Showing papers in "Mineralium Deposita in 2015"

Enrichment of U–Se–Mo–Re–V in coals preserved within marine carbonate successions: geochemical and mineralogical data from the Late Permian Guiding Coalfield, Guizhou, China

Abstract: We present multi-element data on the super-high-organic-sulfur (SHOS; 5.19 % on average) coals of Late Permian age from Guiding, in Guizhou Province, China. The coals, formed on restricted carbonate platforms, are all highly enriched in S, U, Se, Mo, Re, V, and Cr, and, to a lesser extent, Ni and Cd. Although the Guiding coals were subjected to seawater influence, boron is very low and mainly occurs in tourmaline and mixed-layer illite/smectite. Uranium, Mo, and V in the coal are mainly associated with the organic matter. In addition, a small proportion of the U occurs in coffinite and brannerite. The major carrier of Se is pyrite rather than marcasite. Rhenium probably occurs in secondary sulfate and carbonate minerals. The U-bearing coal deposits have the following characteristics: the formation age is limited to Late Permian; concentrations of sulfur and rare metals (U, Se, Mo, Re, V, and in some cases, rare earth elements and Y) are highly elevated; the U-bearing coal beds are intercalated with marine carbonate rocks; organic sulfur and rare metals are uniformly distributed within the coal seams; and the combustion products (e.g., fly and bottom ash) derived from the coal deposits may have potential economic significance for rare metals: U, Se, Mo, Re, V, rare earth elements, and Y.

Cretaceous–Cenozoic tectonic history of the Jiaojia Fault and gold mineralization in the Jiaodong Peninsula, China: constraints from zircon U–Pb, illite K–Ar, and apatite fission track thermochronometry

Abstract: The Jiaojia Fault (JJF) in the Jiaodong area of eastern China is an important NNE-trending structure that is subsidiary to the regional Tancheng–Lujiang (Tan-Lu) Fault Zone, and hosts >1200 t of gold reserves contained in disseminated and stockwork ore, dominantly in the footwall of the fault. We present new zircon U–Pb, apatite fission track, and illite K–Ar data along the JJF and have delineated its tectonic history focusing on its formation and reactivation. Zircon U–Pb dating shows that the Shangzhuang granite is a composite body with ages between 132 ± 1 and 127 ± 1 Ma. Illite K–Ar ages for the fault’s gouge range from 83 ± 2 to 68 ± 2 Ma, and the measured apatite fission track ages for ores are between 55 and 21 Ma. Previous zircon U–Pb geochronology and structural studies suggest that the JJF was originally activated in the Jurassic during 160–150 Ma as a sinistral fault. The JJF was a normal fault in the Early Cretaceous due to NW–SE orientated tension and NE–SW compression, which lasted from 135 to 120 Ma. This was followed by sinistral strike–slip faulting due to NW–SE compression and NE–SW tension during 120–110 Ma, and it changed to normal displacement at ca. 110 Ma. Our apatite fission track data analysis and thermal modeling of representative samples suggest that there was a subsequent dextral reactivation of the fault at ca. 55 Ma. Previous age data of ca. 130–110 Ma for gold mineralization along the JJF coincides with the Early Cretaceous magmatism and is coeval with the transition from normal faulting to sinistral strike–slip faulting of the JJF in Early Cretaceous, which is interpreted to be due to changing direction of the subducting Pacific Plate.

Geochemistry of magnetite from porphyry Cu and skarn deposits in the southwestern United States

Abstract: A combination of petrographic observations, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and statistical data exploration was used in this study to determine compositional variations in hydrothermal and igneous magnetite from five porphyry Cu–Mo and skarn deposits in the southwestern United States, and igneous magnetite from the unmineralized, granodioritic Inner Zone Batholith, Japan. The most important overall discriminators for the minor and trace element chemistry of magnetite from the investigated porphyry and skarn deposits are Mg, Al, Ti, V, Mn, Co, Zn, and Ga—of these the elements with the highest variance for (I) igneous magnetite are Mg, Al, Ti, V, Mn, Zn, for (II) hydrothermal porphyry magnetite are Mg, Ti, V, Mn, Co, Zn, and for (III) hydrothermal skarn magnetite are Mg, Ti, Mn, Zn, and Ga. Nickel could only be detected at levels above the limit of reporting (LOR) in two igneous magnetites. Equally, Cr could only be detected in one igneous occurrence. Copper, As, Mo, Ag, Au, and Pb have been reported in magnetite by other authors but could not be detected at levels greater than their respective LORs in our samples. Comparison with the chemical signature of igneous magnetite from the barren Inner Zone Batholith, Japan, suggests that V, Mn, Co, and Ga concentrations are relatively depleted in magnetite from the porphyry and skarn deposits. Higher formation conditions in combination with distinct differences between melt and hydrothermal fluid compositions are reflected in Al, Ti, V, and Ga concentrations that are, on average, higher in igneous magnetite than in hydrothermal magnetite (including porphyry and skarn magnetite). Low Ti and V concentrations in combination with high Mn concentrations are characteristic features of magnetite from skarn deposits. High Mg concentrations (<1,000 ppm) are characteristic for magnetite from magnesian skarn and likely reflect extensive fluid/rock interaction. In porphyry deposits, hydrothermal magnetite from different vein types can be distinguished by varying Ti, V, Mn, and Zn contents. Titanium and V concentrations are highly variable among hydrothermal and igneous magnetites, but Ti concentrations above 3,560 ppm could only be detected in igneous magnetite, and V concentrations are on average lower in hydrothermal magnetite. The highest Ti concentrations are present in igneous magnetite from gabbro and monzonite. The lowest Ti concentrations were recorded in igneous magnetite from granodiorite and granodiorite breccia and largely overlap with Ti concentrations found in hydrothermal porphyry magnetite. Magnesium and Mn concentrations vary between magnetite from different skarn deposits but are generally greater than in hydrothermal magnetite from the porphyry deposits. High Mg, and low Ti and V concentrations characterize hydrothermal magnetite from magnesian skarn deposits and follow a trend that indicates that magnetite from skarn (calcic and magnesian) commonly has low Ti and V concentrations.

