ALBERT

Description: The Kamoa copper deposit is located in the Katanga Province of the southern Democratic Republic of Congo outside of the area previously considered prospective for large copper deposits. With a 1% Cu cutoff, the hypogene ore zone of Kamoa spans over 81 km 2 , varying in thickness from 2 to 15 m, and is currently laterally unconstrained. The deposit is hosted at the stratigraphic contact between hematitic and locally pyritic Mwashya Subgroup sandstones and overlying fine-grained pyritic diamictites of the Grand Conglomérat unit of the Nguba Group. The Mwashya sandstones appear to have been deposited in a marginal marine to fluvial environment. The Grand Conglomérat diamictite contains glacially derived mass transport and sediment gravity flow deposits. The unit appears to have been deposited in a tectonically active, locally anoxic marine environment. The contact between the Mwashya and Grand Conglomérat units represents a major change in depositional style during what was probably a period of rapid subsidence or sea level rise. The majority of the Kamoa orebody occurs within the lowermost portion of the Grand Conglomérat unit that contains siltstones with high concentrations of diagenetic framboidal and later cubic pyrite that may be indicative of early hydrothermal activity. Later hydrothermal alteration mineral assemblages within the Grand Conglomérat are stratigraphically zoned, changing from a potassic alteration and silicification assemblage in the lowermost stratigraphic units to a dominantly magnesian alteration assemblage higher in the stratigraphy. Ore stage sulfide minerals are zoned vertically away from the Mwashya-Grand Conglomérat contact, changing from chalcocite to bornite to chalcopyrite to pyrite with increased stratigraphic height. Copper sulfide minerals occur as fine-grained disseminations within the diamictite matrix that probably represent replaced diagenetic pyrite as well as in coarse-grained mineral rims on diamictite clasts. The rims are best developed in the lowermost stratigraphic units and gradually lessen in size, vertical elongation, and abundance up stratigraphic section. Sulfur isotope studies indicate that most of the sulfur in the copper sulfides was derived from earlier formed diagenetic iron sulfide. Fluid inclusion analyses indicate that the ore-forming fluid was saline, ~23 to 26 wt % NaCl wt equiv, and had homogenization temperatures (T h ) ranging from 210° to 240°C.

Description: Initial{varepsilon} Nd isotope data for the four granites related to the Ardlethan Sn deposit in the Wagga Sn belt of the central Lachlan fold belt of eastern Australia range from -11.6 to +2.1, increasing with increasing fractionation. The four granites beneath the Renison tin mine in western Tasmania range from -6.1 to -3.9. The data clearly establish the presence of light rare earths of mantle origin in the fractionated granites associated with the Ardlethan porphyry Sn deposits and hint at the same relationship for the Renison granites, though in this case the mineralizing pluton is probably yet to be discovered. Sulfur and carbon isotope data from the Ardlethan and Renison ores, and from similar deposits in Tasmania, suggest that mantle volatiles played a significant role in the genesis of Sn granites and deposits. Mantle input may have occurred either by mixing of crustal and mantle melts or metasomatism by mantle-derived fluids.

