sr0=0.7051–0.707). The slight shift into a higher strontium isotope composition of the hydrothermal minerals of the Shkol'noe deposit in comparison with other deposits and rocks of the Bel'tau-Kurama belt may be ascribed to the contribution of relatively radiogenic strontium from the Karamazar-type granitoids. The mobilization of low radiogenic strontium during propylitic alteration of diabase dikes emplaced after the Au-Ag mineralization could be responsible for comparatively low 87Sr/86Sr ratios in some of the latest post-dike carbonates.

Notes: Abstract Synorogenic veins from the Proterozoic Eastern Mount Isa Fold Belt contain three different types of fluid inclusions: CO2-rich, aqueous two-phase and rare multiphase. Inclusions of CO2 without a visible H2O phase are particularly common. The close association of CO2-rich inclusions with aqueous two-phase, and possibly multiphase inclusions suggests that phase separation of low- to -moderate salinity CO2-rich hydrothermal fluids led to the selective entrapment of the CO2. Microthermometric results indicate that CO2-rich inclusions homogenize between –15.5 and +29.9 °C which corresponds to densities of 0.99 to 0.60 g.cm−3. The homogenization temperatures of the associated aqueous two-phase inclusions are 127–397 °C, with salinities of 0.5 to 18.1 wt.% NaCl equivalent. The rarely observed multiphase inclusions homogenize between 250 and 350 °C, and have salinities ranging from 34.6 to 41.5 wt.% NaCl equivalent. Evidence used to support the presence of fluid immiscibility in this study is mainly derived from observations of coexisting H2O-rich and CO2-rich inclusions in groups and along the same trail. In addition, these two presumably unmixed fluids are also found on adjacent fractures where monophase CO2-rich inclusions are closely related to H2O-rich inclusions. Similar CO2-rich inclusions are widespread in mineral deposits in this region, which are simply metal-enriched synorogenic veins. Therefore, we argue that fluid immiscibility caused volatile species such as CO2 and H2S to be lost from liquid, thus triggering ore deposition by increasing the fluid pH and decreasing the availability of complexing ligands.

Notes: Abstract Integration of geochemical, mineralogical, isotopic, and geochronological data with geodynamic considerations suggests that the Variscan granites in the Erzgebirge-Slavkovský les domain originated from repeated melting events and were emplaced over a period of about 40 Ma (330–290 Ma). Several lines of evidence exist supporting the idea that Erzgebirge granites assigned to different types (biotite granites, two-mica granites, strongly peraluminous P-rich Li-mica granites, and slightly peraluminous P-poor granites) are in most cases not genetically related via continuous fractional crystallization from a common magmatic reservoir. The genesis of the Slavkovsky les granites, however, might be discussed in terms of an uninterrupted fractionation series. Geological models of Sn-W deposits based upon geochemical and structural results imply that the main ore depositional events followed immediately the emplacement and solidification processes of melt via fluid-melt immiscibility, breccia-pipe formation and/or pervasive rock-fluid interactions.

Notes: Abstract The Rammelsberg polymetallic massive sulphide deposit was the basis of mining activity for nearly 1000 y before finally closing in 1988. The deposit is hosted by Middle Devonian pelitic sediments in the Rhenohercynian terrane of the Variscan Orogen. The deposit consists of two main orebodies that have been intensely deformed. Deformation obscures the original depositional relationships, but the regional setting as well as the geochemistry and mineralogy of the mineralisation display many characteristics of the SHMS (sediment-hosted massive sulphide) class of ore deposits. Rammelsberg is briefly compared to the other massive sulphide deposits in the European Variscan, including Meggen and those deposits in the Iberian Pyrite Belt.

Notes: Abstract The Almadén district is the largest mercury concentration in the world, with a total content of about 250 000 t of mercury, nearly one third of the known total mercury resources of the Earth. Mercury has been exploited since the Celtic and Roman times, with peak production during the Renaissance and between 1939–1945. The district is hosted by a Paleozoic synclinorium overlying Precambrian rocks. The Paleozoic sequence comprises epicontinental quartz arenite rocks, including black shales and quartzites. Diatremes, alkaline lavas of different composition, and late tholeiitic diabases account for the Ordovician to Devonian magmatism. The tectonic setting of this complex suite corresponds to the intraplate type. The mercury deposits of Almadén can be classified into two main types: type 1, early stratiform type ores characterized by cinnabar deposition on the lower Silurian quartzites (Criadero quartzite; e.g. the Almadén and El Entredicho deposits), and type 2, late discordant orebodies (e.g. Las Cuevas), largely hosted or related to diatremes (the `frailesca rocks') of alkaline basaltic composition. In type 1 cinnabar was deposited during diagenesis, in relation to hydrothermal circulation driven by magmatic activity. Type 2 include a variety of deposits having in common the discordant character of the orebodies (e.g. veins, stockworks, massive replacements), and their wide dispersion along the stratigraphic column, i.e. from Lower Silurian (e.g. Nueva Concepción) to Upper Devonian (e.g. Corchuelo).

Notes: Abstract The French Massif Central (FMC) represents the whole West European Variscan (WEV) belt, in terms of both the geodynamic evolution and the metallic content. Thus, a study of the metallogenic evolution of the FMC may elucidate the conditions that allow the mineralisation of a collision belt, since recent collision belts, e.g. the Himalayas or the Alps show that mineralisation does not necessarily result from the collision process. The Palaeozoic history of the FMC is divided into three geodynamic stages unevenly involved from the metallogenic view point. The Eo-Variscan stage (Cambrian to Silurian) was not important; the Meso-Variscan stage (Devonian-Early Carboniferous) was of limited importance; and most of the mineralisations formed during the Neo-Variscan stage (Late Carboniferous-Early Permian). In addition, some more mineralisation was produced during the Mesozoic because of the thermal reactivation linked with the Alpine orogenies. The Eo-Variscan stage (Cambrian-Silurian) corresponded to the pre-collision history, marked at the WEV belt scale by a fragmentation of the northern Gondwana (immature crust evolved from the Late Proterozoic Cadomian orogeny), up to the break-up of the crust and the formation of oceanic basins (Cambrian-Ordovician), followed by their resorption by subduction during the Silurian. In the FMC, no subduction-related magmatism is known (being rare at the WEV belt scale), and consequently subduction-related mineralisation, e.g. porphyry copper, is unknown in the WEV belt. Although some ophiolitic remnants are known, they never display Cyprus-type VMS deposits, nor massive podiform chromitites. Beside platformal sedimentary deposits on passive margins, the only deposits formed during the Eo-Variscan stage were of the SEDEX type, linked with the early rifting of the Gondwanian crust. The Meso-Variscan stage (Devonian-Early Carboniferous) corresponded to the collision proper, with the formation of crustal-scale nappe structures and the intrusion of collision-related peraluminous granites. Although these granites were enriched in rare metals they did not yield significant hydrothermal mineralisation, due to the great depth of their emplacement, as the similar granites in the Himalayas. However, they were a source of rare metals (in particular, uranium) for later mineralisation events. At the WEV belt scale Devonian distensive events are coeval with the collision. They were recorded by the formation of sedimentary basins of limited time and space extent, corresponding to the splitting of the continental crust (up to formation of oceanic domains in many cases), and were characterised by a bi-modal (“spilite-keratophyre”) volcanism. These basins formed in transtensional (or pull-apart) settings along major strike-slip faults, a peculiarity of the Variscan collision belt (which may conveniently be described as a “strike-slip orogen”). In such basins, many deposits linked with the volcanic thermal energy were formed: SEDEX deposits of the Meggen-type, iron deposits of the Lahn-Dill-type and VMS base metal deposits, the latter being the only ones known in the FMC (Brévenne deposits). The Neo-Variscan stage corresponded to the “hypercollision” and was characterised by a shift from compressional tectonics (late thickening of the crust during the Sudetian event and long-lasting dextral strike-slip tectonics along NW-SE to NE-SW fault zones) towards extensional tectonics (“basin and range” of the Late Stephanian-Early Permian), as well as by high heat flows, recorded by LP-HT metamorphism, extensive granitisation and granulitisation of the lower crust. These characteristics record the development of a lithospheric delamination process. In response to the energetic input released by this process, numerous hydrothermal deposits were formed in the FMC, as well as in the whole WEV belt, during the Neo-Variscan stage. These are mainly: (1) high-temperature granite-centered tungsten deposits, mainly associated with cordierite-bearing high level intrusions of Namurian-Westphalian age; (2) rare metal granites (and the associated hydrothermal tin mineralisations), resulting from fluid-induced low-degree partial melting of the middle crust in relation with the devolatilisation of the granulitised lower crust; (3) shear-zone hosted gold and antimony deposits, related to crustal-scale hydrothermal circulation, triggered by the transition to extensional tectonics at about 300 Ma; and (4) uranium deposition in extensional settings related to the Early Permian distension. The Post-Variscan mineralising events recorded the renewal of thermal flows in the lithosphere linked with early Alpine events (mainly the Trias-Lias distension in the Tethyan realm and the middle Cretaceous opening of the Bay of Biscay in the Pyrenean realm). They resulted in low-enthalpy geothermal systems, leading to a variety of deposits, mainly: (1) F-Ba districts, reworking F and Ba from Late Variscan granites and ignimbrites; (2) a major uranium deposit (Lodève), reworking uranium from the Permian Lodève basin; and (3) Zn-Pb districts of the MVT-type. Finally, the mineralisation of the Variscan collision belt is mainly the consequence of the Neo-Variscan lithospheric delamination process. By contrast, the absence of such a process in collision belts like the Himalayas or the Alps is the key of them being devoid of mineralisation. It appears that the mechanical energy released by the collision itself is not sufficient to mobilise and concentrate the trace elements involved in the metallogenic processes.

Notes: Abstract The Irish carbonate-hosted base metal deposits have long been an enigmatic subclass of deposit. Some of the Irish deposits (Harberton Bridge, Allenwood) are clearly epigenetic, hosted in breccia pipes and have close affinities to Mississippi Valley-type deposits. Others, are characterised by stratabound and sometimes stratiform mineralisation (Abbeytown, Navan), while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds (Lisheen, Ballinalack). When the stratigraphic and textural evidence is reviewed, it is apparent that all of the base metal mineralisation occurred in already compacted sediments. Hence, mineralisation probably occurred at depths of at least several 100 m depth below the sea bed. The mineralisation occurred either during transtensional Lower Carboniferous basin development, or subsequently, during the onset of Variscan shortening. Fluids may have been derived from dewatering of Variscan-driven deformation to the south of the carbonate platform, with fluid flow through major fracture zones and basal clastic aquifers. Alternatively, high heat flow produced by Lower Carboniferous extension may have driven the mineralising system. Syn-genetic models are extremely difficult to sustain.

