ALBERT

Description: Magmatic differentiation and/or assimilation and related segregation of immiscible sulfide liquid are generally believed to be critical processes in the formation of the majority of orthomagmatic Ni sulfide deposits. In recent years, a new class of Ni sulfide deposits formed by metasomatic and/or hydrothermal modification of peridotites has been recognized. The serpentinite-hosted Avebury Ni sulfide deposit (Tasmania, Australia), the largest known non-magmatic sulfide deposit, provides an unprecedented opportunity to understand sources of metals and fluids responsible for this style of economic mineralization. Our study shows that serpentinization of the Ni-bearing olivine in the Cambrian peridotites of the McIvor Hill complex was followed by metasomatic transformation assisted by heat and fluids supplied by the nearby Late Devonian granite intrusion. The role of the above in the formation of an economic concentration of Ni sulfides is supported by (1) abundant Ni-Fe alloys and sulfides associated with serpentinization of peridotitic olivine, (2) metasomatic olivine containing inclusions of serpentine and metalliferous brines, and (3) the Late Devonian age of the Ni sulfide deposit. The Avebury metasomatic olivine is Ni-depleted and enriched in Mn relative to olivine of similar Fo content in nearby unmineralized peridotites, and to olivine in subduction-related mafic magmas generally. The unusual minor element chemistry of olivine is matched by a unique set of olivine-hosted multiphase inclusions composed of fibrous Mg-silicates and various Na-, K-, Fe-, Ca-, Mn-, and Ba-bearing chlorides/hydrochlorides, sulfides, arsenides magnetite, REE minerals, and Fe-Ni alloys. Peridotite whole-rock Sr-Nd-Pb isotope data and U-Pb dating of metasomatic titanite support earlier suggestions that Ni mineralization is temporally and genetically related with the intrusion of the nearby 360 Ma Heemskirk Granite. It appears that the multiphase inclusions in metasomatic olivine demonstrate chemical signatures of both in situ serpentinites (entrapped alloys, sulfides, arsenides, and magnetite) and distal fluids (enrichment in Pb, Bi, Sn, Sb, Sr, Ba, Rb, Cs, and Ce). We propose that magmatic olivine in large ultramafic bodies provides almost infinite Ni to replacive serpentinites and constitutes a major reservoir of disseminated Ni mineralization. In the case of Avebury Ni was locally redistributed from olivine in the Cambrian peridotites to mainly Fe-Ni alloys and sulfides during serpentinization in the early Paleozoic. In the Devonian reheating and interaction with a granitic fluid in the contact aureole of the Heemskirk Granite led to de-serpentinization and formation of metasomatic high-Mn, low-Ni olivine with inclusions of serpentine and entrapped alloys, sulfides, arsenides, and magnetite, and metalliferous brines rich in "granitic" elements. Nickel released from serpentinite in this process was re-deposited near the margins of the peridotite to form the Avebury Ni orebody. Our model of serpentinization-related release of Ni from magmatic olivine, in situ precipitation of metallic, sulfide, and arsenide Ni-minerals, and their redistribution and recrystallization in hydrothermal conditions represents an alternative to Ni remobilization from magmatic sulfides.

Description: Mantle polymict breccias sampled by kimberlite magmas are complex mixtures of mantle minerals and rock clasts, cemented together by olivine, phlogopite, orthopyroxene, ilmenite, rutile and sulphides. Because of the kimberlite-like texture (i.e. mineral clasts of diverse origin and composition set in a magmatic matrix) and the large geochemical heterogeneity preserved in polymict breccias, these rocks are believed to derive from primitive or precursor kimberlite magmas. Therefore, the study of such xenoliths can provide constraints on the processes occurring in the mantle during the early stages of kimberlite ascent, and possibly on the composition of kimberlite melts. To constrain the petrogenesis of these unusual rocks, we have studied two samples of polymict breccia from the Bultfontein kimberlite (Kimberley, South Africa) and compared our results with published data for other polymict breccias. The most abundant phase in the matrix of the studied samples is olivine with a narrow range in Mg# (~88–89), but variable Ni–Mn–Ca contents. Similar compositions are characteristic of magmatic olivine in the Bultfontein and nearby De Beers kimberlites. Orthopyroxene is the dominant phase in the matrix of polymict breccias surrounding clinopyroxene clasts, which, like the other silicate mineral clasts, are highly resorbed. The matrix orthopyroxene exhibits variable compositions, with significant enrichment in Ca, Na, Cr, Sr, Ba and light rare earth elements in the grains adjacent to clinopyroxene. The other main matrix phases (phlogopite, ilmenite and rutile) also display variable compositions. Matrix olivine hosts primary carbonate-rich inclusions similar to those observed in polymict breccia ilmenite. These inclusions were previously interpreted as an alkali-carbonate melt trapped during ilmenite growth. This alkali-carbonate melt may represent the parental melt to the matrix minerals of the polymict breccias. The variable composition of the matrix minerals is attributed to rapid, small-scale (centimetre to millimetre) variations in the melt composition owing to clast dissolution, possibly coupled with wall-rock assimilation, closely followed by fast cooling. Partial digestion of silicate porphyroclasts increased the Si content of the matrix melt, thus allowing crystallization of orthopyroxene. Further arguments in favour of a genetic relationship between polymict breccias and kimberlite magmas are provided by (1) similar Hf isotope compositions of polymict breccia ilmenite and South African kimberlites, (2) overlapping olivine compositions in polymict breccias and the host Bultfontein kimberlite, and (3) the occurrence of alkali-carbonate inclusions in polymict breccia and kimberlite minerals. Polymict breccias are interpreted as failed kimberlite intrusions, which metasomatized the magmatic conduit through which subsequent pulses of kimberlite magmas ascended. These wall-rock interactions would limit reactions between later pulses of kimberlite melt and mantle wall-rocks, thus enhancing the ability of kimberlite magmas to reach the surface.