Did the massive magnetite “lava flows” of El Laco (Chile) form by magmatic or hydrothermal processes? New constraints from magnetite composition by LA-ICP-MS

Abstract: The El Laco magnetite deposits consist of more than 98 % magnetite but show field textures remarkably similar to mafic lava flows. Therefore, it has long been suggested that they represent a rare example of an effusive Fe oxide liquid. Field and petrographic evidence, however, suggest that the magnetite deposits represent replacement of andesite flows and that the textures are pseudomorphs. We determined the trace element content of magnetite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from various settings at El Laco and compared them with magnetite from both igneous and hydrothermal environments. This new technique allows us to place constraints on the conditions under which magnetite in these supposed magnetite “lava flows” formed. The trace element content of magnetite from the massive magnetite samples is different to any known magmatic magnetite, including primary magnetite phenocrysts from the unaltered andesite host rocks at El Laco. Instead, the El Laco magnetite is most similar in composition to hydrothermal magnetite from high-temperature environments (>500 °C), such as iron oxide-copper-gold (IOCG) and porphyry-Cu deposits. The magnetite trace elements from massive magnetite are characterised by (1) depletion in elements considered relatively immobile in hydrothermal fluids (e.g. Ti, Al, Cr, Zr, Hf and Sc); (2) enrichment in elements that are highly incompatible with magmatic magnetite (rare earth elements (REE), Si, Ca, Na and P) and normally present in very low abundance in magmatic magnetite; (3) high Ni/Cr ratios which are typical of magnetite from hydrothermal environments; and (4) oscillatory zoning of Si, Ca, Mg, REE and most high field strength elements, and zoning truncations indicating dissolution, similar to that formed in hydrothermal Fe skarn deposits. In addition, secondary magnetite in altered, brecciated host rock, forming disseminations and veins, has the same composition as magnetite from the massive lenses. Euhedral magnetite lining both open-spaced veins in the brecciated host rock and along the walls of large, hollow chimneys in the massive magnetite lenses also displays oscillatory zoning and most likely formed by fluctuating composition and/or physio-chemical conditions of the fluid. Thus, the chemical fingerprint of magnetite from the supposed El Laco magnetite lava flows supports the hydrothermal model of metasomatic replacement of andesite lava flows, by dissolution and precipitation of magnetite from high-temperature fluids, rather than a magmatic origin from an effusive Fe oxide liquid.

Metabasalts as sources of metals in orogenic gold deposits

Abstract: Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 ± 0.5 ppb) and upper greenschist facies (1.0 ± 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 ± 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.

Sediment and weathering control on the distribution of Paleozoic magmatic tin–tungsten mineralization

Abstract: The formation of major granite-hosted Sn and/or W deposits and lithium–cesium–tantalum (LCT) type pegmatites in the Acadian, Variscan, and Alleghanian orogenic belts of Europe and Atlantic Northern America involves weathering-related Sn and W enrichment in the sedimentary debris of the Cadomian magmatic arc and melting of these sedimentary source rocks during later tectonic events, followed by magmatic Sn and W enrichment. We suggest that within this, more than 3,000-km long late Paleozoic belt, large Sn and/or W deposits are only found in regions where later redeposition of the Sn–W-enriched weathered sediments, followed by tectonic accumulation, created large volumes of Sn–W-enriched sedimentary rocks. Melting of these packages occurred both during the formation of Pangea, when continental collision subjected these source rocks to high-grade metamorphism and anatexis, and during post-orogenic crustal extension and mantle upwelling. The uncoupling of source enrichment and source melting explains (i) the diachronous occurrence of tin granites and LCT pegmatites in this late Paleozoic orogenic belt, (ii) the occurrence of Sn and/or W mineralizations and LCT pegmatites on both sides of the Rheic suture, and (iii) the contrasting tectonic setting of Sn and/or W mineralizations within this belt. Source enrichment, sedimentary and tectonic accumulation of the source rocks, and heat input to mobilize metals from the source rocks are three unrelated requirements for the formation of Sn and/or W granites. They are the controlling features on the large scale. Whether a particular granite eventually generates a Sn and/or W deposit depends on local conditions related to source melting, melt extraction, and fractionation processes.

A modified genetic model for the Huangshandong magmatic sulfide deposit in the Central Asian Orogenic Belt, Xinjiang, western China

Abstract: The Huangshandong Ni–Cu deposit is the largest magmatic sulfide deposit discovered to date in the Central Asian Orogenic Belt in northern Xinjiang, western China The host intrusion is a 274-Ma composite mafic–ultramafic intrusion consisting of four separate intrusive units: a large layered gabbroic sequence (phase I), a sheet-like ultramafic body (phase II), a dyke-like gabbronorite body (phase III), and an irregular ultramafic unit (phase IV) Important sulfide mineralization is present in the last three intrusive units, predominantly as disseminated and net-textured sulfides (pyrrhotite, pentlandite, and chalcopyrite) The Huangshandong mafic–ultramafic intrusive rocks are characterized by arc-like geochemical signatures such as low Ca content in olivine and negative Nb–Ta anomalies in whole rocks This, together with a post-subduction setting for the East Tianshan in the Permian, suggests that the source mantle was modified previously by slab-derived fluids in the Carboniferous The mantle-derived magma was ponded in a staging chamber in the lower part of the newly formed arc crust The first batch of magma to arrive at Huangshandong was most fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo67, <300 ppm Ni) The second batch of magma was more primitive, crystallizing more primitive olivine (Fo81–84) The third batch of magma was also highly fractionated and depleted in Ni, crystallizing Fe-rich and Ni-depleted olivine (Fo72, ~600 ppm Ni) The final batch of magma became more primitive again, crystallizing the most primitive olivine (Fo81–86) The occurrence of rounded sulfide inclusions in olivine primocrysts in the Huangshandong ultramafic rocks indicates that immiscible sulfide liquid droplets were present during olivine crystallization The Ni tenors of disseminated sulfide ores in the gabbronorite dyke vary mainly between 5 and 8 wt%, which are too high to have been produced by the parental magma of the dyke The Ni, Cu, and platinum-group elements (PGE) tenors of disseminated sulfide ores in the dyke (phase III) and the ultramafic sheet (phase II) are remarkably similar These observations, together with the sequence of magma emplacement, suggest that the sulfide liquids entrapped in the magma of the dyke formed at depth by a previous pulse of more primitive magma The estimated parental magma for the most primitive lherzolites in the Huangshandong intrusion contains 10 wt% MgO Modeling shows that sulfide saturation in the parental magma of the Huangshandong lherzolites could have resulted from fractional crystallization Significant PGE depletions relative to Ni and Cu in the disseminated sulfide ores of the Huangshandong deposit may be due to sulfide retention in the source mantle