Notes: Abstract Caledonian metallogeny has endowed Scandinavia with abundant metalliferous mineral resources of several genetical and compositional types, many of which have been exploited at various scales from the seventeenth century onwards. Because of this long history of exploration/exploitation, coupled with excellent exposures, the Caledonian orogen in Scandinavia can be taken as a model to illustrate the relationship between metallogenic evolution and plate tectonics. Its orogenic and metallogenic development can be related to a period of plate movements which started about 700–600 Ma ago with rifting and break-up of a Proterozoic megacontinent (Rodinia) followed by the opening-up of a wide ocean (Iapetus), and ending with collision between the Laurentian and Baltic continents (the climactic Scandian phase of orogeny) in Silurian times. Fragments of this sedimentary, magmatic and tectonic history are recorded in various autochthonous, and allochthonous units which were ultimately thrusted eastwards above the Baltic plate margin. The present consensus is that sequences deposited on the margin of the Laurentian plate are represented in the Uppermost Allochthon (UmA) mostly in northern Norway. The Neoproterozoic and Cambrian history of this margin started with the rifting of Rodinia, followed by the development of an Atlantic-type margin which was characterised by an easterly thickening, continental, shelf-wedge of basal quartz sandstones and an overlying blanket of thick carbonate banks reflecting the drift of Laurentia towards equatorial paleo-latitudes. This setting is considered to have been host to two characteristic types of stratabound-stratiform ores: massive to disseminated Zn-Pb-Cu and Cu-Zn sulphides in sedimentary and mixed sedimentary-volcanic lithologies with original ore reserves up to 5–6 Mt (Bleikvassli, Mofjellet); and numerous, laterally extensive, magnetite-hematite deposits in marble-metapelite lithologies. The economically most important of the latter group are the deposits of the Dunderlandsdal area where resources of around 500 Mt were present. Early rift basins preserved on the Baltic side of the rifted megacontinent, were filled with coarse clastites prior to establishment of a passive plate margin in Cambrian times. The passive margin was characterised by shallow marine sandstones and later bituminous Alum Shales deposited in a stable, epicontinental sea. Platform sedimentation was accompanied by local magmatic activity of rift-type tholeiitic through to carbonatitic compositions. This plate margin is presently represented in autochthonous and allochthonous sequences in the lower parts of the Caledonian nappe pile. Metalliferous mineral formation related to this time and setting was of limited importance and comprised previously worked Nb, P and Fe ores in carbonatites of the Fen Complex, vast but uneconomic resources of U, Mo, Ni and V in the Alum Shales, and minor stratabound base metal sulphides and orthomagmatic Cr and Ni-Cu-(PGE) occurrences. Continental margin and oceanic successions, probably developed along the edge of a microcontinent within the Iapetus ocean, are represented in the Gula, Støren and equivalent, sequences in the Upper Allochthon (UA), tectonostratigraphically above and west of the Baltic platform lithologies. These host many Cu-Zn sulphide ores intimately related to tholeiitic basalt units in sedimentary sequences dominated by pelitic and psammitic material, as well as bituminous shales and quartzites of possible ribbon-chert origin. Original ore reserves around 20 Mt were known at the now abandoned Tverrfjell mine and the recently closed Joma deposit. Minor Cu-Ni ores occur in subvolcanic, mafic-ultramafic intrusions. Major plate convergence is first recorded in the Middle to Late Cambrian (about 505 Ma), when subduction along the outer margin of Baltica affected rocks which are now found in or below the basal parts of the UA. The metallogenic significance of this subduction is uncertain; related magmatic and ore-forming processes have yet to be documented. The earliest subduction-related sequences of major ore-forming importance, however, are slightly younger (Early Ordovician) and are presently found in the UA and partly in the UmA. One of these is the Stekenjokk-Fundsjø arc sequence (about 490 Ma) which forms a central unit in the mountain chain, comprising bimodal, immature arc-type volcanites and high-level felsic intrusions. The sequence formed in a primary oceanic setting outboard of the Baltica plate while this lay in an approximately east-west position opposite to, and at some distance from, the Siberian plate, and was amalgamated with the Gula Complex in Ordovician times. Abundant VMS deposits are associated with thick, often graphitic, tuffites and are of the Zn-Cu type with generally high Zn/Cu ratios. The biggest known deposit (Stekenjokk-Levi) contained 26 Mt of ore, four others had individual tonnages of about 3 Mt and have been important base metal producers. Another series of arc sequences are found at higher levels in the nappe pile, in southwestern Norway, the western Trondheim Region, the western Grong District, Leka and Lyngen. Mafic or bimodal volcanic and plutonic lithologies comprise many types characteristic of immature arcs and often include ophiolitic successions. They all formed in oceanic arc-marginal basin systems on the Laurentian side of the Iapetus ocean between 500 and 480 Ma. VMS deposits of the Cu-Zn type are abundant and several of these were important base metal producers in the past. The biggest deposit is Løkken which contained 30 Mt of ore; six fall in the 1–10 Mt class and include the Skorovas and other deposits in the western Grong District and in southwest Norway. Orthomagmatic ores occur in the form of PGE mineralisation in ultramafic cumulates in the basal parts of ophiolite successions and Cu-Ni-PGE mineralisation related to high-level intrusions of boninitic affinity; none of these have hitherto been of economic significance. The immature arc-marginal basin systems near Laurentia continued to evolve and in Early to Middle Ordovician times magmatism gradually changed to predominantly calc-alkaline igneous activity characteristic of an active continental margin. Major VMS ores seem to be scarce in these sequences which are most extensively developed in central and southwest Norway. Known metalliferous deposits are predominantly iron-formations ranging from magnetite- to pyrite-dominated types with varying base-metal contents; Zn-Pb-Cu mineralisation is subordinate. Major granodioritic plutons which intruded the mature-arc sequences at an advanced stage of development (c.460 Ma) occasionally contain vein- and stockwork-type Cu-Mo mineralisation, so far with no economic significance. By the end of the Ordovician, convergence of the Laurentian and Baltic continents had resulted in considerable narrowing of the Iapetus ocean and obduction of the arc/arc-basin systems on to the edge of the Laurentian plate. Material derived from the uplifted sequences supplied detritus to the remaining, narrow, basin. At about the Ordovician/Silurian boundary these thick clastic, often calcareous, sequences and their crustal substrata were intruded by rift-type, mainly tholeiitic to alkaline magmas and minor subduction-related melts; local volcanism also accompanied this activity. This important magmatic event is interpreted to reflect a paleotectonic setting characterised by transcurrent movements and development of local transtensional regimes and fault-controlled sedimentary basins during the initial, oblique, interference of the Baltic and Laurentian plate margins. Its metallogenic manifestation is seen partly in occasional Ni-Cu deposits in mafic intrusions, including the Bruvann, Råna, deposit presently being exploited. More abundant, however, are stratabound sulphides which are associated with the volcanic rocks, or hosted in the sedimentary successions often closely r

Notes: Abstract The Iberian Pyrite Belt, located in the SW Iberian Peninsula, contains many Paleozoic giant and supergiant massive sulphide deposits, including the largest individual massive sulphide bodies on Earth. Total ore reserves exceed 1500 Mt, distributed in eight supergiant deposits (〉100 Mt) and a number of other smaller deposits, commonly with associated stockwork mineralizations and footwall alteration haloes. Massive sulphide bodies largely consist of pyrite, with subordinated sphalerite, galena and chalcopyrite and many other minor phases, although substantial differences occur between individual deposits, both in mineral abundance and spatial distribution. These deposits are considered to be volcanogenic, roughly similar to volcanic-hosted massive sulphides (VHMS). However, our major conclusion is that the Iberian type of massive sulphides must be considered as a VHMS sub-type transitional to SHMS. This work is an assessment of the geological, geochemical and metallogenic data available up to date, including a number of new results. The following points are stressed; (a) ore deposits are located in three main geological sectors, with the southern one containing most of the giant and supergiant orebodies, whereas the northern one has mainly small to intermediate-sized deposits; (b) ore deposits differ one from another both in textures and mineral composition; (c) Co and Bi minerals are typical, especially in stockwork zones; (d) colloidal and other primary depositional textures are common in many localities; (e) a close relation has been found between ore deposits and some characteristic sedimentary horizons, such as black shales. In contrast, relationships between massive sulphides and cherts or jaspers remains unclear; (f) footwall hydrothermal alterations show a rough zoning, the inner alteration haloes being characterized in places by a high Co/Ni ratio, as well as by mobility of Zr, Y and REE; (g) 18O and D values indicate that fluids consist of modified seawater, whereas 34S data strongly suggest the participation of bacterial-reduced sulphur, at least during some stages of the massive sulphide genesis, and (h) lead isotopes suggest a single (or homogeneized) metal source, from both the volcanic piles and the underlying Devonian rocks (PQ Group). It is concluded that, although all these features can be compatible with classical VHMS interpretations, it is necessary to sketch a different model to account for the IPB characteristics. A new proposal is presented, based on an alternative association between massive sulphide deposits and volcanism. We consider that most of the IPB massive orebodies, in particular the giant and supergiant ones, were formed during pauses in volcanic activity, when hydrothermal activity was triggered by the ascent and emplacement of late basic magmas. In these conditions, deposits formed which had magmatic activity as the heat source; however, the depositional environment was not strictly volcanogenic, and many evolutionary stages could have occurred in conditions similar to those in sediment-hosted massive sulphides (SHMS). In addition, the greater thickness of the rock pile affected by hydrothermal circulation would account for the enormous size of many of the deposits.

Notes: Abstract The Kupferschiefer ore series, between the Lower Permian (Rotliegendes) terrestrial redbeds/volcanics and the Upper Permian (Zechstein) marine sequence, is developed as dark-grey organic matter-rich and metal sulphide-containing deposits (reduced zone) and as red-stained organic matter-depleted and iron oxide-bearing sediments (oxidized zone = Rote Fäule). The transition zone from oxidized to reduced rocks occurs both vertically and horizontally. This zone is characterized by sparsely disseminated remnant copper sulphides within hematite-bearing sediments, replacements of copper sulphides by iron oxides and covellite, and oxide pseudomorphs after framboidal pyrite. These textural features and copper sulphide replacements after pyrite in reduced sediments imply that the main oxide/sulphide mineralization postdated formation of an early-diagenetic pyrite. Hematite-dominated sediments locally contain enrichments of gold and PGE. The Kupferschiefer mineralization resulted from upward and laterally flowing fluids which oxidized originally pyritiferous organic matter-rich sediments to form hematitic Rote Fäule areas, and which emplaced base and noble metals into reduced sediments. It is argued that long-lived and large-scale lateral fluid flow caused the cross-cutting relationships, expansion of the hematitic alteration front, redistribution of noble metals at the outer parts of oxidized areas, and the location of copper orebodies directly above and around oxidized and gold-bearing areas. The Rote Fäule may be a guide to favourable areas for both the Cu-Ag and new Au-Pt-Pd Kupferschiefer-type deposits.

Notes: Abstract The two Alpine orogenic phases of the Eastern Alps, in the Cretaceous and in the Tertiary, were both accompanied by the formation of mineral deposits. However, subduction-related magmatic belts as well as the typical “Andean” ore deposits are missing. Therefore, the role of metamorphism in East Alpine metallogeny was tentatively explored for more than 60 y, although for a long time without tangible results. Microthermometric, geochemical and isotopic investigations of fluid inclusions from selected Alpine mineral deposits presented allow a preliminary confirmation of the involvement of metamorphic fluids in their origin. Deposits which were formed immediately after the first, Cretaceous orogeny, were produced at high pressures by fluids of very high salinity and high density, and with an isotopic composition of the water falling into the metamorphic field. These fluids are best understood as products of metamorphic de-volatilization of rocks of the subducted South Pennine domain. In contrast to this, the deposits formed after the second, Tertiary orogeny, originated at relatively low pressures from fluids with an appreciable content of CO2 and of low to moderate salinities. Isotopic compositions of this carbon indicate a deep crustal or even mantle source for CO2, while the water is isotopically more heterogeneous and may have mixed sources, both surficial and metamorphic. Tectonic control of these mineralizations is late-orogenic trans-tensional faulting, which exposed hot metamorphic rocks to fluid convection along brittle structures. These deposits conform best to the model of metamorphogenic metallogenesis by retrograde leaching, although ponded metamorphic fluids and mantle volatiles may also have been involved.

Notes: Abstract Epithermal precious metal deposits have only quite recently been added to the metallogenic view of Italy. It was only in the late 1980s that two districts were recognized, in Sardinia and southern Tuscany, previously long known for a large variety of other commodities. The Sardinian epithermal precious metal district is associated with the Oligocene-Miocene Sardinian calc-alkaline magmatic cycle. The most relevant areas include the Au deposit at Furtei, already in production, and the Osilo prospect, where extensive exploration is under way. The deposit at Furtei contains at least six tonnes of gold metal. The mineralogy of the deposit and of the alteration assemblages is typical of the acid-sulfate (high-sulfidation) class of volcanic-hosted epithermal deposits. Fluids associated with alteration and mineralization have moderate temperatures (200–300 °C) and low salinities (less than 6% wt. NaCl equivalent); high-temperature, high-salinity fluids similar to porphyry-style systems also circulated at Furtei. At Osilo, a number of quartz veins containing up to several ppm Au have been identified. The alteration assemblage includes adularia and illite, i.e. is typical of low-sulfidation deposits. Fluid inclusion homogenization temperatures range from 198 to 270 °C, with salinities of less than 4% wt. NaCl equivalent. In southern Tuscany, a number of carbonate-hosted (“Carlin type”) gold showings occur at the edges of the geothermal fields of Larderello, Amiata and Latera, an area previously known for Sb mineralization. In fact, many showings coincide with former Sb mines, and stibnite, along with pyrite, is the most common sulfide mineral. Gold is typically invisible. Fluids hosted in a variety of minerals from these occurrences span a relatively large temperature range (132 to 245 °C), with constantly low salinities (less than 7% wt. NaCl equivalent). The onset of (presumably meteoric) hydrothermal fluid circulation can be ascribed to the emplacement of Neogene Tuscan magmatic rocks, but the ultimate source of gold remains speculative.