Description: Hyperspectral cathodoluminescence (CL) mapping, combined with electron probe microanalysis (EPMA) and Fourier transform infrared spectroscopy, was used for the reconstruction of crystallization conditions of quartz from porphyry environments. Quartz eyes from the two porphyry deposits Rio Blanco (Chile) and Climax (U.S.A.) were studied. Three peaks are found to be responsible for the total CL emission: 1.93, 2.05, and 2.72 eV. The first two peaks are assigned to O-M (with M being an alkali ion) and oxygen vacancies, respectively. The 2.72 eV peak shows a linear correlation with the Ti concentration determined by EPMA point measurements. In addition, a negative correlation between the 1.93 eV emission and the Al concentration was observed. Quartz grains often form clusters in which adjacent grains show identical CL patterns, indicating that they crystallized attached to each other and were not disturbed later. Quartz cores display sector zoning and enrichment in Li, OH, and sometimes Al, which points to rapid crystallization from an extremely evolved melt. Quartz rims show high Ti, and low Li and OH contents, indicating crystallization from a less evolved melt either at higher temperatures or at higher titanium activities. The Al and Ti distribution patterns are frequently not correlated and both show uneven distribution indicating fast growth from inhomogeneous melts. Only Ti displays sharp transitions and fine oscillatory zoning, which can be explained by the higher mobility of Al in the quartz lattice. The quartz eyes crystallized after magma emplacement under non-equilibrium conditions. It is likely that the crystallization occurred from the melt enriched in Al, Li, and OH and probably other metals and/or volatiles on the brink of fluid exsolution. Subsequent fluid exsolution brought about disequilibrium to the system, resulting in dissolution of quartz and redistribution of elements between the melt and the fluid. The OH, Li, and other alkali metals and volatiles partitioned into the fluid, whereas Ti and Al remained in the melt. Resorption of quartz caused by the fluid exsolution continued until equilibrium was reached again, after which crystallization of quartz rims began from the water-, alkali-, and volatile-poor melt with higher Ti activity. Further accumulation of Al and Ti in the residual melt led to crystallization of extremely Al- and Ti-rich quartz.

Description: Mantle xenoliths sampled by kimberlite and alkali basalt magmas show a range of metasomatic styles, but direct evidence for the nature of the metasomatising fluids is often elusive. It has been suggested that carbonate-rich melts produced by partial melting of carbonated peridotites and eclogites play an important role in modifying the composition of the lithospheric mantle. These mantle-derived carbonate melts are often inferred to be enriched in alkali elements; however, alkali-rich carbonate fluids have only been reported as micro-inclusions in diamonds and as unique melts involved in the formation of the Udachnaya-East kimberlite (Yakutia, Russia). In this paper we present the first direct evidence for alkali-carbonate melts in the shallow lithospheric mantle (~110–115 km), above the diamond stability field. These alkali-carbonate melts are preserved in primary multiphase inclusions hosted by large metasomatic ilmenite grains contained in a polymict mantle xenolith from the Bultfontein kimberlite (Kimberley, South Africa). The inclusions host abundant carbonates (magnesite, dolomite, and K-Na-Ca carbonates), kalsilite, phlogopite, K-Na titanates, and phosphates, with lesser amounts of olivine, chlorides, and alkali sulfates. Textural and chemical observations indicate that the alkali-carbonate melt likely derived from primary or precursor kimberlite magmas. Our findings extend the evidence for alkali-carbonate melts/fluids permeating the Earth mantle outside the diamond stability field and provide new insights into the chemical features of previously hypothesized melts. As metasomatism by alkali-rich carbonate melts is often reported to affect mantle xenoliths, and predicted from experimental studies, the fluid type documented here likely represent a major metasomatising agent in the Earth’s lithospheric mantle.