Timing of multiple hydrothermal events in the iron oxide-copper-gold deposits of the Southern Copper Belt, Carajás Province, Brazil

Abstract: The Southern Copper Belt, Carajas Province, Brazil, hosts several iron oxide–copper–gold (IOCG) deposits, including Sossego, Cristalino, Alvo 118, Bacuri, Bacaba, Castanha, and Visconde. Mapping and U–Pb sensitive high-resolution ion microprobe (SHRIMP) IIe zircon geochronology allowed the characterization of the host rocks, situated within regional WNW–ESE shear zones. They encompass Mesoarchean (3.08–2.85 Ga) TTG orthogneiss, granites, and remains of greenstone belts, Neoarchean (ca. 2.74 Ga) granite, shallow-emplaced porphyries, and granophyric granite coeval with gabbro, and Paleoproterozoic (1.88 Ga) porphyry dykes. Extensive hydrothermal zones include albite–scapolite, biotite–scapolite–tourmaline–magnetite alteration, and proximal potassium feldspar, chlorite–epidote and chalcopyrite formation. U–Pb laser ablation multicollector inductively coupled mass spectrometry (LA-MC-ICP-MS) analysis of ore-related monazite and Re–Os NTIMS analysis of molybdenite suggest multiple Neoarchean (2.76 and 2.72–2.68 Ga) and Paleoproterozoic (2.06 Ga) hydrothermal events at the Bacaba and Bacuri deposits. These results, combined with available geochronological data from the literature, indicate recurrence of hydrothermal systems in the Southern Copper Belt, including 1.90–1.88-Ga ore formation in the Sossego–Curral ore bodies and the Alvo 118 deposit. Although early hydrothermal evolution at 2.76 Ga points to fluid migration coeval with the Carajas Basin formation, the main episode of IOCG genesis (2.72–2.68 Ga) is related to basin inversion coupled with Neoarchean (ca. 2.7 Ga) felsic magmatism. The data suggest that the IOCG deposits in the Southern Copper Belt and those in the Northern Copper Belt (2.57-Ga Salobo and Igarape Bahia–Alemao deposits) do not share a common metallogenic evolution. Therefore, the association of all IOCG deposits of the Carajas Province with a single extensive hydrothermal system is precluded.

Mineralogical siting of platinum-group elements in pentlandite from the Bushveld Complex, South Africa

Abstract: The Bushveld Complex in South Africa hosts the world’s largest resources of platinum-group elements (PGEs), which are mainly mined from three ore bodies, namely the Merensky Reef, the UG-2 chromitite, and the Platreef. In these ores, the PGEs are bimodally distributed, occurring both as discrete platinum-group minerals (PGMs) and hosted by sulfides. The presence of PGEs in sulfides has been demonstrated by electron probe microanalysis, laser ablation induced coupled plasma mass spectrometry, secondary ion mass spectrometry, and particle-induced X-ray emission. However, evidence is lacking on the mineralogical siting of the PGEs, e.g., whether they occur in solid solution, as nano-inclusions, and/or micro-inclusions. Therefore, in the present study, a combination of focused ion beam and transmission electron microscopy was used which allows to obtain crystal structural relationships between the host mineral and incorporated trace elements and revealing the physicochemical state of the PGE in sulfides. The present study confirms the existence of micrometer-sized discrete PGMs in the ores. Further, the PGEs occur in a number of forms, namely (1) as discrete nano-inclusions of PGMs, (2) as patchily distributed solid solution, (3) ordered within the pentlandite crystal structure, substituting for Ni and/or Fe (superlattice), and (4) as homogenous solid solution. Nanometer-sized PGMs (nPGMs) show no orientation relationship with the host sulfide mineral. Consequently, they are discrete phases, which were trapped within pentlandite during sulfide growth. Heterogeneous and patchy distributions of Rh and Ir within the pentlandite lattice suggest that Rh and Ir were already present within the sulfide liquid. The absence of possible reaction partners (e.g., Bi, As, and Sn) necessary for the formation of discrete PGMs forced Rh and Ir to remain in the crystal lattice of pentlandite and down-temperature exsolution caused patchy distribution patterns of Rh and Ir. High concentrations of Rh and Ir in pentlandite initiate ordering of the randomly distributed PGE in form of nanometer-sized lamellae resulting in the formation of a superlattice. Palladium is homogenously distributed within the pentlandite lattice, even at high Pd concentrations, and in addition also occurs as nPGMs.