Notes: Abstract This whole rock and silicate mineral study focuses on the genesis of the Merensky reef sequence, as well as the footwall and hanging wall norites at an area of Rustenburg Platinum Mines in a demonstrably normal (undisturbed) environment. Continuous sampling provides major and trace element variations and mineral compositions and allows an evaluation of the post- liquidus processes which affected the sequence. Following the formation of liquidus phases three stages are envisaged to have modified the rocks. These are (a) migration of fluid during early compaction of cumulates, (b) circulation of fluids within the crystal mush, and (c) reaction and solidification of trapped liquid. Liquidus compositions are nowhere preserved in the sequence. A strong link is demonstrated between orthopyroxene compositions (e.g. Mg# and TiO2) and the incompatible trace element content of the whole rocks. The final amount of trapped liquid is shown to have been variable but never exceeded 10%. Calculated liquidus (pre-equilibration) orthopyroxene compositions show an up- sequence progression of evolving compositions from the footwall norite to the hanging wall norite. Initial Sr isotopic values do not support a simple magma mixing model by which radiogenic Main Zone magma mixes with that of the Critical Zone at the level of the Merensky reef. There is evidence that the hanging wall norite formed from a much more evolved magma. These conclusions have implications for the distribution and origin of the PGE-enriched Merensky reef package.

Notes: Abstract The Waziristan ophiolite complex is located along the western margin of the Indian plate in northwestern Pakistan. The Mn-deposits in the Saidgi and Shuidar areas are part of this ophiolite complex. These deposits, both banded and massive in nature, are hosted by metachert and are generally overlying metavolcanics. Braunite and cryptocrystalline quartz are the main constituents of the manganese ores in both areas. Hematite occurs in the Shuidar deposits as a minor phase. Metacherts are microcrystalline aggregates of microcrystalline quartz, chalcedony, and lesser hematite. Chemically, the studied Mn-deposits and associated metacherts are very similar to those formed by submarine hydrothermal effusive processes. The deposits originated along sea-floor spreading centers (mid-ocean ridges) within the Neo-Tethys Ocean and were later obducted as part of the Waziristan ophiolite complex.

Notes: Abstract Mineralisation at the Zarshuran, NW Iran, occurs on the flank of an inlier of Precambrian rocks hosted in black silty calcareous and carbonaceous shale with interbedded dolomite and limestone varying in thickness from 5 to 60 m and extending along strike for approximately 5–6 km. Two major, steeply dipping sets of faults with distinct trends occur in the Zarshuran: (1) northwest (310–325) and (2) southwest (255–265). The main arsenic mineralisation occurs at the intersection of these faults. The mineral assemblage includes micron to angstrom-size gold, orpiment, realgar, stibnite, getchellite, cinnabar, thallium minerals, barite, Au-As-bearing pyrite, base metal sulphides and sulphosalts. Hydrothermal alteration features are developed in black shale and limestone around the mineralisation Types of alteration include: (1) decalcification, (2) silicification, (3) argillisation, (4) dolomitisation, (5) oxidation and acid leaching and (6) supergene alteration. The early stage of mineralisation involved removal of carbonates from the host rocks, followed by quartz precipitation. The main stage includes massive silicification associated with argillic alteration. In the late stage veining became more dominant and the main arsenic ore was deposited along fault cross cuts and gouge. These characteristics are typical of Carlin-type sediment-hosted disseminated gold deposits. The early stage of mineralisation contains only two-phase aqueous fluid inclusions. The main stage has two groups of three-phase CO2-bearing inclusions with minor CH4 ± N2, associated with high temperature, two-phase aqueous inclusions. During the late stage, fluids exhibit a wide range in composition, salinity and temperature, and CH4 becomes the dominant carbonic fluid with minor CO2 associated with a variety of two-phase aqueous fluid inclusions. The characteristics of fluids at the Zarshuran imply the presence of at least two separate fluids during mineralisation. The intersections of coexisting carbonic and aqueous inclusion isochores, together with stratigraphic and mineral stability evidence, indicate that mineralisation occurred at 945 ± 445 bar and 243 ± 59 °C, implying a depth for mineralisation of at least 3.8 ± 1.8 km (assuming a lithostatic pressure gradient). Fluid density fluctuations and the inferred depth of formation suggest that the mineralisation occurred at the transition between overpressured and normally pressured regimes. Geochronologic studies utilising K/Ar and Ar/Ar techniques on hydrothermal argillic alteration (whole rock and separated clay size fractions) and on volcanic rocks, indicates that mineralisation at Zarshuran formed at 14.2 ± 0.4 Ma, and was contemporaneous with nearby Miocene volcanic activity, 13.7 ± 2.9 Ma. It is proposed that mineralisation was the result of the infiltration of hydrothermal fluids containing a magmatic gas component, and that it was localised in the Zarshuran Unit because of the redox boundary that it provided and/or because it lay between an overpressured region at depth and a zone of circulating, hydrostatically pressured fluids above.

Notes: Abstract Several precious metal-bearing, low sulfidation epithermal veins occur in the rolling topography of the Osilo area, northern Sardinia. The Sa Pala de Sa Fae and the Sa Pedra Bianca veins were subject to intense diamond drilling exploration in the mid 1990 s. The veins extend for 1–3 km, dip steeply, and range from 1 to 10 m in width. High K-calc-alkaline volcanic deposits containing plagioclase phenocrysts (along with lesser pyroxene, amphibole, magnetite, olivine and sanidine) form the main host rocks. Gold grades in drill intersections range from 〈0.1 to 〈20 ppm, with silver-gold ratios of around 4 to 7. Mineralogical studies show a systematic distribution of three hydrothermal mineral assemblages. At distances 〉50 m from the vein, the assemblage albite + Fe-chlorite + illite + pyrite (± montmorillonite ± calcite ± K-feldspar) prevails regionally, and its formation is attributed to minor metasomatism of the country rock involving the addition of water, carbon dioxide and hydrogen sulfide. At distances 〈10 m from the vein, the assemblage quartz + K-feldspar + pyrite ± illite dominates, forming an alteration envelope that cross cuts regional alteration. Quartz and K-feldspar increase in abundance towards the vein. Quartz is the main vein mineral, and it displays a range of morphologies and textures including crustiform colloform banding, quartz pseudomorphs of platy calcite, breccias and coarse euhedral crystals. Electrum and argentite which are the main gold and silver minerals deposited during the early stages of vein mineralization with rhomb-shaped crystals of K-feldspar (adularia). Pyrite, plus lesser marcasite, arsenopyrite, stibnite and sphalerite, are the other sulfide phases in veins. Kaolinite ± halloysite ± jarosite form a late assemblage overprinting earlier hydrothermal alteration. It is mostly restricted to shallow depths of a few meters, except near veins. Most of this assemblage likely formed from weathering and oxidation of sulfides. Microthermometric measurements were made on quartz-hosted, two-phase (liquid + vapor) inclusions, containing ∼75% liquid; mean homogenization temperatures (∼750 measurements) range from 220 to 250 °C, and ice-melting temperatures (∼550 measurements) range from 0.0 to −2.3 °C. The presence of co-existing vapor-rich and liquid rich inclusions, with quartz pseudomorphs of platy calcite, indicate that boiling conditions existed. Slight vapor-bubble expansion of a few fluid inclusions subjected to crushing experiments indicates inclusion fluids contained variable but low concentrations of dissolved gas. This study shows that gold-silver mineralization formed in subvertical channels from ascending solutions at 250 °C at around 300 to 450 m below the paleo-water table in a typical low-sulfidation epithermal environment. Hydrothermal solutions that produced vein mineralization and related alteration were dilute (〈4.1 equivalent wt.% NaCl and 〈4 wt.% CO2), near neutral pH, reduced and, at times, boiling.

Notes: Abstract The BanskáŠtiavnica ore district is in the central zone of the largest stratovolcano in the Central Slovakia Neogene Volcanic Field, which is situated at the inner side of the Carpathian arc over the Hercynian basement with the Late Paleozoic and Mesozoic sedimentary cover. Volcanic rocks of the High-K orogenic suite are of the Badenian through Pannonian age (16.5–8.5 Ma). Their petrogenesis is closely related to subduction of flysch belt oceanic basement underneath the advancing Carpathian arc and to back-arc extension processes. The stratovolcano includes a large caldera 20 km in diameter and a late-stage resurgent horst in its centre, exposing a basement and extensive subvolcanic intrusive complex. The following stages have been recognized in the evolution of the stratovolcano: (1) formation of a large pyroxene/hornblende-pyroxene andesite stratovolcano; (2) denudation, emplacement of a diorite intrusion; (3) emplacement of a large granodiorite bell-jar pluton within the basement; (4) emplacement of granodiorite/quartz-diorite porphyry stocks and dyke clusters around the pluton; (5) caldera subsidence and its filling by biotite-hornblende andesite volcanics, emplacement of quartz-diorite porphyry sills and dykes at the subvolcanic level; (6) renewed activity of andesites from dispersed centres on slopes of the volcano; (7) uplift of a resurgent horst accompanied by rhyolite volcanics and granite porphyry dykes. The following types of ore deposits (mineralizations) have been identified in the BanskáŠtiavnica ore district: 1. Quartz-pyrophyllite-pyrite high-sulphidation system at Šobov, related to the diorite intrusion. 2. Magnetite skarn deposits and occurrences at contacts of the granodiorite pluton with Mesozoic carbonate rocks. Magnetite ores occur as lenses in the calcic skarns. 3. Stockwork/disseminated base metal deposit along an irregular network of fractures in apical parts of the granodiorite pluton and in remnants of basement rocks. Mineral paragenesis is simple, with leading sphalerite and galena and minor chalcopyrite and pyrite. In overlying andesites the mineralization is accompanied by metasomatic quartzites and argillites with pyrophyllite, kaolinite, illite and pyrite. 4. Porphyry/skarn copper deposits and occurrences related to granodiorite/quartz-diorite porphyry dyke clusters and stocks around the granodiorite intrusion. The mineralized zone is represented by accumulations of chalcopyrite in exo- and endo-skarns, usually of the magnesian type affected by serpentinization. Besides chalcopyrite, pyrhotite, minor bornite, chalcosite, tennantite and magnetite, rare molybdenite and gold are present. The alteration pattern around productive intrusions includes an external zone of propylitization, a zone of argillitic alteration (kaolinite – illite – pyrite) and an internal zone of phyllic alteration (quartz – sericite – pyrite). Biotitization is rare and limited to porphyry intrusions. 5. Intrusion related “mesothermal” gold deposit in an andesitic environment just above the granodiorite intrusion. Gold of high fineness with base metal mineralization is contained in brecciated and/or banded quartz veins of subhorizontal orientation, parallel to the surface of granodiorite pluton. At least the first phase of mineralization is older than quartz-diorite porphyry sills, which separate granodiorite and blocks of mineralized andesite. 6. Hot spring type advanced argillic systems in the caldera filling. Silicites and opalites accompanied by kaolinite, alunite and pyrite grade downward into smectite dominated argillites. 7. Vein type epithermal precious/base metal deposits and occurrences as a result of the long lasting interaction among structural evolution of the resurgent horst and evolving hydrothermal system, extensive intrusive complex and deep seated siliceous magma chamber serving as heat and magmatic fluid source. Three types of epithermal veins occur in a zonal arrangement: (a) base metal veins ± Au with transition to Cu ± Bi mineralization at depth in the east/central part of the horst, (b) Ag – Au veins with minor base metal mineralization and (c) Au – Ag veins located at marginal faults of the horst. Isotopic composition of oxygen and hydrogen in hydrothermal fluids indicate mixing of magmatic and meteoric component (with generally increasing proportion of meteoric component towards younger mineralization periods?). Veins are accompanied by zones of silicification, adularization and sericitization, indicating a low sulphidation environment. 8. Replacement base metal mineralization of a limited extent in the Mesozoic carbonate rocks next to sulphide rich epithermal base metal veins.