Description: Clark volcano of the Kermadec arc, northeast of New Zealand, is a large stratovolcano comprised of two coalescing volcanic cones; an apparently younger, more coherent, twin-peaked edifice to the northwest and a relatively older, more degraded and tectonized cone to the southeast. High-resolution water column surveys show an active hydrothermal system at the summit of the NW cone largely along a ridge spur connecting the two peaks, with activity also noted at the head of scarps related to sector collapse. Clark is the only known cone volcano along the Kermadec arc to host sulfide mineralization. Volcano-scale gravity and magnetic surveys over Clark show that it is highly magnetized, and that a strong gravity gradient exists between the two edifices. Modeling suggests that a crustal-scale fault lies between these two edifices, with thinner crust beneath the NW cone. Locations of regional earthquake epicenters show a southwest-northeast trend bisecting the two Clark cones, striking northeastward into Tangaroa volcano. Detailed mapping of magnetics above the NW cone summit shows a highly magnetized "ring structure" ~350 m below the summit that is not apparent in the bathymetry; we believe this structure represents the top of a caldera. Oblate zones of low (weak) magnetization caused by hydrothermal fluid upflow, here termed "burn holes," form a pattern in the regional magnetization resembling Swiss cheese. Presumably older burn holes occupy the inner margin of the ring structure and show no signs of hydrothermal activity, while younger burn holes are coincident with active venting on the summit. A combination of mineralogy, geochemistry, and seafloor mapping of the NW cone shows that hydrothermal activity today is largely manifest by widespread diffuse venting, with temperatures ranging between 56° and 106°C. Numerous, small (≤30 cm high) chimneys populate the summit area, with one site host to the ~7-m-tall "Twin Towers" chimneys with maximum vent fluid temperatures of 221°C (pH 4.9), consistent with 34 S anhydrite-pyrite values indicating formation temperatures of ~228° to 249°C. Mineralization is dominated by pyrite-marcasite-barite-anhydrite. Radiometric dating using the 228 Ra/ 226 Ra and 226 Ra/Ba methods shows active chimneys to be 〈20 with most 〈2 years old. However, the chimneys at Clark show evidence for mixing with, and remobilizing of, barite as old as 19,000 years. This is consistent with Nd and Sr isotope compositions of Clark chimney and sulfate crust samples that indicate mixing of ~40% seawater with a vent fluid derived from low K lavas. Similarly, REE data show the hydrothermal fluids have interacted with a plagioclase-rich source rock. A holistic approach to the study of the Clark hydrothermal system has revealed a two-stage process whereby a caldera-forming volcanic event preceded a later cone-building event. This ensured a protracted (at least 20 ka yrs) history of hydrothermal activity and associated mineral deposition. If we assume at least 200-m-high walls for the postulated (buried) caldera, then hydrothermal fluids would have exited the seafloor 20 ka years ago at least 550 m deeper than they do today, with fluid discharge temperatures potentially much hotter (~350°C). Subsequent to caldera infilling, relatively porous volcaniclastic and other units making up the cone acted as large-scale filters, enabling ascending hydrothermal fluids to boil and mix with seawater subseafloor, effectively removing the metals (including remobilized Cu) in solution before they reached the seafloor. This has implications for estimates for the metal inventory of seafloor hydrothermal systems pertaining to arc hydrothermal systems.

Description: The northern Shoshone and Toiyabe Ranges in north-central Nevada expose numerous areas of mineralized Paleozoic rock, including major Carlin-type gold deposits at Pipeline and Cortez. Paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting, but here we present new data that suggest major post-Eocene extension along west-dipping normal faults. Tertiary rocks in the northern Shoshone Range crop out in two W-NW–trending belts that locally overlie and intrude highly deformed Lower Paleozoic rocks of the Roberts Mountains allochthon. Tertiary exposures in the more extensive, northern belt were interpreted as subvertical breccia pipes (intrusions), but new field data indicate that these "pipes" consist of a 35.8 Ma densely welded dacitic ash flow tuff (informally named the tuff of Mount Lewis) interbedded with sandstones and coarse volcaniclastic deposits. Both tuff and sedimentary rocks strike N-S and dip 30° to 70° E; the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes. The southern belt along Mill Creek, previously mapped as undivided welded tuff, includes the tuff of Cove mine (34.4 Ma) and unit B of the Bates Mountain Tuff (30.6 Ma). These tuffs dip 30° to 50° east, suggesting that their west-dipping contacts with underlying Paleozoic rocks (previously mapped as depositional) are normal faults. Tertiary rocks in both belts were deposited on Paleozoic basement and none appear to be breccia pipes. We infer that their present east tilt is due to extension on west-dipping normal faults. Some of these faults may be the northern strands of middle Miocene (ca. 16 Ma) faults that cut and tilted the 34.0 Ma Caetano caldera ~40° east in the central Shoshone Range (〈5 km south of Mill Creek), but further mapping is necessary to trace the faults through the highly deformed Paleozoic rocks that surround the isolated Tertiary outcrops. Significant post-Eocene extensional faulting in the northern Shoshone Range may have important implications for both the structure of the Roberts Mountains allochthon and the exposure of potentially mineralized rocks in its lower plate, both of which were likely east-tilted and repeated by west-dipping faults together with overlying Tertiary rocks.