Extreme variation of sulfur isotopic compositions in pyrite from the Qiuling sediment-hosted gold deposit, West Qinling orogen, central China: an in situ SIMS study with implications for the source of sulfur

Abstract: High spatial resolution textural (scanning electron microscope (SEM)), chemical (electron microprobe (EMP)) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)), and sulfur isotopic (secondary ion mass spectrometry (SIMS)) analyses of pyrite from the Qiuling sediment-hosted gold deposit (232 ± 4 Ma) in the West Qinling orogen, central China were conducted to distinguish pyrite types and gain insights into the source and evolution of sulfur in hydrothermal fluids The results reveal an enormous variation (−271 to +696 ‰) in sulfur isotopic composition of pyrite deposited during three paragenetic stages Pre-ore framboidal pyrite, which is characterized by low concentrations of As, Au, Cu, Co, and Ni, has negative δ34S values of −271 to −76 ‰ that are interpreted in terms of bacterial reduction of marine sulfate during sedimentation and diagenesis of the Paleozoic carbonate and clastic sequences, the predominant lithologies in the deposit area, and the most important hosts of many sediment-hosted gold deposits throughout the West Qinling orogen The ore-stage hydrothermal pyrite contains high concentrations of Au, As, Cu, Sb, Tl, and Bi and has a relatively narrow range of positive δ34S values ranging from +81 to +152 ‰ The sulfur isotope data are comparable to those of ore pyrite from many Triassic orogenic gold deposits and Paleozoic sedimentary exhalative (SEDEX) Pb-Zn deposits in the West Qinling orogen, both being hosted mainly in the Devonian sequence This similarity indicates that sulfur, responsible for the auriferous pyrite at Qiuling, was largely derived from the metamorphic devolatization of Paleozoic marine sedimentary rocks Post-ore-stage pyrite, which is significantly enriched in Co and Ni but depleted in Au and As, has unusually high δ34S values ranging from +374 to +696 ‰, that are interpreted to result from thermochemical reduction of evaporite sulfates in underlying Cambrian sedimentary rocks with very high δ34S values The variations in Au content and sulfur isotopic compositions across a single ore-stage pyrite grain may reflect displacement of indigenous groundwater with low δ34S values by auriferous metamorphic fluids with high δ34S values The very low-grade metamorphism of the host rocks and the metamorphic derivation of sulfur for the ore pyrite indicate that the Qiuling sediment-hosted gold deposit is an epizonal manifestation of an orogenic gold system in the West Qinling orogen

Quantified, multi-scale X-ray fluorescence element mapping using the Maia detector array: application to mineral deposit studies

Abstract: The Maia large solid-angle detector array and imaging system is capable of collecting high-resolution images of up to ∼100 M pixels in size with dwell times of less than 0.2 ms per pixel and thus it is possible to document variation in textures associated with trace element chemistry by collecting quantified elemental maps of geological samples on the scale of entire thin sections in a short time frame (6–8 hr). The analysis is nondestructive and allows variation to be recognised on a centimetre scale while also recognising zonations at the micron scale. Studies of ore systems require microanalysis of samples to collect information on mineral chemistry in order to understand physiochemical conditions during ore genesis and alteration. Such studies contribute to the debate on whether precious metals are remobilised or introduced in multiple hydrothermal events. In this study we demonstrate the microanalytical capabilities of the Maia large solid-angle detector array and imaging system on the X-ray fluorescence microscopy beamline at the Australian Synchrotron to provide data for these studies. We present a series of case studies from orogenic gold deposits that illustrate the power of the Maia detector for constraining chemical zonations in sulphides and associated alteration minerals, which can be used to decipher ore-forming processes associated with gold deposition. A series of large-area (<7 cm2) elemental maps were collected with 2 to 4 μm pixel size using the Maia detector array. The data was processed using the GeoPIXE™ software package which allows variation in trace, minor and major element chemistry to be visualised in element maps. These maps are used to target further investigation with bulk spectra extracted and fitted for specific mineral grains and transects drawn through regions of interest. Analysis using the Maia detector offers a complementary method to map element distribution in geological samples that is both relatively fast and has a low detection limit for many elements of interest.

Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit, Loma Caribe peridotite (Dominican Republic)

Abstract: Two Ni-laterite profiles from the Loma Caribe peridotite (Dominican Republic) have been investigated for their platinum group element (PGE) geochemistry and mineralogy. One profile (Loma Peguera) i ...

Geochemistry of magnetite from Proterozoic Fe-Cu deposits in the Kangdian metallogenic province, SW China

Abstract: Fe-Cu deposits in the Kangdian Fe-Cu metallogenic province, SW China, are hosted in Paleoproterozoic meta-volcanic-sedimentary sequences and are spatially associated with coeval mafic intrusions. Several well-known examples are the giant Lala, Dahongshan, and Yinachang deposits. They have a common paragenetic sequence of an early Fe-oxide stage associated with sodic alteration and a late Cu-sulfide stage associated with potassic-carbonate alteration. Magnetite dominates the Fe-oxide stage of these deposits but is also present in the Cu-sulfide stage of the Lala deposit. This study uses trace element compositions of magnetite to examine the nature and origin of the ore-forming fluids. The magnetite has variable concentrations of Ti, Al, Mg, Mn, Si, V, Cr, Ca, Co, Ni, Sc, Zn, Cu, Mo, Sn, and Ga, which are thought to have been controlled mainly by fluid compositions and/or intensive parameters (e.g., temperature and oxygen fugacity (fO2)). Fluid-rock interaction and coprecipitating mineral phases appear to be less important in controlling the magnetite compositions. Magnetite grains in the Fe-oxide stage of the Lala and Dahongshan deposits have comparable trace element compositions and were likely precipitated from chemically similar fluids. High Ni contents of magnetite in both deposits, coupled with previous isotopic data and the fact that the two deposits are spatially associated with coeval mafic intrusions, strongly suggest that the ore-forming fluids were genetically related to the mafic magmas that formed the intrusions. Magnetite grains in the Fe-oxide stage of the Yinachang deposit have much lower V and Ni but higher Sn and Mo contents than those of the Lala and Dahongshan deposits and are thus thought to have precipitated from more oxidized and Mo-Sn-rich fluids that may have evolved from relatively felsic magmas. Magnetite grains from the Cu-sulfide and Fe-oxide stages of the Lala deposit are broadly similar in composition, but those in the Cu-sulfide stage have slightly higher Cu, Zn, and Mn and are thought to have crystallized from relatively low-temperature and Cu-Zn-Mn-rich fluids evolved from the fluids of the early Fe-oxide stage. Our results show that magnetite from the Fe-Cu deposits in the Kangdian Province, banded iron formation, Fe skarn deposits, diabase-hosted hydrothermal Fe deposits, and magmatic deposits has significantly different compositions. We propose that covariations of Co-Ni, Zn-Sn, and Co/Ni-Mn can be used to effectively discriminate different deposit types.