Notes: Abstract Ultramafic rocks in Jamaica are dunites with minor lherzolite, often serpentinised, and are part of a dismembered ophiolite complex. In Tobago, dunites, wehrlites, pyroxenites and hornblendites form the lower part of a plutonic complex of island arc affinity. The mineral assemblages and chemistry reflect these differences. Chromite in Jamaica is high in Al and Mg, whereas in Tobago it is rich in Fe, as in Alaskan-type intrusives. Ni-Cu-PGE assemblages in Jamaica are pentlandite, with later low temperature heazlewoodite, awaruite and native copper, the latter with Pt and Pd. In Tobago an assemblage of pentlandite, pyrrhotite, pyrite and chalcopyrite is much less affected by later alteration. PGE phases also occur. The dunites in Jamaica have sufficient MgO to be a potential source of olivine. The higher Fe in olivine from Tobago indicates that olivine cumulates in plutonics from island arc settings are a less suitable source of the mineral. Ni-laterites in Jamaica are unlikely because of high topographic relief. The prospect for Ni-laterites in Tobago is low as there is little Ni in the olivines. Chrysotile asbestos, talc and magnesite are absent in both islands. This is probably a consequence of the lack of secondary serpentine recrystallisation to form fibrous chrysotile veins, the deep tectonic level and lack of hydrothermal circulation for magnesite to form, and the absence of metamorphic/metasomatic events and/or late stage extension tectonics which might have yielded talc.

Notes: Abstract P–T conditions during skarn formation in the 75.5 Ma old Ocna de Fier-Dognecea (SW Romania) ore district are assessed in this work using a combination of petrogenetic grids, Berman's TWEEQU programme, and several independent geothermobarometers. These were applied both to hornfelses surrounding the skarn and to the granodiorite which caused the skarn and contact metamorphism. The results are consistent and point to a peak metamorphic temperature of 700 ± 50 °C, decreasing away from the contact, and to a pressure of 2.8 ± 1 kbar, equivalent to ∼10 km depth in the region. These results quantify the qualitative idea that skarn mineralisation normally forms in a high T, low P contact metamorphic environment.

Notes: Abstract The granodiorite intrusion at Ocna de Fier-Dognecea in the western South Carpathians, Romania, triggered the formation of a classic Fe-(Pb-Zn) skarn deposit. The intrusive is related to the larger composite Bocşa Laccolith five kilometres north that is part of the regional Banatite Suite. Previous work indicated a K/Ar age of 65–57 Ma and postulated an Andean-type subduction related tectonic setting for the intrusions. We report ion probe U/Pb zircon ages of 79.6 ± 2.5 Ma for the Bocşa Laccolith and 75.5 ± 1.6 Ma for the Ocna de Fier Pluton, which date their emplacement. Fission track dating on titanite gives slightly younger ages: 78 ± 4 Ma for Bocşa and 73 ± 4 Ma for Ocna de Fier. Together with zircon and apatite data from the same samples, average cooling rates of 52 °C/Ma and 83 °C/Ma are calculated for the Bocşa and Ocna de Fier intrusives respectively. A post-collision tectonic setting is proposed on the basis of field evidence, the timing of intrusions in the context of regional tectonic evolution, and trace element geochemistry.

Notes: Abstract The Paleoproterozoic Kauhajärvi gabbro is one of several Fe-, Ti-, and P-rich mafic intrusions associated with granitoids in the Fennoscandian shield in western Finland. The gabbro is cut by the late-orogenic Lauhanvuori granite (ca.1870 Ma), whereas the surrounding area is composed of synorogenic, collision-related granitoids and calc-alkaline volcanic rocks (ca. 1890 Ma) belonging to the Mid Finland Granitoid Complex. The mafic intrusions were probably emplaced into a Svecofennian rift zone. They are characterized by a high phosphorus content; the common occurrence of ilmenite as separate grains; and the coeval crystallization of apatite, Fe-Ti oxides, and Fe-Mg silicates. The Kauhajärvi gabbro is composed of two geochemically and structurally distinct zones. The basal zone is composed of poorly-layered, fine- to medium-grained gabbro, which represents an early intrusion of tholeiitic magma, and has rather high concentrations of chromium, magnesium and silica. Typically, the concentrations of iron, titanium and phosphorus are low, except for the top that is enriched in apatite and ilmenite. During most of the crystal-liquid fractionation of the basal zone magma, low f O2 limited the crystallization of Fe-Ti-oxides. Instead, titanium became enriched in the uppermost layer of the basal zone. The main zone represents a later injection of more evolved tholeiitic magma and makes up 80 to 90%of the total intrusion volume. Peridotite is common, along with gabbro and gabbronorite, in the lower and middle parts of the main zone, and anorthosite is common near the top of the main zone. The Mg:Fe ratio in mafic minerals and vanadium concentrations in magnetite decrease upwards. The variation within the main zone can be explained by crystal-liquid fractionation of a single batch of a parental magma under conditions of relatively high f O2. Titanium is not progressively enriched. The ratio of titanium to iron (TiO2/Fe2O3 = 0.16 to 0.20; Fe total as Fe2O3) is constant in the main zone and normal for mafic intrusions. Titanium is sited in separate ilmenite grains and in lamella within ilmenomagnetite (Ti-bearing magnetite). The high phosphorus content in the main zone is interpreted to result in crystallization of ilmenite and ilmenomagnetite instead of Ti-rich magnetite under relatively high f O2 conditions. High concentrations of titanium, iron and phosphorus in rocks of the main zone can be explained by pre-emplacement crystal-melt fractionation in a deep magma reservoir and/or contamination of mantle-derived mafic magmas by granitic magmas from partial melting of crustal rocks. A low-grade Fe-Ti-P resource at Kauhajärvi consists of layers with as much as 20 wt. % combined ilmenite (usually 8 to 11 wt. %), apatite (1 to 8 wt. %) and magnetite (1 to 9 wt. %). Mineralized layers are of variable thickness (2 m to 30 m) and occur in variable host rocks (peridotite or gabbro). The Fe-Ti oxides are most abundant in peridotite and pyroxene- or olivine-rich gabbronorite within the main zone. The contact between mineralized rocks (4%TiO2) and non- or slightly-mineralized rocks is gradual. The deposit as a whole consists of three to five mineralized layers with maximum combined thickness of 70 m. Apatite is most abundant in the oxide-rich layers, but is locally also concentrated in anorthosite with low Fe-Ti oxide contents. The weight ratio of ilmenite to magnetite is 3:2. The ratio of total Ti-Fe-oxides to apatite averages 4.0, with the range of 1.5 to 〉15.

Notes: Abstract Giant iron-ore deposits, such as those in the Hamersley Province of northwestern Australia, may contain more than a billion tonnes of almost pure iron oxides and are the world's major source of iron. It is generally accepted that these deposits result from supergene oxidation of host banded iron formation (BIF), accompanied by leaching of silicate and carbonate minerals. New textural evidence however, shows that formation of iron ore at one of those deposits, Mount Tom Price, involved initial high temperature crystallisation of magnetite-siderite-iron silicate assemblages. This was followed by development of hematite- and ferroan dolomite-bearing assemblages with subsequent oxidation of magnetite, leaching of carbonates and silicates and crystallisation of further hematite. Preliminary fluid inclusion studies indicate both low and high salinity aqueous fluids as well as complex salt-rich inclusions with the range of fluid types most likely reflecting interaction of hydrothermal brines with descending meteoric fluids. Initial hematite crystallisation occurred at about 250 °C and high fluid pressures and continued as temperatures decreased. Although the largely hydrothermal origin for mineralisation at Mount Tom Price is in conflict with previously proposed supergene models, it remains consistent with interpretations that the biosphere contained significant oxygen at the time of mineralisation.

Notes: Abstract Re-Os isotopes were used to constrain the source of the ore-forming elements of the Tharsis and Rio Tinto mines of the Iberian Pyrite Belt, and the timing of mineralization. The pyrite from both mines has simila]r Os and Re concentrations, ranging between 0.05–0.7 and 0.6–66 ppb, respectively. 187Re/188Os ratios range from about 14 to 5161. Pyrite-rich ore samples from the massive ore of Tharsis and two samples of stockwork ore from Rio Tinto yield an isochron with an age of 346 ± 26 Ma, and an initial 187Os/188Os ratio of about 0.69. Five samples from Tharsis yield an age of 353 ± 44 Ma with an initial 187Os/188Os ratio of about 0.37. A sample of massive sulfide ore from Tharsis and one from Rio Tinto lie well above both isochrons and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma of the black shales in which the ores are disseminated. Our data do not permit us to determine whether the Re-Os isochron yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasonable Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-Os geochronologic system. The highly radiogenic initial Os isotopic ratio agrees with previous Pb isotopic studies. If the initial ratio is recording the initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The continental margin sediments that underlie the deposits (phyllite-quartzite group) or the volcanic rocks (volcanogenic-sedimentary complex) in which the ores occur are plausible sources for the ore-forming metals and should constrain the models for the genesis of these deposits.

Notes: Abstract The Sungun porphyry copper deposit is hosted in a Diorite/granodioritic to quartz-monzonitic stock that intruded Eocene volcanosedimentary and Cretaceous carbonate rocks. Copper mineralization is associated mainly with potassic alteration and to a lesser extent with sericitic alteration. Based on previously published fluid inclusion and isotopic data by Hezarkhani and Williams-Jones most of the copper is interpreted to have deposited during the waning stages of orthomagmatic hydrothermal activity at temperatures of 400 to 300 °C. These data also indicate that the hydrothermal system involved meteoric waters, and boiled extensively. In this work, thermodynamic data are used to delineate the stability fields of alteration and ore assemblages as a function of fS2, fO2 and pH. The solubility of chalcopyrite was evaluated in this range of conditions using recently published experimental data. During early potassic alteration (〉450 °C), Copper solubility is calculated to have been 〉50 000 ppm, whereas the copper content of the initial fluid responsible for ore deposition is estimated, from fluid inclusion data, to have been 1200–3800 ppm. This indicates that initially the fluid was highly undersaturated with respect to chalcopyrite, which agrees with the observation that veins formed at T 〉 400 °C contain molybdenite but rarely chalcopyrite. Copper solubility drops rapidly with decreasing temperature, and at 400 °C is approximately 1000 ppm, within the range estimated from fluid inclusion data, whereas at 350 °C it is only 25 ppm. These calculations are consistent with observations that the bulk of the chalcopyrite deposited at Sungun is hosted by veins formed at temperatures of 360 ± 60 °C. Other factors that, in principle, may reduce chalcopyrite solubility are increases in pH, and decreases in fO2 and aCl−. Our analysis shows, however, that most of the change in pH occurred at high temperature when chalcopyrite was grossly undersaturated in the fluid, and that the direction of change in fO2 increased chalcopyrite solubility. We propose that the Sungun deposit formed mainly in response to the sharp temperature decrease that accompanied boiling, and partly as a result of the additional heat loss and decrease in aCl−, which occurred as a result of mixing of acidic Cu-bearing magmatic waters with cooler meteoric waters of lower salinity.

Notes: Abstract Carbonate staining has long been utilized with thin sections. Similar techniques are also extremely useful for distinguishing the mineralogy and texture of different carbonate minerals in drill cores. The simplest method for routine staining of whole drill core utilizes a dilute hydrochloric acid solution containing both alizarin red S and potassium ferricyanide. This solution is painted on to the drill core to allow for the rapid identification of calcite, ferroan calcite, ferroan dolomite (ankerite), and rhodochrosite. Production-scale staining of whole drill core rapidly provides a wealth of information on carbonate mineralogy and textural relationships in a number of deposit types. Examples from the Irish Zn-Pb district, the McArthur River Zn-Pb-Ag district, the Ruby Creek Cu-Co deposit, and the Kansanshi copper deposit are presented to illustrate the type of information that can be gained through use of this inexpensive and rapid technique.

Notes: Abstract The Lower Cretaceous volcanic formations of Copiapó (Chile) host major stratiform, disseminated, vein, and stockwork mineral deposits. Among these are the Punta del Cobre district (Cu-Fe) and the neighbouring, large Candelaria deposit (Cu-Fe-Au). These deposits are likely to have formed during a major extensional event that was accompanied by granitoid emplacement that triggered important contact metamorphism, metasomatic activity and mineralization processes. We suggest here that alteration-mineralization processes developed within the upper-plate domain of a NNE-trending, ESE-dipping extensional system, while granitoids were accomodated within the lower-plate. If this model prove to be right, `Candelaria-type' deposits may be hidden beneath the present, higher structural position of the Punta del Cobre district.