Description: The Enterprise nickel deposit (40 Mt of 1.07% Ni) is located on the eastern edge of the Kabompo dome in the North-Western Province of Zambia. The deposit area contains basement schists overlain by Neoproterozoic metasedimentary rocks. Nickel sulfides are hosted within a sequence of quartz-, carbonate-, and carbon-rich metasedimentary rocks that interfinger with and overlie siliciclastic metasedimentary rocks. The host rocks contain significant kyanite, talc, and magnesian chlorite. Silicification and magnesian metasomatism occurred prior to or concurrent with a regional metamorphic event (590–500 Ma). Mineralization resulted in the precipitation of nickel and iron-nickel sulfides in veins and as semimassive replacements of the host rocks. Nickel sulfides precipitated in two main stages: a millerite-vaesite-pyrite assemblage that formed disseminations and semimassive replacements in vuggy textured rocks, and a later millerite-bravoite-(molybdenite) assemblage in quartz-kyanite veins and local semimassive replacements. The deposit contains minor copper and trace amounts of cobalt and platinum group elements (PGEs). A discrete zone of copper sulfides underlies a portion of the nickel sulfide zone. Re-Os geochronology on molybdenite yields a 540.6 ± 1.8 Ma age for mineralization at Enterprise, the approximate age of metamorphism. Sulfur isotope results indicate that the sulfur at Enterprise was derived largely from Neoproterozoic marine sulfate by thermochemical sulfate reduction. Significant volumes of mafic igneous rocks are not present in the immediate area of the Enterprise deposit. No evidence of prealteration concentrations of nickel exists within the sedimentary rock sequence at the deposit. The alteration and mineralization style of the Enterprise deposit is similar to the much less metamorphosed nickel-bearing Shinkolobwe uranium deposit in the Democratic Republic of Congo (DRC), though the Enterprise deposit does not contain significant uranium. The sediment-hosted nickel-rich deposits of Central African Copperbelt exemplified by Enterprise appear to represent a new style of hydrothermal nickel mineralization.

Description: Textural and compositional data for magnetite from nine iron skarn deposits in Canada, Romania, and China show that most samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization. The dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined, whereas the oxy-exsolution occurs only in high-Ti magnetite, forming exsolution lamellae of Fe-Ti-Al oxides. Electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite, notably Si, Mg, Ca, Al, Mn, and Ti, whereas oxy-exsolution is effective in decreasing the Ti content of high-Ti magnetite. Many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations (BIF), iron oxide–copper-gold (IOCG), or porphyry Cu fields using the Ti + V versus Ca + Al + Mn discrimination diagram. This pattern suggests that trace element data for magnetite that has unusual composition and/or reequilibrated cannot be reliably used as a petrogenetic indicator. Mixing of externally derived saline fluids with Fe-rich magmatic-hydrothermal solutions, an increase in temperature, and local decreasing pressure and f O 2 are considered the most important causes for the dissolution and reprecipitation, or recrystallization, of the magnetite; increasing f O 2 and decreasing temperature may facilitate oxy-exsolution of Fe-Ti-Al oxides in high-Ti magnetite. Results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits.