The timing of gold mineralization across the eastern Yilgarn craton using U–Pb geochronology of hydrothermal phosphate minerals

Abstract: The highly mineralized Eastern Goldfields of the eastern Yilgarn craton is an amalgamation of dominantly Neoarchaean granitoid-greenstone terranes and domains that record a history of early rifting, followed by westward directed collision with initial arc formation, collision and clastic basin formation, and final accretion to the western Yilgarn proto-craton between 2.66 and 2.60 billion years ago. The gold deposits that define this region as a world-class gold province are the product of orogenic processes that operated during accretion late in the tectonic history, after initial compressional deformation (D1–D2) and the majority of granitoid magmatism. Minor gold was also deposited throughout the entire tectonic history in magmatic-hydrothermal-related systems. However, such mineralization (mostly < 0.3 g/t gold) is nowhere economic unless it overprints, or is overprinted by, much higher-grade orogenic gold lodes. Robust SHRIMP U–Pb geochronology of gold-related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events (D3–D4), shortly before cratonization. However, westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m.y. between c. 2.65 Ma in the east (including Laverton District, Kurnalpi Terrane) to c. 2.63 Ma in the west (including Kalgoorlie Terrane) across the eastern part of the craton. The robust geochronology refutes previous suggestions that significant gold mineralization events extended from DE to D4 in the evolution of the orogen and that the Kalgoorlie gold deposits formed over a period of 45 m.y. The crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous, but must be modified to account for diachroneity of orogenic events and gold mineralization across the Eastern Goldfields.

First whiffs of atmospheric oxygen triggered onset of crustal gold cycle

Abstract: Lack of a suitable gold source has long been regarded as a major argument against a palaeoplacer model for the exceptionally well-endowed Witwatersrand gold province. By comparing worldwide Witwatersrand-type deposits/occurrences, ranging in age from 3.1 to 1.8 Ga, we propose that the primary concentration of gold in the continental crust resulted from atmospheric and biological evolution in the Mesoarchaean. A high Au flux off the Archaean land surface (orders of magnitude greater than today’s) was a consequence of the chemistry of the Mesoarchaean atmosphere and hydrosphere. When early life gradually changed from anaerobic anoxygenic to oxygenic photosynthesizers at around 3.0 Ga, the first ‘whiffs’ of free oxygen produced by photosynthesis under an overall reducing atmosphere provided the ideal trap for Au dissolved in the huge reservoir in meteoric and shallow sea water. Oxidative precipitation of gold on the surface of the O2-producing microbes, probably cyanobacteria, could fix huge amounts of gold over large areas. Some of this microbially mediated gold is still preserved in thin kerogen layers ("carbon seams") that typically developed on erosional unconformities, scour surfaces and bedding planes in near-shore environments at around 2.9 Ga. This gold provided the principal source for the very rich placer deposits that formed by the subsequent sedimentary reworking of the delicate microbial mats on aeolian deflation surfaces, into fluvial channels and delta deposits, represented by Meso- to Neoarchaean auriferous conglomerates. Tectonic reworking of these first gold-rich sediments by orogenic processes explains the temporal peak of orogenic-type gold formation at around 2.7 to 2.4 Ga. With time, the strongly gold-enriched Archaean sediments became progressively eroded, covered or tectonically reworked, and their role as potential source of younger placer deposits diminished. This explains why Witwatersrand-type deposits younger than 2.4 Ga are rare and far less well-endowed and effectively missing in the rock record after 1.8 Ga.

Gold and silver in PGE–Cu–Ni and PGE ores of the Noril’sk deposits, Russia

Abstract: Gold and silver contents in Noril’sk ore are controlled by the amount of sulphides and bulk Cu grade. Relative concentrations, re-calculated to 100 % sulphide, depend on type of ore: they are higher for disseminated ore than for massive ore and are the highest for low-sulphide platinum ore. Gold occurs mainly as high-fineness Au–Ag alloy in pyrrhotite-rich ore, whereas silver enters chalcopyrite mainly as solid solution. Increase in Cu grade correlates with an increase in the concentration of silver in chalcopyrite. Gold and silver form discrete minerals such as Au–Cu alloys, Au–Ag alloys, tellurides, sulphides, selenides, sulphobismuthides, Ag and Ag–Pd chlorides in Cu-rich ores; they also enter the structures of complex platinum-group minerals. The Au–Ag mineralisation is related to the post-magmatic hydrothermal stage under temperature conditions of 350–50 °C. Silver entered crystallizing chalcopyrite in solid solution in the late-magmatic stage, while all of the gold and the remainder of the silver and some platinum-group elements were transported predominantly as chloride and hydrosulphide complexes in hydrothermal fluids.