Notes: Abstract  In order to confirm the possible existence of FeGeO3 perovskite, we have performed in situ X-ray diffraction measurements of FeGeO3 clinopyroxene at pressures up to 40 GPa at room temperature. The transition of FeGeO3 clinopyroxene into orthorhombic perovskite is observed at about 33GPa. The cell parameters of FeGeO3 perovskite are a=4.93(2) Å, b=5.06(6) Å, c=6.66(3) Å and V=166(3) Å3 at 40 GPa. On release of pressure, the perovskite phase transformed into lithium niobate structure. The previously reported decomposition process of clino-pyroxene into Fe2GeO4 (spinel)+GeO2 (rutile) or FeO (wüstite) +GeO2 (rutile) was not observed. This shows that the transition of pyroxene to perovskite is kinetically accessible compared to the decomposition processes under low-temperature pressurization.

Notes: Abstract  The nature of OH species in natural clear quartz was investigated by means of in-situ IR measurements over the temperature range –185 to 1000 °C. Reversible thermal behavior of OH species was examined for a sample pre-heated to 1000 °C for 1 hour. At room temperature, the IR spectrum of the quartz sample examined includes an intense absorption peak at 3379 cm–1 which has been assigned to an OH stretching vibration associated with Al substituting for Si (OH(Al)). The major spectral changes of the OH(Al) bond involve a systematic shift of its peak position and a decrease in its integral absorbance with temperature. A quasi-linear increase of the peak position from –185 to 400 °C is interpreted to be due to the change in the vibrational frequency of OH(Al) with hydrogen bond (H bond) distance. At higher temperatures, the IR frequency shows only a slight change, indicating a small influence of the H bond. On the other hand, the gradual decrease of the integral absorbance of OH(Al) with temperature indicates a decrease of this defect’s molar absorptivity without any reduction in defect concentration. This is interpreted to result from a decrease in dipole moment of OH(Al) with temperature. A sudden shift of the vibrational frequency from 3396 to 3386 cm–1 between 550 and 560 °C and a constant value of the integral absorbance from 535 to 570 °C were considered to be related to the change in H bond distance during the structural transformation of α-quartz to its β-form. The local environment of OH(Al) begins to change at temperatures below 570 °C, where the crystallographic α–β transition occurs.

Notes: Abstract  The high-pressure equation of state of the normal spinels MgCr2O4 (picrochromite), MnCr2O4 and ZnCr2O4, and their reaction of decomposition into Cr2O3 (eskolaite) and MO (rocksalt-type) component oxides, were investigated by periodic unrestricted Hartree-Fock calculations. All-electron basis sets, and an a posteriori correction for the electron correlation energy, based on Density-Functional-Theory, were employed. Interpolation of results by the P-V Murnaghan equation of state yielded the equilibrium volume and energy, and the bulk modulus and its pressure derivative, for each of the seven phases (three spinels, three rocksalt oxides and eskolaite) considered. The simulated behaviour of interatomic distances vs pressure shows similar compressibilities of M-O bonds in both octahedral and tetrahedral coordinations. Binding energies and formation enthalpies of spinels from oxides are also computed and compared to available experimental data. The predicted decomposition pressures of Mg, Mn and Zn chromium spinels are 19, 23 and 34 GPa, respectively. The greater stability of ZnCr2O4 is related to Zn2+ being better suited to tetrahedral coordination than the other M2+ cations. Such results are strongly supported by the excellent agreement previously obtained between simulated (11 GPa) and experimental (13 GPa) pressures of the decomposition of MgAl2O4 spinel into corundum and periclase.

Notes: Abstract  Phase D is a dense hydrous magnesium silicate (ideal formula MgSi2H2O6) which contains silicon cations exclusively in octahedral coordination. Measurements of the unit cell parameters of phase D were made to pressures of 30 GPa using a diamond anvil cell and employing synchrotron X-ray diffraction. A neon pressure medium was used. Using a third order Birch-Murnaghan equation of state the isothermal bulk modulus of phase D was determined as 166(±3) GPa with K′ equal to 4.1(±0.3). The compression of phase D is anisotropic with the c-axis twice as compressible as the a-axis. Above 20 GPa, however, the c/a ratio becomes pressure independent.

Notes: Abstract  The existence of an incomplete solid solution series between loparite (NaLREETi2O6), a member of the perovskite mineral group, and thorutite (ThTi2O6) is established on the basis of experimental and mineralogical data. The products of low- and high-pressure synthesis in the system NaLaTi2O6– ThTi2O6 were studied by energy-dispersive spectrometry, X-ray diffractometry and Rietveld analysis. At atmospheric pressure, Th is incorporated in loparite as both ThTi2O6 and Na2ThTi3O9. In synthetic systems, the maximum determined ThTi2O6 content of loparite is 18 mol%, with a corresponding A-site cation deficiency of 9%. The structure of such loparite is tetragonal and presumably derived from the cubic aristotype by octahedral rotation [I4/mcm, a=5.4652(1) Å, c=7.7476(2) Å]. At a pressure of 6 GPa, no solubility between loparite and ThTi2O6 is observed, and Th is accommodated in the loparite structure entirely as Na2ThTi3O9 (up to 30 mol%). Naturally occurring loparite contains up to 29 mol% ThTi2O6, based on the conventional method of analysis recalculation, or 23.5 mol% ThTi2O6, assuming the presence of protons at the vacant A-sites. ThTi2O6 synthesized by the solid-state reaction, crystallizes with monoclinic symmetry [C2/m, a=9.8140(2) Å, b=3.8228(1) Å, c=7.0313(2) Å,β=118.82(1)°]. Atomic coordinates for ThTi2O6 obtained in this study from X-ray powder data, as well as structural parameters derived from the new data, are in a good agreement with those known from single-crystal refinement. ThTi2O6 does not crystallize at high pressure, and Th is accommodated in perovskite-type compounds and cubic ThO2 that provide a twelve- and eight-fold coordination site for Th, respectively.

Notes: Abstract  Diffusion coefficients of Co2+ and Ni2+ in synthetic single crystal forsterite along the c-axis were determined in the temperature ranges, 700–1200 °C and 800–1300 °C, respectively. The synthesized forsterite specimens were coated with thin evaporated films of CoO and NiO on the c-surface and annealed for diffusion experiments. The short penetration distance of diffusing ions in forsterite was measured by secondary ion mass spectrometry using the depth profile method. The diffusion coefficients of Co (700–1200 °C) and Ni (800–1300 °C) are given by: and The observed diffusion coefficient values show good linear relationships in Arrhenius plots and the activation energy values obtained agree well with the previous values, although the diffusion coefficient values observed at the high temperature end of the experimental range deviate from the previous values. These results indicate that Co and Ni diffuse in olivine with a single mechanism within the temperature range observed, possibly with an extrinsic in nature as in the case of Mg tracer diffusion observed by Chakraborty et al. 1994 and of Fe-Mg interdiffusion by Chakraborty.

Notes: Abstract The infrared spectrum of CaAl2Si2O7 · H2O-lawsonite, has been characterized to pressures of 20 GPa at 300 K. Our results constrain the response to compression of the silicate tetrahedra, hydroxyl units, and water molecules in this material. The asymmetric and symmetric stretching and bending vibrations of the Si2O7 groups (at zero pressure frequencies between 600 and 1000 cm−1) increase in frequency with pressure at rates between 3.6 and 5.9 cm−1/GPa. All silicate modes appear to shift continuously with pressure to 20 GPa, although the lowest frequency stretching vibration becomes unresolvable above 18 GPa, and a splitting of the main bending vibration is observed near this pressure. The O-H stretches of the hydroxyl units exhibit a discontinuity in their mode shifts at ∼8–9 GPa, which we interpret to be produced by a pressure-induced change in hydrogen bonding. The stretching and bending vibrations of the water molecule are relatively unaffected by compression to 20 GPa, thus demonstrating that the structural cavities in which water molecules reside are relatively rigid. Significant changes in the amplitude of the O-H stretches of the hydroxyl and water units are observed at this pressure as well; nevertheless, our results demonstrate that the dominant structural units in lawsonite persist metastably at 300 K with only modest structural modifications well beyond the known stability field of this phase.

Notes: Abstract The high-pressure behaviour of chromous orthosilicate, Cr2SiO4, has been studied by means of single-crystal X-ray diffraction and electronic absorption spectroscopy. X-ray diffraction data show that the structure remains orthorhombic to the highest pressure reached of 9.22 GPa. The compressibility of the unit-cell is strongly anisotropic with the c axis approximately six times more compressible than the a and b axes. A third-order Birch-Murnaghan equation of state fitted to the volume-pressure data yields V 0 = 610.10(3) Å3, K = 94.7(4) GPa, K′ = 8.32(14). Cr2SiO4 is therefore more compressible than the isostructural Cd analogue, even though its molar volume is smaller. This unusual behaviour can be attributed to the fact that the Cr atom is too small for the six-coordinated site that it occupies, and the site is therefore strongly distorted. Structure refinements indicate that under high pressures the Cr atom remains strongly displaced from the central position of the octahedron. Polarized and unpolarized electronic absorption spectra include a strong absorption band occuring at 18.300 cm−1 for E//c (which is parallel to the shortest Cr-Cr vector in the structure) which has an unusually large half width (5000 cm−1), indicative of electronic interaction between metal centres. Deconvolution of unpolarized high-pressure spectra show that the relative integrated intensity of this component increases linearly from 40% at 1 bar to 60% at 11.2 GPa. Both the structural changes and the absorption spectra at high pressures suggest that pairs of adjacent Cr atoms in chromous orthosilicate form chromium dimers with a weak metal-metal bond, which is consistent with the diamagnetic response found at ambient pressure.

Notes: Abstract The hydration state of Na- and K-exchanged clinoptilolite from Castle Creek (Idaho, U.S.A.) has been measured by a pressure titration method to 300 °C and P H2O〈30 bars. The water content of clinoptilolite can be predicted as a function of water activity and temperature with the equation: a H2O = [exp[[−ΔH h ∘/nRT] + [ΔS h ∘/nR] − 1/nRT· [W1 X h + W2 X h 2]− ln(X a/X h)]]−1 where T is degrees in Kelvin, ΔH h ∘ is the standard molal enthalpy of hydration, ΔS h ∘ is the entropy of hydration, X h and X a are, respectively, the mole fractions of the hydrous and anhydrous components of the solid solution, W 1 and W 2 are interaction parameters, n is the maximum number of moles of H2O per formula unit (based on 12 oxygens), and R is the gas constant. This equation can be used to locate clinoptilolite-H2O isohydrons in a H2O-T space below the liquid-vapor equilibrium curve of water. The standard molal Gibbs free energy of hydration is −47.62 ± 5.52 kJ/mol H2O and −5.40 ± 2.71 kJ/mol H2O for the Na- and K-clinoptilolite, respectively. These standard-state thermodynamic properties of clinoptilolite hydration are in good agreement with previous data at low H2O pressures. The experiments indicate that clinoptilolite progressively dehydrates with increasing temperature at pressures along the liquid-vapor equilibrium curve. Kinetic data above 150 °C show that clinoptilolite dehydration and hydration reactions are fast and reversible and that steady-state hydration states are attained in minutes.

Notes: Abstract The high temperature (β) phases of SiO2 cristobalite and quartz are studied by performing molecular dynamics simulations using a model which allows easy analysis of tetrahedral motions. The dynamic nature of the disordered high-temperature phase of cristobalite is attributed to rigid unit mode (RUM) excitations, and it is found that the entire spectrum of RUMs is responsible for the disorder. Comparisons of the results of β-cristobalite with those of β-quartz lead to the conclusion that framework structures with high degrees of geometric flexibility, and hence many RUMs, are free to deform through cooperative tetrahedral rotations even in the limit of extremely large tetrahedral stiffnesses.