Description: Brothers caldera volcano is a submarine volcano of dacitic composition, located on the Kermadec arc, New Zealand. It hosts the NW caldera vent field perched on the steep slope of the caldera walls and includes numerous, active, high-temperature (max 302°C) chimneys and a greater amount of dead, sulfide-rich spires. Petrographic studies of these chimneys show that three main zones can occur within the chimneys: a chalcopyrite-rich core, surrounded by a sulfate-dominated zone, which is in turn mantled by an external rind of Fe oxides, calcite, and silicates. Four chimney types are identified based on the relative proportions of the chalcopyrite and sulfate layers and the presence or absence of anhydrite. Two are Cu rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys, and two are Zn rich, i.e., sphalerite-barite and sphalerite-chalcopyrite. Chimney growth begins with the formation of a sulfate wall upon which sulfides precipitate. Later, zone refining results in a chalcopyrite-rich core with pyrite/marcasite and sphalerite occurring predominantly near the outer margins. In chalcopyrite-bornite chimneys, the chalcopyrite core rapidly loses permeability and limits the thickness of the surrounding sulfate layer. In these chimneys, bornite, chalcocite, and covellite form along the outer margin of the chalcopyrite zone as a result of oxidation by seawater. Zinc-rich chimneys display a more vertical zonation and their growth involves an upward-advancing barite cap followed by chalcopyrite deposition (if present) nearer the base. The vertical zonation and lack of anhydrite in these chimneys also implies that larger chalcopyrite and anhydrite deposits may exist subsea floor. The different chimney types are related to subsea-floor permeability, the amount of fluid mixing that occurs prior to venting, and heterogeneous fluid compositions. The occurrence of specular hematite and Bi or Au tellurides associated with chalcopyrite are consistent with magmatic contributions to the NW caldera vent site. These tellurides are the first gold-bearing phase to be identified in these chimneys, and the Bi-Au association suggests that gold enrichment up to 91 ppm is due to scavenging by liquid bismuth. The presence of tellurides in Brothers chimneys have implications for other telluride-bearing deposits, such those in the Urals. Likewise, other aspects of the mineralogy (i.e., textures) and zonation, including the implied subsea-floor deposition, presented here from an active, undeformed environment can aid in understanding ancient volcanogenic massive sulfide (VMS) deposits that have undergone various degrees of metamorphism.

Description: Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) trace element maps of pyrite and gold from the Carbon Leader Reef in the Witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal, derived from the West Rand Group or equivalent shales stratigraphically below this reef. Rounded detrital pyrite grains in the Carbon Leader Reef vary from compact to porous to sooty, with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the West Rand Group. Detrital porous and sooty pyrites contain 0.4 to 11.3 ppm solid-solution gold, sulfur isotope 34 S values from –17 to +16%, and high levels of As and Te, plus a wide range of other trace elements (in decreasing order: Ni, Co, Sb, Cr, Cu, U, Pb, Bi, Mo, Zn and Ag). The sooty pyrite is the most Au, Te, and Mo rich and is intergrown with alumino-silicates and organic matter. The detrital pyrite textures, S-isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions. The LA-ICP-MS maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro-inclusions of gold, brannerite, and Ni-As sulfides. The pyrite overgrowths are also enriched in solid-solution gold, tellurium, and other trace elements including, in decreasing order, As, Co, Ni, Cr, Cu, Mn, Pb, Bi, and Ag. Fractured and brecciated pyrite associated with brittle bedding-parallel fracture zones in the Carbon Leader Reef are also enriched in these elements due to alteration of pyrite surrounding the fractures. Laser ICP-MS Pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 Ma with hydrothermal overgrowths originating between 2100 and 2020 Ma incorporating highly radiogenic Pb. This study demonstrates that both of the two competing theories for the origin of the Witwatersrand gold reefs are likely to be correct. We suggest that the hydrothermal event was widespread (kilometer scale) and involved basinal fluids that scavenged gold, tellurium, arsenic, and trace elements (Co, Ni, Cr, Cu, Mn, Pb, Bi, Ag) from gold-bearing sedimentary units in the Central Rand Group.