Vanadium-rich ruby and sapphire within Mogok Gemfield, Myanmar: implications for gem color and genesis

Abstract: Rubies and sapphires are of both scientific and commercial interest. These gemstones are corundum colored by transition elements within the alumina crystal lattice: Cr3+ yields red in ruby and Fe2+, Fe3+, and Ti4+ ionic interactions color sapphires. A minor ion, V3+ induces slate to purple colors and color change in some sapphires, but its role in coloring rubies remains enigmatic. Trace element and oxygen isotope composition provide genetic signatures for natural corundum and assist geographic typing. Here, we show that V can dominate chromophore contents in Mogok ruby suites. This raises implications for their color quality, enhancement treatments, geographic origin, exploration and exploitation and their comparison with rubies elsewhere. Precise LA-ICP-MS analysis of ruby and sapphire from Mogok placer and in situ deposits reveal that V can exceed 5,000 ppm, giving V/Cr, V/Fe and V/Ti ratios up to 26, 78, and 97 respectively. Such values significantly exceed those found elsewhere suggesting a localized geological control on V-rich ruby distribution. Our results demonstrate that detailed geochemical studies of ruby suites reveal that V is a potential ruby tracer, encourage comparisons of V/Cr-variation between ruby suites and widen the scope for geographic typing and genesis of ruby. This will allow more precise comparison of Asian and other ruby fields and assist confirmation of Mogok sources for rubies in historical and contemporary gems and jewelry.

Geology, mineralization, and fluid inclusion characteristics of the Kumbel oxidized W–Cu–Mo skarn and Au–W stockwork deposit in Kyrgyzstan, Tien Shan

Abstract: The Kumbel deposit is located within a metallogenic belt of W–Mo, Cu–Mo, Au–W, and Au deposits along the Late Paleozoic active continental margin of Tien Shan. The deposit is related to a Late Carboniferous multiphase pluton, with successive intrusive phases from early olivine monzogabbro through monzonite–quartz monzonite to granodiorite and granite, with the latest monzogabbro–porphyry dikes. The deposit represents an example of a complex W–Cu–Mo–Au magmatic–hydrothermal system related to magnetite-series high-K calc–alkaline to shoshonitic igneous suite. It contains large bodies of W–Cu–Mo oxidized prograde and retrograde skarns, with abundant andradite garnet, magnetite, and especially hematite, as well as K-feldspar, molybdoscheelite, chalcopyrite, and molybdenite, with transitions to zones of intense quartz–K-feldspar (with minor andradite and hematite) veining. The skarns are cut by quartz–carbonate ± adularia ± sericite veins (locally sheeted) and stockworks bearing scheelite and minor Cu, Zn, Pb sulfides, as well as Au, Bi, Te, and As mineralization. The association of these veins with the oxidized skarns and magnetite-series intrusion is consistent with the general oxidized, intrusion-related W–Mo–Cu–Au type of deposit, with an affinity to the alkalic (silica-saturated) Cu–Au ± Mo porphyry deposits. The fluid inclusion data show the predominance of magmatic–hydrothermal aqueous chloride fluid during the formation of skarns and quartz–carbonate–scheelite–sulfide veins. The high fluid pressures (∼1,750 bars), together with their high temperature (up to 600 °C) and high salinity (∼50–60 wt% NaCl-equiv.), suggest the formation of skarns and quartz–K-feldspar–andradite–hematite veins under conditions typical of magmatic–hydrothermal transition (depth of ≥4–5 km) of intrusion-related mineralized system, possibly by exsolution of the fluids from crystallizing magma. The auriferous quartz–carbonate–scheelite–sulfide veins formed from high to moderate salinity (from ∼40 to <20 wt% NaCl-equiv.) and high pressure (from ∼1,200 bars to 850–900 bars) aqueous chloride fluids under decreasing temperature (from ∼370 to 120 °C). The massive deposition of molybdoscheelite in retrograde skarn and scheelite in quartz–carbonate–scheelite–sulfide veins could correspond to enrichment of fluids in Ca (up to 18–25 wt% CaCl2), likely from crystallizing magma.

Diagenetic origin of the stratiform Cu–Co deposit at Kamoto in the Central African Copperbelt

Abstract: Sediment-hosted stratiform ore deposits that underwent metamorphism and orogenesis are interpreted either to have a multiphase origin that might in part predate these events or to be exclusively related to fluid migration during the orogeny. This controversy concerns the formation of many world-class sediment-hosted ore deposits such as in the Central African Copperbelt. Here we present Re–Os dating results for disseminated and stratiform Cu–Co sulfide pseudomorphs after anhydrite in nodules and layers from the Copperbelt. Results demonstrate that at least some of the ores formed at around 800 Ma during rifting and basin development, and thus predate the Pan-African Lufilian orogeny. Younger Re–Os ages, i.e., between 682 ± 28 and 230 ± 36 Ma, may be due to the replacement of the early Cu–Co sulfides by younger sulfide phases. Alternatively, these younger ages may result from the disturbance or resetting of the Re–Os system during recrystallization/metamorphism or the transition of chalcocite from its high- to its low-temperature polymorph.

Sulfur isotope and trace element data from ore sulfides in the Noranda district (Abitibi, Canada): implications for volcanogenic massive sulfide deposit genesis

Abstract: We examine models for volcanogenic massive sulfide (VMS) mineralization in the ~2.7-Ga Noranda camp, Abitibi subprovince, Superior Province, Canada, using a combination of multiple sulfur isotope and trace element data from ore sulfide minerals. The Noranda camp is a well-preserved, VMS deposit-rich area that is thought to represent a collapsed volcanic caldera. Due to its economic value, the camp has been studied extensively, providing a robust geological framework within which to assess the new data presented in this study. We explore previously proposed controls on mineralization within the Noranda camp and, in particular, the exceptional Au-rich Horne and Quemont deposits. We present multiple sulfur isotope and trace element compositional data for sulfide separates representing 25 different VMS deposits and “showings” within the Noranda camp. Multiple sulfur isotope data for this study have δ34SV-CDT values of between −1.9 and +2.5 ‰, and Δ33SV-CDT values of between −0.59 and −0.03 ‰. We interpret the negative Δ33S values to be due to a contribution of sulfur that originated as seawater sulfate to form the ore sulfides of the Noranda camp VMS deposits. The contribution of seawater sulfate increased with the collapse and subsequent evolution of the Noranda caldera, an inference supported by select trace and major element analyses. In particular, higher concentrations of Se occur in samples with Δ33S values closer to 0 ‰, as well as lower Fe/Zn ratios in sphalerite, suggesting lower pressures and temperatures of formation. We also report a relationship between average Au grade and Δ33S values within Au-rich VMS deposits of the Noranda camp, whereby higher gold grades are associated with near-zero Δ33S values. From this, we infer a dominance of igneous sulfur in the gold-rich deposits, either leached from the volcanic pile and/or directly degassed from an associated intrusion.