Notes: Abstract Mössbauer spectra (MS) of anapaite (Ca2 Fe2+(PO4)2 · 4H2O) and of a sample after being immersed in a 4% H2O2 solution at room temperature (RT) over 12 days (hereafter an4ox) were collected at temperatures in the range 4.2 to 420 K and 11 to 300 K respectively. All MS consist of symmetrical doublets, hence magnetic ordering was not observed. The temperature dependencies of the Fe2+ centre shifts of anapaite and an4ox were analysed with the Debye model for the lattice vibrations. The characteristic Mössbauer temperatures were found as 370 K ± 25 K and 340 K ± 25 K, and the intrinsic isomer shifts as 1.427 ± 0.005 mm/s and 1.418 ± 0.005 mm/s respectively. From the external-field (60 kOe) MS recorded at 4.2 and 189 K for the non-treated sample, the principal component V zz of the electric field gradient (EFG) is determined to be positive and the asymmetry parameter η ≈ 0.2 and 0.4 respectively. The temperature variations of the quadrupole splittings, ΔE Q(T), cannot be interpreted on the basis of the thermal population of the 5 D electronic levels resulting from the tetragonal compression of the O6 co-ordination. The low-temperature linear behaviour of ΔE Q(T) is attributed to a strong orbit-lattice coupling. A field of 60 kOe applied to anapaite at 4.2 K produces magnetic hyperfine splitting with effective hyperfine fields of −136, −254 and −171 kOe along the principal axes Ox, Oy and Oz of the EFG tensor respectively. Additional oxidation treatments in solutions with various H2O2 concentrations up to 20% and subsequent Mössbauer experiments at room temperature, have revealed that the anapaite structure is not sensitive to oxidation since eventually only a small amount of Fe2+ (∼6.5%) is converted into Fe3+.

Notes: Abstract The crystal structures, lattice dynamics and magnetic properties of synthetic Co-cordierite, Co2Al4Si5O18, and Mn-cordierite, Mn2Al4Si5O18 have been studied by neutron powder diffraction, infrared spectroscopy and magnetisation measurements. Due to different synthesis conditions, the Co-cordierite used here crystallised in the hexagonal α-cordierite structure with a disordered Si/Al distribution in the framework, while for the Mn-cordierite the orthorhombic β-structure has been determined. The experimentally determined paramagnetic moments, exp (Mn) = 5.47(6) B and exp (Co) = 3.88(4) B , are in good agreement with theoretical predictions for octahedrally coordinated Mn2+ and Co2+, respectively. In both compounds there is no magnetic long-range order down to at least 1.5 K. However, the onset of an anti-ferromagnetic short-range correlation of magnetic moments along [001] has been observed for Mn-cordierite by magnetisation and neutron diffraction measurements. This short-range magnetic correlation becomes evident from diffuse scattering observed at 2 K. The diffuse scattering has been interpreted in terms of a Blech-Averbach function.

Notes: Abstract Iron pressurized to 60 gigapascal (GPa) was heated with laser up to temperatures of over 2200 K. The structural changes were determined in-situ using third generation synchrotron X-ray source; the changes were recorded on an imaging plate with a monochromatic beam. The results strongly support the existence of a phase transformation of the hexagonal close-packed (hcp) structure to the new polymorph (β-phase of iron) at high pressure and temperature. We interpret the X-ray data as belonging to the double hexagonal close-packed (dhcp) structure distorted by stress due to laser heating.

Notes: Abstract Local heterogeneities in pyrope-almandine, almandine-grossular and pyrope-grossular solid solutions have been investigated using IR-powder absorption spectroscopy. Correlations of the wavenumber shifts and line broadening systematics with the thermodynamic mixing properties were found. Wavenumber shifts of the highest energy modes correlate closely with the Si-O bond distances and give an indirect view of the average distortions across the three solid solutions. They have a linear behaviour for Py-Alm, but show positive variations from linearity for Alm-Gr and Py-Gr systems. An effective line width (Δcorr) of the absorption bands over a given wavenumber interval was obtained using the autocorrelation function. Line broadening is associated with local heterogeneities arising from cation substitution in the structure of samples at intermediate compositions. Non-linearities of the line broadening were found for Alm-Gr and Py-Gr and have a shape similar to the enthalpy of mixing, ΔHmix. An empirical analysis was therefore carried out to compare ΔHmix and Δcorr quantitatively. Low-temperature far-IR spectra were recorded for the end-members pyrope, almandine and grossular and far-IR and mid-IR low-temperature spectra for Py60Gr40 in the temperature range 292–44 K. Softening of the lowest energy band with decreasing temperature was observed in the spectrum of pyrope and more enhanced in the spectrum of Py60Gr40. The same softening occurs by substitution of grossular component into pyrope. High energy modes of Py60Gr40 show the effect of saturation below 110–130 K, which correlates with the volume saturation at low temperature. This could provide an alternative explanation for the heat capacity anomaly found for Py-Gr solid solution at low-temperatures.

Notes: Abstract Various X-ray diffraction methods have been applied to study the compression behavior of gibbsite, Al(OH)3, in diamond cells at room temperature. A phase transformation was found to take place above 3 GPa where gibbsite started to convert to its high-pressure polymorph. The high-pressure (HP) phase is quenchable and coexists with gibbsite at the ambient conditions after being unloaded. This HP phase was identified as nordstrandite based on the diffraction patterns obtained at room pressure by angle dispersive and energy dispersive methods. On the basis of this structural interpretation, the bulk modulus of the two polymorphs, i.e., gibbsite and nordstrandite, could be determined as 85 ± 5 and 70 ± 5 GPa, respectively, by fitting a Birch-Murnaghan equation to the compression data, assuming their Ko ′ as 4. Molar volume cross-over occurs at 2 GPa, above which the molar volume of nordstrandite is smaller than that of gibbsite. The differences in the molar volume and structure between the two polymorphs are not significant, which accounts for the irreversibility of the phase transition. In gibbsite, the axial compressibility behaves as c/c o 〉 a/a o 〉 b/b o. This is due to the fact that the dioctahedral sheets along the c-axis are held by the relatively weak hydrogen bonding, which results in the greater compressibility along this direction. In nord- strandite, the axial compressibility is b/b o 〉 c/c o 〉 a/a o, which can also be interpreted as resulting from the the existence of hydrogen bonds along the b-axis.

Notes: Abstract The structure of grain boundaries in a granite-origin ultramylonite, composed mainly of fine-grained feldspar and quartz, was studied by high-resolution electron microscopy (HREM). At most of the boundaries, not only between the same minerals but also between different minerals, lattice fringes in adjacent grains meet at the interface with no other appreciable phases. In these boundaries, some of the straight segments correspond to a low-index plane of one of the connected grains. Boundaries containing voids, with a spheroidal shape elongated along the boundaries, were observed only between quartz grains. It is suggested that these boundaries were formed by healing of microcracks. The structural width of major boundaries, deduced from lattice-fringe imaging, is less than about 0.5 nm.

Notes: Abstract Beta-eucryptite as grown by the flux method has been investigated by in-situ cold- and hot-stage transmission electron microscopy (TEM). Using electron diffraction and dark-field TEM imaging, we found no evidence for the merohedral twinning that accompanies structural collapse at the β-to-α-quartz transition, suggesting a true hexagonal symmetry for the aluminosilicate framework of β-eucryptite. Selected-area electron diffraction (SAED) patterns exhibited a variety of incommensurate structures along the three a axes with an average modulation period of about 6.5 a. These modulated structures arise from superperiodic stacking parallel to (100) of two structural units with different Li configurations. In-situ heating experiments show that the modulated structures undergo a reversible transition to the disordered modification at elevated temperatures. This transition arises from positional order-disorder of Li ions along the main structural channels. In addition, it appears that at constant low temperature, electron beam irradiation alone can induce a sequence of transitions similar to those that are thermally induced from the ordered modification to modulated states.

Notes: Abstract The mechanical twinning of calcite is compared with ferroelastic behavior. For calcite a paraelastic prototype phase does not exist, therefore a virtual prototype is defined with cubic m3m symmetry. Using this condition the strain tensors of all domain states of mechanical twinning are calculated. With the use of the strain tensors, application of Sapriel's strain compatibility law gives the crystallographic orientations of all possible twin planes between different domain states. The findings indicate that the twin walls are {1 0 0} and {1 1 0} planes with respect to the morphological rhombohedral lattice. These twin plane orientations from the equated strain tensors are in accordance with the r-and e-twin systems commonly observed in calcite.

Notes: Abstract The high-pressure and temperature equation of state of majorite solid solution, Mj0.8Py0.2, was determined up to 23 GPa and 773 K with energy-dispersive synchrotron X-ray diffraction at high pressure and high temperature using the single- and double-stage configurations of the multianvil apparatuses, MAX80 and 90. The X-ray diffraction data of the majorite sample were analyzed using the WPPD (whole-powder-pattern decomposition) method to obtain the lattice parameters. A least-squares fitting using the third-order Birch-Murnaghan equation of state yields the isothermal bulk modulus, K T0  = 156 GPa, its pressure derivative, K′ = 4.4(±0.3), and temperature derivative (∂K T /∂T) P = −1.9(±0.3)× 10−2 GPa/K, assuming that the thermal expansion coefficient is similar to that of pyrope-almandine solid solution.

Notes: Abstract Grain growth kinetics in CaTiO3-perovskite + FeO-wüstite aggregates were studied at the conditions of T = 1223–1623 K, P = 0.1 MPa and P = 200 MPa. Starting samples were fabricated by hot-pressing mechanically mixed powders of CaTiO3 + FeO with FeO = 0%, 1%, 3%, 6%, 10%, 20% and 100% by weight in a gas-medium apparatus at 1323 K and 300 MPa for 5 h. The increase of grain size (G) of CaTiO3 with time (t) follows a growth law: G n −G n 0 = κ·t(κ=κ0exp(−(Q/RT)). Two grain growth regimes are observed at T 〈 1523 K and T ≥ 1523 K. For T 〈 1523 K, the best fits of the data to the growth law yield growth exponents of n = 2.2 ± 0.2, 3.0 ± 0.3 and 3.5 ± 0.3 for samples with FeO = 0%, 3% and 10% respectively. Under these conditions the rate constants, κ, obey an Arrhenius relation with Q = 206 ± 35 kJ/mol and 385 ± 65 kJ/mol for samples with FeO = 3% and 10%. Grain growth of CaTiO3 becomes sluggish when FeO content exceeds 6%. For T ≥ 1523 K, the best fits of the data to the growth law yield n = 2.5 ± 0.2 for both samples with FeO = 3% and 10%. The activation energies (Q ) were determined as 71 ± 30 kJ/mol and 229 ± 45 kJ/mol for samples with FeO = 3% and 10%, respectively. The TEM observations show a remarkable difference in the distribution and geometry of FeO below and above 1523 K: nanometer-sized particles of FeO were observed along CaTiO3 grain boundaries in samples annealed at T 〈 1523 K. No FeO particles were detected along CaTiO3 grain boundaries in samples annealed at T ≥ 1523 K, but large clusters of FeO particles are observed locally indicating a fast separation of FeO from CaTiO3. Thus we conclude that the slow growth rate of CaTiO3 at T 〈 1523 K is due to the pinning by FeO particles at grain boundary, and that the change of grain growth kinetics in CaTiO3 at T ≥ 1523 K may relate to the separation of FeO from CaTiO3, which we interpret as due to the phase transformation of CaTiO3 at around 1523 K.

Notes: Abstract Semiempirical band structure calculations were performed on several skutterudite-type compounds by using the extended Hückel method. Starting with the molecular orbital calculations on isolated P4 and As4 rings, the reason for the band dispersions of the skutterudites was found to be the interactions between the nonmetal atoms. Both the intermolecular and the intramolecular interactions between the phosphorus atoms are stronger than those between the arsenic atoms. Hence, the dispersion of the bands in CoP3 is larger than that in CoAs3. The COOP (crystal orbital overlap population) integrals of the intramolecular P-P bonds reveal the relation between the valence electron count and the observed bond lengths. The P-P bonds in the skutterudite-type compounds like TP3 (T = Co, Rh, Ir) become stronger by reduction as in NiP3 and weaker by oxidation as in RT4X12 (X = P, As, Sb; R = alkaline earth or rare earth metals) because the bands near the Fermi level are bonding. The electronic reason for the geometric distortion of the Ge2Y2 (Y = S, Se) units of mixed skutterudites TGe1.5Y1.5 is caused by an electron pair gap on germanium, which corresponds to low electron density perpendicular to the ring plane on the germanium atoms.