Description: The Iron Range iron oxide ± Cu ± Au deposits in southeastern British Columbia comprise massive lenses and veins of hematite and martite with lesser magnetite in the Iron Range fault zone, which crosscuts the Proterozoic Aldridge Formation and Moyie sills. The mineralized zones are flanked by albite-quartz-iron oxide breccia within sedimentary rocks and by chlorite-altered iron oxide breccia where they are in contact with Moyie sills. Oxygen isotope analyses indicate 340° to 400°C precipitation temperatures for the albite-quartz-magnetite assemblages in the mineralized zones. Magnetite trace element compositions closely compare with those in iron oxide-(copper-gold) (IOCG) to porphyry-type mineralization worldwide. Paleomagnetic studies show consistent paleopole orientations concordant with Cretaceous poles and support links to a porphyry-type genesis associated with phases of the nearby 80 to 105 Ma magnetite-bearing Bayonne Suite plutonic rocks. The Iron Range iron oxide mineralized zones share many characteristics of major IOCG deposits, with the exception of economic Cu (± Au) concentrations in the exposed rocks; however, recent drilling intersected minor chalcopyrite, pyrite, and gold.

Description: The Iberian Pyrite Belt is one of the most outstanding European ore provinces and hosts one of the largest concentrations of massive sulfide deposits today, totaling 1,850 million metric tons (Mt) in more than 90 deposits. Lagoa Salgada is a small orebody (estimated to have at least 4 Mt) and, as yet, an unexploited orebody found within this ore province. It is located 80 km northwest of Neves Corvo and occurs ~130 m beneath sediments of the Sado Tertiary basin, limiting interpretation to drill hole data. Lagoa Salgada is folded, faulted, and interpreted to occur mostly on the subvertical-overturned and intensely faulted limb of a southwest-verging anticline. It is represented by a central stockwork zone and a massive sulfide lens zone in the northwestern part of the orebody. Mineralization is mainly composed of pyrite with minor sphalerite, tetrahedrite-tennantite, arsenopyrite, chalcopyrite, galena, stannite, and supergene minerals. The orebody is hosted by a volcanic succession of rhyodacitic composition. These small orebodies and some of the other abandoned mines within the Iberian Pyrite Belt may represent interesting and feasible mining projects as a result of the added value generated by the presence of trace metals, such as In, serving a significant future demand for the high-tech industry. Lagoa Salgada is one such case. Indium is a significant trace metal in the ores of Lagoa Salgada as indicated by whole-rock analyses. This element is preferentially contained by sphalerite. Electron probe microanalyses (EPMA) of In contents within sphalerite show a large variability, ranging from below detection limits to an obtained maximum of 0.8 percent In. Discrete inclusions of In-bearing minerals have not been observed, thus favoring the idea that In occurs either dispersed or in nanodomains within the host mineral.

Description: The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithofacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account.

Description: The Kangerlussuaq Alkaline Complex, East Greenland, is one of the largest alkaline complexes in the world. It is known to host a number of occurrences of base and precious metal mineralization, including the Flammefjeld porphyry Mo deposits and a suite of epithermal style base and precious metal veins in an area around the Søndre Syenitgletscher. Here we describe and date a previously unknown occurrence of molybdenite-pyrite-scheelite–bearing quartz veins associated with alkali basalt dikes that intrude the main syenites of the complex. The veins appear to be cogenetic with the dikes, which crosscut and brecciate granodiorites of the Cirque 1320 complex. Sulfur isotope signatures of molybdenite and pyrite in the veins give a tight range with mean δ34S of 2.2 ± 0.7‰, consistent with a magmatic S source related to the intrusion of the dikes. Molybdenite from the veins gave an Re-Os age of 52.74 ± 0.26 Ma, some 13 m.y. older than molybdenite at the Flammefjeld porphyry deposit, thus distinguishing this metallogenic episode as a distinct event, temporally unrelated to the known Mo mineralization at Flammefjeld. Significantly, in terms of the timing of regional metallogenesis and magmatism, our date suggests that the age of the Kangerlussuaq Alkaline Complex itself may be slightly older than the 50 Ma currently accepted, based on 40Ar-39Ar dating from biotites in the main syenites. Given the common discrepancy in ore systems of 40Ar-39Ar, which produce slightly younger dates than those by Re-Os and U-Pb, we suggest the actual age of the Kangerlussuaq Alkaline Complex may be closer to 53 to 52 Ma.