In situ Sr isotope analysis of apatite by LA-MC-ICPMS: constraints on the evolution of ore fluids of the Yinachang Fe-Cu-REE deposit, Southwest China

Abstract: Apatite is a ubiquitous accessory mineral in a variety of rocks and hydrothermal ores. Strontium isotopes of apatite are well known to retain petrogenetic information and have been widely used to investigate the origin of igneous rocks, but such attempts have rarely been made to constrain ore-forming processes of hydrothermal systems. We here report in situ LA-MC-ICPMS Sr isotope data of apatite from the ~1660-Ma Yinachang Fe-Cu-REE deposit, Southwest China. The formation of this deposit was coeval to the emplacement of regionally distributed doleritic intrusions within a continental-rift setting. The deposit has a paragenetic sequence consisting of sodic alteration (stage I), magnetite mineralization (stage II), Cu sulfide and REE mineralization (stage III), and final barren calcite veining (stage IV). The stage II and III assemblages contain abundant apatite, allowing to investigate the temporal evolution of the Sr isotopic composition of the ore fluids. Apatite of stage II (Apt II) is associated with fluorite, magnetite, and siderite, whereas apatite from stage III (Apt III) occurs intimately intergrown with ankerite and Cu sulfides. Apt II has 87Sr/86Sr ratios varying from 0.70377 to 0.71074, broadly compatible with the coeval doleritic intrusions (0.70592 to 0.70692), indicating that ore-forming fluids responsible for stage II magnetite mineralization were largely equilibrated with mantle-derived mafic rocks. In contrast, Apt III has distinctly higher 87Sr/86Sr ratios from 0.71021 to 0.72114, which are interpreted to reflect external radiogenic Sr, likely derived from the Paleoproterozoic strata. Some Apt III crystals have undergone extensive metasomatism indicated by abundant monazite inclusions. The metasomatized apatite has much higher 87Sr/86Sr ratios up to 0.73721, which is consistent with bulk-rock Rb-Sr isotope analyses of Cu ores with 87Sr/86Sri from 0.71906 to 0.74632. The elevated 87Sr/86Sr values of metasomatized apatite and bulk Cu ores indicate that later fluids were dominated by highly radiogenic Sr equilibrated with the Paleoproterozoic country rocks. Results of this study highlight the utilization of in situ Sr isotope analysis of apatite in unraveling the evolution of hydrothermal systems.

Formation of albitite-hosted uranium within IOCG systems: the Southern Breccia, Great Bear magmatic zone, Northwest Territories, Canada

Abstract: Uranium and polymetallic U mineralization hosted within brecciated albitites occurs one kilometer south of the magnetite-rich Au–Co–Bi–Cu NICO deposit in the southern Great Bear magmatic zone (GBMZ), Canada. Concentrations up to 1 wt% U are distributed throughout a 3 by 0.5 km albitization corridor defined as the Southern Breccia zone. Two distinct U mineralization events are observed. Primary uraninite precipitated with or without pyrite–chalcopyrite ± molybdenite within magnetite–ilmenite–biotite–K-feldspar-altered breccias during high-temperature potassic–iron alteration. Subsequently, pitchblende precipitated in earthy hematite–specular hematite–chlorite veins associated with a low-temperature iron–magnesium alteration. The uraninite-bearing mineralization postdates sodic (albite) and more localized high-temperature potassic–iron (biotite–magnetite ± K-feldspar) alteration yet predates potassic (K-feldspar), boron (tourmaline) and potassic–iron–magnesium (hematite ± K-feldspar ± chlorite) alteration. The Southern Breccia zone shares attributes of the Valhalla (Australia) and Lagoa Real (Brazil) albitite-hosted U deposits but contains greater iron oxide contents and lower contents of riebeckite and carbonates. Potassium, Ni, and Th are also enriched whereas Zr and Sr are depleted with respect to the aforementioned albitite-hosted U deposits. Field relationships, geochemical signatures and available U–Pb dates on pre-, syn- and post-mineralization intrusions place the development of the Southern Breccia and the NICO deposit as part of a single iron oxide alkali-altered (IOAA) system. In addition, this case example illustrates that albitite-hosted U deposits can form in albitization zones that predate base and precious metal ore zones in a single IOAA system and become traps for U and multiple metals once the tectonic regime favors fluid mixing and oxidation-reduction reactions.