Notes: Abstract Over thirty samples of natural Ti-bearing amphiboles with Ti- and Fe-contents ranging from 0.111 to 0.729 atom per formula unit (a.p.f.u.) and from 0.479 to 2.045 a.p.f.u., respectively, were studied by means of optical absorption spectroscopy and microprobe analysis. Thirteen samples were also studied by Mössbauer spectroscopy. A strong pleochroic absorption edge, causing the dark brown colours of Ti-bearing amphiboles, is attributed to ligand-metal and metal-metal charge transfer transitions involving both iron and titanium ions (O2−→ Fe3+, Fe2+, O2−→ Ti4+ and Fe2+ + Ti4+→ Fe3+ + Ti3+). A broad intense Y-polarized band ∼22 000 cm−1 (ν1/2 ≈ 3700 cm−1) in spectra of two low iron amphiboles with a relatively low Fe3+/Fetotal ratio, both from eclogite-like rocks in kimberlite xenoliths, was attributed to electronic Fe2+(M3) + Ti4+(M2)→Fe3+(M3)+Ti3+(M2) IVCT transitions. The IVCT bands of other possible ion pairs, involving Ti4+ and Fe2+ in M2 and M1, M4 sites, respectively, are presumed to be at higher energies, being obscured by the absorption edge.

Notes: Abstract To assist in the assignment and interpretation of 23Na NMR spectra in silicate and aluminosilicate minerals and glasses we have calculated the 23Na NMR shieldings and the electric field gradients (EFG) at the Na for a number of Na-containing species. Included are Na(OH2) n +, n = 1, 2, 4, 5, 6 and 8, and Na+ complexes with SiH3OH, SiH3ONa and O(SiH3)2. We have also evaluated shieldings and EFGs for Na-centered clusters extracted from crystalline Na2SiO3 and anhydrous sodalite, Na6[AlSiO4]6. Using 6-31G* SCF optimized geometries and the GIAO method with a 6-31G* basis set [and 6-311(2d,p) bases for the smaller clusters] we find a calculated increase in shielding with coordination number (CN) for the Na(OH2) n +, n = 4, 6, 8 series that agrees reasonably well with experimental trends. Calculated changes in the Na shielding as water is replaced by bridging or nonbridging silicate O atoms are also consistent with experimental observations. The deshielding of Na (with respect to gas-phase Na+) which is produced by an O-containing ligand is a strongly decreasing function of the R(Na–O) and a weakly decreasing function of the underbonding or free valence of the O. Deshielding contributions to the isotropic shielding from different ligands are additive to good approximation for low CN species, so that the total deshielding can be calculated accurately by summing the contributions from the individual ligands. However for the larger CN species the directly calculated deshieldings are substantially smaller than those obtained using such an additivity approximation. We further test this approximation by calculating the deshieldings for Na in 12 different sites in silicate and aluminosilicate minerals which have recently been studied experimentally, using our calculated deshielding contributions for individual O-containing ligands and experimental values for the Na–O distances. Correlation coefficients between the experimental shifts and the calculated deshieldings are around 0.9 and the slope of the correlation is almost 1.0 . Calculations on large Na-centered clusters extracted from the crystal structures of Na2SiO3 and anhydrous sodalite reproduce the experimental values for both NMR shieldings and electric field gradients but at considerable computational cost. Comparison with recent 23Na NMR studies on hydrous albite glasses indicates that coordination of either H2O or OH− to the Na could give the magnitude of deshielding observed, depending upon the detailed Na–O distances within the hydrous glass.

Notes: Abstract Electrical conductivity of the lower mantle-like assemblage (Mg,Fe)SiO3 perovskite-(Mg,Fe)O magnesiowüstite is usually analyzed using the quasi-chemical Arrhenian approach of diffusion. The conductivity of this assemblage has often been attributed to hopping of small polarons, because of the low value of the activation energy and the small negative activation volume. However, the solid-state physics approach can provide more arguments, for or against conduction by polarons. We have tried to bridge the gap between the two approaches and identify the physical quantities entering the phenomenological activation parameters. In particular, we have investigated the pressure dependence of the activation energy, and the physical meaning of the activation volume. Hopping is controlled by the binding energy of the polaron and by the value of the exchange integral, which increases with pressure causing the observed decrease of the activation energy. From the physical theory and the results of experiments at pressures up to 40 GPa and temperatures up to 400 ∘C, we have estimated the values of parameters characteristic of polarons: radius, mobility, time between jumps and adiabaticity. These values are compatible with conduction by small adiabatic polarons. The consequences for extrapolations to lower mantle conditions of the presence of a temperature dependent preexponential term in the expression for conductivity have been examined. It was found that the extrapolations are not significantly different from those using the Arrhenius equation.

Notes: Abstract The high-temperature cell parameters of lime (CaO), periclase (MgO), corundum (Al2O3), and spinel (MgAl2O4) have been determined from 300 up to 3000 K through X-ray diffraction experiments with synchrotron radiation. The good agreement found with dilatometric results suggests that vacancy-type defects do not make a large contribution to thermal expansion for these oxides, even near the melting point, justifying the use of X-ray diffraction for determining volume properties up to very high temperatures. Thermal expansion coefficients were determined from the measured cell volumes with equations of the form α=α0 + α1 T + α2/T 2. Along with available isobaric heat capacity and compressibility data, these derived coefficients clearly show that anharmonic effects contribute little to the isochoric heat capacities (C v ) of CaO, MgO, and Al2O3, which do not depart appreciably from the 3nR Dulong and Petit limit.

Notes: Abstract We present the results of atomistic simulations to calculate the effect of impurities on vacancy migration at the {4 1 0} tilt grain boundary of MgO. We show that impurities within the mantle could significantly modify vacancy migration within the mantle due to segregation to grain boundaries. This segregation increases with increasing impurity size and with increasing pressure. The impurities have little effect on vacancy migration at 0 GPa but at higher pressures the impurities alter the activation energies such that the vacancies prefer to migrate towards them. This suggests that at mantle conditions the vacancies will be pinned both by the compression of the boundary and also the impurities, slowing diffusion.

Notes: Abstract Near-infrared (NIR) absorption bands related to total water (4000 and 7050 cm−1), OH groups (4500 cm−1) and molecular H2O (5200 cm−1) were studied in two polymerised glasses, a synthetic albitic composition and a natural obsidian. The water contents of the glasses were determined using Karl Fischer titration. Molar absorption coefficients were calculated for each of the bands using albitic glasses containing between 0.54 and 9.16 wt.% H2O and rhyolitic glasses containing between 0.97 and 9.20 wt.% H2O. Different combinations of baseline type and intensity measure (peak height/area) for the combination bands at 4500 and 5200 cm−1 were used to investigate the effect of evaluation procedure on calculated hydrous species concentrations. Total water contents calculated using each of the baseline/molar absorption coefficient combinations agree to within 5.8% relative for rhyolitic and 6.5% relative for albitic glasses (maximum absolute differences of 0.08 and 0.15 wt.% H2O, respectively). In glasses with water contents 〉1 wt.%, calculated hydrous species concentrations vary by up to 17% relative for OH and 11% relative for H2O (maximum absolute differences of 0.33 and 0.43 wt.% H2O, respectively). This variation in calculated species concentrations is typically greater in rhyolitic glasses than albitic. In situ, micro-FTIR analysis at 300 and 100 K was used to investigate the effect of varying temperature on the NIR spectra of the glasses. The linear and integral molar absorption coefficients for each of the bands were recalculated from the 100 K spectra, and were found to vary systematically from the 300 K values. Linear molar absorption coefficients for the 4000 and 7050 cm−1 bands decrease by 16–20% and integral molar absorption coefficients by up to 30%. Depending on glass composition and baseline type, the integral molar absorption coefficients for the absorption bands related to OH groups and molecular H2O change by up to −5.8 and +7.4%, respectively, while linear molar absorption coefficients show less variation, with a maximum change of ∼4%. Using the new molar absorption coefficients for the combination bands to calculate species concentrations at 100 K, the maximum change in species concentration is 0.08 wt.% H2O, compared with 0.39 wt.% which would be calculated if constant values were assumed for the combination band molar absorption coefficients. Almost all the changes in the spectra can therefore be interpreted in terms of changing molar absorption coefficient, rather than interconversion between hydrous species.

Notes: Abstract  The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total volume of 109 m3 and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated to the 2350 B.P. eruption, although the post-eruption avalanche may have its origins in the over-steepened slopes created by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results from deposition in a narrow, steep-sided mountain valley containing a major river.

Notes: Abstract  The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera, caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous, pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera.

Notes: Abstract  The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km3) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche blocks range in size from 〈35 m proximally to 〈10 m in the distal zone and consist dominantly of fragile materials. Debris-avalanche matrix percentages increase from 35–60% in the proximal zone to 95% in the distal zone. The debris-avalanche matrix is greater in volume (80–90%) at the bottom and margins of the deposit. Normal grading of large clasts and reverse grading of wood logs and branches occur within the debris-avalanche matrix. Preferred orientation of 311 wood logs and branches within the deposit coincide with the interpreted local flow direction. The basal part of the deposit is characterized by (1) erosional features and incorporated clasts of underlying material; (2) a higher proportion (30–50%) of incorporated clasts than the upper part; and (3) reverse grading of clasts. The valley-filling Kaida debris avalanche deposit (50 000 y B.P., 〉0.3 km3) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks range in size from 〈25 m proximally to 〈7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults, and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a higher percentage of plucked clasts from the adjacent underlying formations. A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted to result from shearing along the bottom and valley walls.

Notes: Abstract  GOES provides thermal data for all of the Hawaiian volcanoes once every 15 min. We show how volcanic radiance time series produced from this data stream can be used as a simple measure of effusive activity. Two types of radiance trends in these time series can be used to monitor effusive activity: (a) Gradual variations in radiance reveal steady flow-field extension and tube development. (b) Discrete spikes correlate with short bursts of activity, such as lava fountaining or lava-lake overflows. We are confident that any effusive event covering more than 10,000 m2 of ground in less than 60 min will be unambiguously detectable using this approach. We demonstrate this capability using GOES, video camera and ground-based observational data for the current eruption of Kīlauea volcano (Hawai'i). A GOES radiance time series was constructed from 3987 images between 19 June and 12 August 1997. This time series displayed 24 radiance spikes elevated more than two standard deviations above the mean; 19 of these are correlated with video-recorded short-burst effusive events. Less ambiguous events are interpreted, assessed and related to specific volcanic events by simultaneous use of permanently recording video camera data and ground-observer reports. The GOES radiance time series are automatically processed on data reception and made available in near-real-time, so such time series can contribute to three main monitoring functions: (a) automatically alerting major effusive events; (b) event confirmation and assessment; and (c) establishing effusive event chronology.

Notes: Abstract  The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, California, comprises a thick volcaniclastic sequence deposited in a submarine island-arc setting and penetrated by numerous related hypabyssal intrusions. A composite andesite-diorite intrusive complex ≥4.5 km long and ≥1.5 km thick was emplaced while the host Tuttle Lake sediments were still wet and unconsolidated. Large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff and tuff-breccia. The southern half of the complex consists of augite-phyric andesite containing peperite in numerous small, isolated pockets and in more extensive, laterally continuous zones. The peperites comprise three main types recognized previously in other peperite studies. Fluidal peperite consists of small (≤30 cm), closely spaced, at least partly interconnected, globular to amoeboid andesite bodies enclosed by tuff. This peperite type developed during intrusion of magma into fine-grained wet sediment along unstable interfaces, and fluidization of the sediment facilitated development of complex intrusive geometries. Blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic stressing of rigid margins, possibly aided by small steam explosions. The northern half of the intrusive complex consists predominantly of a different type of peperite, in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoidal, elongate, or angular, polyhedral shapes are closely packed to widely dispersed within disrupted host sediment. Textural features suggest the andesite clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits are represented by elongate, sinuous bodies up to 30 m or more in length. Disruption and dispersal of the andesite clasts are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits penetrating the host sediments. Closely packed peperite is present adjacent to mappable intrusions of coherent andesite, and along the margin of a large mass of coarse-grained diorite. These coherent intrusions are considered to be major feeders for this part of the complex. Examples of magma/wet sediment interaction similar in scale to the extensive peperites described here occur elsewhere in ancient island-arc strata in the northern Sierra Nevada. Based on these and other published examples, large-scale peperites probably are more common than generally realized and are likely to be important in settings where thick sediment sequences accumulate during active volcanism. Careful mapping in well-exposed terrains may be required to recognize large-scale peperite complexes of this type.

Notes: Abstract  We analyze digital topographic data collected in September 1993 over a ∼500-km2 portion of K*lauea Volcano, Hawai'i, by the C-band (5.6-cm wavelength) topographic synthetic aperture radar (TOPSAR) airborne interferometric radar. Field surveys covering an ∼1-km2 area of the summit caldera and the distal end of an ∼8-m-thick 'a'* flow indicate that the 10-m spatial resolution TOPSAR data have a vertical accuracy of 1–2 m over a variety of volcanic surfaces. After conversion to a common datum, TOPSAR data agree favorably with a digital elevation model (DEM) produced by the U.S. Geological Survey (USGS), with the important exception of the region of the ongoing eruption (which postdates the USGS DEM). This DEM comparison gives us confidence that subtracting the USGS data from TOPSAR data will produce a reasonable estimate of the erupted volume as of September 1993. This subtraction produces dense rock equivalent (DRE) volumes of 392, 439, and 90×106 m3 for the Pu'u '*'*, K*pa'ianah*, and episode 50–53 stages of the eruption, respectively. These are 124, 89, and 94% of the volumes calculated by staff of the Hawaiian Volcano Observatory (HVO) but do not include lava of K*pa'ianah* and episodes 50–53 that flowed into the ocean and are thus invisible to TOPSAR. Accounting for this lava increases the TOPSAR volumes to 124, 159, and 129% of the HVO volumes. Including the ±2-m uncertainty derived from the field surveys produces TOPSAR-derived volumes for the eruption as a whole that range between 81 and 125% of the USGS-derived values. The vesicularity- and ocean-corrected TOPSAR volumes yield volumetric eruption rates of 4.5, 4.5, and 2.7 m3/s for the three stages of the eruption, which compare with HVO-derived values of 3.6, 2.8, and 2.1 m3/s, respectively. Our analysis shows that care must be taken when vertically registering the TOPSAR and USGS DEMs to a common datum because C-band TOPSAR penetrates only partially into thick forest and therefore produces a DEM within the tree canopy, whereas the USGS DEM is adjusted for vegetation.

Notes: Abstract  Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions. The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic magma by mixing between relatively high (1000–1200  °C)- and low (∼1000  °C)- temperature (T) end members. Oscillatory zoning in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature (high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions sequentially tapped these magma pockets.

Notes: Abstract  Remote monitoring of active lava domes provides insights into the duration of continued lava extrusion and detection of potentially associated explosive activity. On inactive flows, variations in surface texture ranging from dense glass to highly vesicular pumice can be related to emplacement time, volatile content, and internal structure. Pumiceous surface textures also produce changes in thermal emission spectra that are clearly distinguishable using remote sensing. Spectrally, the textures describe a continuum consisting of two pure end members, obsidian and vesicles. The distinct spectral features of obsidian are commonly muted in pumice due to overprinting by the vesicles, which mimic spectrally neutral blackbody emitters. Assuming that this energy combines linearly in direct proportion to the percentage of vesicles, the surface vesicularity can be estimated by modeling the pumice spectrum as a linear combination of the glass and blackbody spectra. Based on this discovery, a linear retrieval model using a least-squares fitting approach was applied to airborne thermal infrared data of the Little Glass Mountain and Crater Glass rhyolite flows at Medicine Lake Volcano (California) as a case study. The model produced a vesicularity image of the flow with values from 0 to ∼70%, which can be grouped into three broad textural classes: dense obsidian, finely vesicular pumice, and coarsely vesicular pumice. Values extracted from the image compare well with those derived from SEM analysis of collected samples as well as with previously reported results. This technique provides the means to accurately map the areal distributions of these textures, resulting in significantly different values from those derived using aerial photographs. If applied to actively deforming domes, this technique will provide volcanologists with an opportunity to monitor dome-wide degassing and eruptive potential in near-real-time. In July 1999 such an effort will be possible for the first time when repetitive, global, multispectral thermal infrared data become available with the launch of the Advanced Spaceborne Thermal Emission and Reflectance Radiometer (ASTER) instrument aboard the Earth Observing System satellite.

Notes: Abstract  We introduce a 3D model for near-vent channelized lava flows. We assume the lava to be an isothermal Newtonian liquid flowing in a rectangular channel down a constant slope. The flow velocity is calculated with an analytical steady-state solution of the Navier-Stokes equation. The surface velocity and the flow rate are calculated as functions of the flow thickness for different flow widths, and the results are compared with those of a 2D model. For typical Etna lava flow parameters, the influence of levees on the flow dynamics is significant when the flow width is less than 25 m. The model predicts the volume flow rate corresponding to the surface velocity, taking into account that both depend on flow thickness. The effusion rate is a critical parameter to evaluate lava flow hazard. We propose a model to calculate the effusion rate given the lava flow width, the topograhic slope, the lava density, the surface flow velocity, and either the lava viscosity or the flow thickness.

Notes: Abstract  During the 1944 eruption of Vesuvius a sudden change occurred in the dynamics of the eruptive events, linked to variations in magma composition. K-phonotephritic magmas were erupted during the effusive phase and the first lava fountain, whereas the emission of strongly porphyritic K-tephrites took place during the more intense fountain. Melt inclusion compositions (major and volatile elements) highlight that the magmas feeding the eruption underwent differentiation at different pressures. The K-tephritic volatile-rich melts (up to 3 wt.% H2O, 3000 ppm CO2, and 0.55 wt.% Cl) evolved to reach K-phonotephritic compositions by crystallization of diopside and forsteritic olivine at total fluid pressure higher than 300 MPa. These magmas fed a very shallow reservoir. The low-pressure differentiation of the volatile-poor K-phonotephritic magmas (H2O〈1 wt.%) involved mixing, open-system degassing, and crystallization of leucite, salite, and plagioclase. The eruption was triggered by intrusion of a volatile-rich magma batch that rose from a depth of 11–22 km into the shallow magma chamber. The first phase of the eruption represents the partial emptying of the shallow reservoir, the top of which is within the volcanic edifice. The newly arrived magma mixed with that resident in the shallow reservoir and forced the transition from the effusive to the lava fountain phase of the eruption.

Notes: Abstract  Personnel from the U.S. Geological Survey's Cascades Volcano Observatory conducted first-order, class-II leveling surveys near Lassen Peak, California, in 1991 and at Newberry Volcano, Oregon, in 1985, 1986, and 1994. Near Lassen Peak no significant vertical displacements had occurred along either of two traverses, 33 and 44 km long, since second-order surveys in 1932 and 1934. At Newberry, however, the 1994 survey suggests that the volcano's summit area had risen as much as 97±22 mm with respect to a third-order survey in 1931. The 1931 and 1994 surveys measured a 37-km-long, east–west traverse across the entire volcano. The 1985 and 1986 surveys, on the other hand, measured only a 9-km-long traverse across the summit caldera with only one benchmark in common with the 1931 survey. Comparison of the 1985, 1986, and 1994 surveys revealed no significant differential displacements inside the caldera. A possible mechanism for uplift during 1931–1994 is injection of approximately 0.06 km3 of magma at a depth of approximately 10 km beneath the volcano's summit. The average magma supply rate of approximately 1×10–3 km3/year would be generally consistent with the volcano's growth rate averaged over its 600,000-year history (0.7–1.7×10–3 km3/year).

Notes: Abstract  Volcanoes in humid tropical environments are frequently cloud covered, typically densely vegetated and rapidly eroded. These factors complicate field and laboratory studies and even the basic identification of potentially active volcanoes. Numerous previous studies have highlighted the potential value of radar remote sensing for volcanology in equatorial regions. Here, cloud- and vegetation-penetrating LHH-band (λ≈24 cm) synthetic aperture radar (SAR) data from the Japanese Earth Resources Satellite (JERS-1) are used to investigate persistently active volcanoes and prehistoric calderas in East Java, Indonesia. The LHH-band JERS-1 SAR produces high-spatial-resolution (18 m) imagery with relatively high incidence angle that highlights structures and topographic variations at or greater than the wavelength scale while minimising geometrical distortions such as layover and foreshortening. These images, along with Internet browse data derived from the Canadian RADARSAT mission, provide new evidence relating regional tectonics to volcanism throughout East Java. Volcanic events, such as caldera collapse at the Tengger caldera, appear to have been partly controlled by northwest-aligned faults related to intra-arc sedimentary basins. Similar regional controls appear important at historically active Lamongan volcano, which is encircled by numerous flank maars and cinder cones. A previously undocumented pyroclastic sheet and debris avalanche deposit from the Jambangan caldera complex is also manifested in the synoptic radar images. At the currently active Semeru volcano these data permit identification of recent pyroclastic flow and lahar deposits. Radar data therefore offer a valuable tool for mapping and hazard assessment at late Quaternary volcanoes. The criteria developed in the analysis here could be applied to other regions in the humid tropics.

Notes: Abstract  Five new stepwise-heating 40Ar/39Ar ages and one new high-sensitivity 14C date of ash-fall and ash-flow deposits from late Quaternary silicic volcanoes in northern Central America document the eruption rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango) located north of the basalt, andesite, and dacite stratovolcanoes of the Central American volcanic front. These deposits form extensive time-stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valuable framework for age determinations of more localized volcanic and nonvolcanic events. The new data, especially when integrated with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because of their locations near the highly vulnerable economic hearts of Guatemala and El Salvador, the risks of eruptions from these calderas should be carefully considered along with risks of major earthquakes and volcanic front volcanoes, which are much more frequent but inflict less severe and extensive damage. This investigation also includes some examples of dating efforts that failed to produce reasonable results.

Notes: Abstract  Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580  °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550  °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550  °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525  °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences.

Notes: Abstract  On 20 April 1988 a landslide of approximately 200,000 m3 occurred on the northeastern flank of the volcano La Fossa on the island of Vulcano. The landslide fell into the sea, producing a small tsunami in the bay between Punte Nere and Punta Luccia that was observed locally in the neighbouring harbour called Porto Levante. The slide occurred during a period of unrest at the volcano that was monitored very accurately. The study of this event is composed of two parts, the simulation of the landslide and the simulation of the ensuing tsunami; the former is studied by means of a Lagrangian-type numerical model in which the landslide is seen as a multibody system, an ensemble of material-deforming blocks interacting together during their motion; the latter is simulated according to the Eulerian view by solving the shallow-water approximation to Navier-Stokes equations of fluid dynamics, with the incorporation of a forcing term depending on the slide motion. Technically, the slide evolution is computed first, and this result is then used to evaluate the excitation term of the hydraulic equations and to calculate the tsunami propagation. Computed wave fronts radiate both toward the open sea, with rapid amplitude decay, and along the shore, in the form of edge waves that lose energy slowly. Comparison between model outputs and observations can be carried out only in a qualitative way owing to the absence of tide-gauge records, and results are satisfactory.

Notes: Abstract  This paper illustrates some problems involved in the quantitative compositional study of pyroclastic deposits and proposes criteria for selecting the main petrographic and textural classes for modal analysis. The relative proportions of the different classes are obtained using a point-counting procedure applied to medium-coarse ash samples that reduces the dependence of the modal composition on grain size and avoids tedious counting of different grain-size fractions. The major purposes of a quantified measure of component distributions are to: (a) document the nature of the fragmenting magma; (b) define the eruptive dynamics of the eruptions on a detailed scale; and (c) ensure accuracy in classifying pyroclastic deposits. Compositional modes of the ash fraction of pyroclastic deposits vary systematically, and their graphical representation defines the compositional and textural characteristics of pyroclastic fragments associated with different eruptive styles. Textural features of the glass component can be very helpful for inferring aspects of eruptive dynamics. Four major parameters can be used to represent the component composition of pyroclastic ash deposits: (a) juvenile index (JI); (b) crystallinity index (CrI); (c) juvenile vesicularity index (JVI); and (d) free crystal index (FCrI). The FCrI is defined as the ratio between single and total crystal fragments in the juvenile component (single crystals+crystals in juvenile glass). This parameter may provide an effective estimate of the mechanical energy of eruptions. Variations in FCrI vs JVI discriminate among pyroclastic deposits of different origin and define compositional fields that represent ash derived from different fragmentation styles.

Notes: Abstract  The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately 500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions (18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units. In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting the aquifer system, strongly enhanced the explosivity of the eruptions.

Notes: Abstract  The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic flow deposits). These eruptions identify a brief (〈ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre.

Notes: Abstract  Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively, at temperatures close to feldspar liquidus (750–700  °C), were carried out to determine the modality of water exsolution and vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (≈0.5 vol.%) of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius, area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5–1000 μm were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic coordinates. The slope of this regression is approximately 2.8±0.1. This dependence is in striking contrast with distributions theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous nucleation mechanisms.