The early Cambrian Chahmir shale-hosted Zn–Pb deposit, Central Iran: an example of vent-proximal SEDEX mineralization

Abstract: The Chahmir zinc–lead deposit (1.5 Mt @ 6 % Zn + 2 % Pb) in Central Iran is one among several sedimentary-exhalative Zn–Pb deposits in the Early Cambrian Zarigan–Chahmir basin (e.g., Koushk, Darreh-Dehu, and Zarigan). The deposit is hosted by carbonaceous, fine-grained black siltstones, and shales interlayered with volcaniclastic sandstone beds. It corresponds to the upper part of the Early Cambrian volcano-sedimentary sequence (ECVSS), which was deposited on the Posht-e-Badam Block during back-arc rifting of the continental margin of Central Iran. Based on crosscutting relationships, mineralogy, and texture of sulfide mineralization, four different facies can be distinguished: stockwork (feeder zone), massive ore, bedded ore, and distal facies (exhalites with barite). Silicification, carbonatization, sericitization, and chloritization are the main wall-rock alteration styles; alteration intensity increases toward the proximal feeder zone. Fluid inclusion microthermometry was carried out on quartz associated with sulfides of the massive ore. Homogenization temperatures are in the range of 170–226 °C, and salinity is around 9 wt% NaCl eq. The size distribution of pyrite framboids of the bedded ore facies suggests anoxic to locally suboxic event for the host basin. δ34S(V-CDT) values of pyrite, sphalerite, and galena range from +10.9 to +29.8 ‰. The highest δ34S values correspond to the bedded ore (+28.6 to +29.8 ‰), and the lowest to the massive ore (+10.9 to +14.7 ‰) and the feeder zone (+11.3 and +12.1 ‰). The overall range of δ34S is consistent with a sedimentary environment where sulfide sulfur was derived from two sources. One of them was corresponding to early ore-stage sulfides in bedded ore and distal facies, consistent with bacterial reduction from coeval seawater sulfate in a closed or semiclosed basin. However, the δ34S values of late ore-stage sulfides, observed mainly in massive ore, interpreted as a hydrothermal sulfur component, leached from the lower part of the ECVSS. Sulfur isotopes, along with the sedimentological, textural, mineralogical, fluid inclusion, and geochemical characteristics of the Chahmir deposit are in agreement with a vent-proximal (Selwyn type) SEDEX ore deposit model.

Zn and Pb mobility during metamorphism of sedimentary rocks and potential implications for some base metal deposits

Abstract: Comprehension of the genesis of Pb-Zn ore systems is currently limited by a poor understanding of where these metals are sourced from. Our study of metal mobility during regional metamorphism in the Mt. Lofty Ranges, South Australia, demonstrates that in staurolite-absent siliciclastic metasedimentary rocks, biotite contains >80 % of the bulk rock Zn, as well as a considerable proportion of the total Pb. Fluid flow through these metasedimentary rocks led to a continuous depletion of Pb and Zn on a mineral and bulk rock scale during prograde regional metamorphism. We calculate that ∼80 % of the bulk rock Zn and ∼50 % of the bulk rock Pb were mobilised, mainly through reactions involving biotite. These reactions led to a calculated Pb and Zn “loss” of ∼2.7 and 27 Mt, respectively, in the high-grade metamorphic zone. Halogen contents of apatite and biotite and bulk rock Zn isotope data provide evidence that Cl-rich metamorphic fluids were important for metal transport. Hence, fluid flow accompanying prograde metamorphism of typical sedimentary rocks can mobilise base metals to the degree required to potentially supply significant Pb-Zn ore systems.

Timing of porphyry (Cu-Mo) and base metal (Zn-Pb-Ag-Cu) mineralisation in a magmatic-hydrothermal system—Morococha district, Peru

Abstract: The Morococha district in central Peru is characterised by economically important Cordilleran polymetallic (Zn-Pb-Ag-Cu) vein and replacement bodies and the large Toromocho porphyry Cu-Mo deposit in its centre. U-Pb, Re-Os, and 40Ar/39Ar geochronology data for various porphyry-related hydrothermal mineralisation styles record a 3.5-Ma multi-stage history of magmatic-hydrothermal activity in the district. In the late Miocene, three individual magmatic-hydrothermal centres were active: the Codiciada, Toromocho, and Ticlio centres, each separated in time and space. The Codiciada centre is the oldest magmatic-hydrothermal system in the district and consists of a composite porphyry stock associated with anhydrous skarn and quartz-molybdenite veins. The hydrothermal events are recorded by a titanite U-Pb age at 9.3 ± 0.2 Ma and a molybdenite Re-Os age at 9.26 ± 0.03 Ma. These ages are indistinguishable from zircon U-Pb ages for porphyry intrusions of the composite stock and indicate a time span of 0.2 Ma for magmatic-hydrothermal activity. The small Ticlio magmatic-hydrothermal centre in the west of the district has a maximum duration of 0.3 Ma, ranging from porphyry emplacement to porphyry mineralisation at 8.04 ± 0.14 Ma (40Ar/39Ar muscovite cooling age). The Toromocho magmatic-hydrothermal centre has a minimum of five recorded porphyry intrusions that span a total of 1.3 Ma and is responsible for the formation of the giant Toromocho Cu-Mo deposit. At least two hydrothermal pulses are identified. Post-dating a first pulse of molybdenite mineralisation, wide-spread hydrous skarn covers an area of over 6 km2 and is recorded by five 40Ar/39Ar cooling ages at 7.2–6.8 Ma. These ages mark the end of the slowly cooling and long-lived Toromocho magmatic-hydrothermal centre soon after last magmatic activity at 7.26 ± 0.02 Ma. District-wide (50 km2) Cordilleran base metal vein and replacement bodies post-date the youngest recorded porphyry mineralisation event at Toromocho by more than 0.5 Ma. Polymetallic veins (5.78 ± 0.10 and 5.72 ± 0.18 Ma; 40Ar/39Ar ages) and the Manto Italia polymetallic replacement bodies (6.23 ± 0.12 and 6.0 ± 0.2 Ma; 40Ar/39Ar ages) are interpreted to have been formed by a single hydrothermal pulse. Hydrothermal activity ceased after the formation of the base metal vein and replacement bodies. Overlapping monazite U-Pb (8.26 ± 0.18 Ma) and muscovite 40Ar/39Ar ages (8.1 ± 0.5 Ma) from the early base metal stage of one Cordilleran vein sample in the Sulfurosa area provide evidence that a discrete hydrothermal pulse was responsible for polymetallic vein formation 2.6 Ma prior to the district-wide polymetallic veins. These ages pre-date those of Toromocho porphyry Cu-Mo formation and show that Zn-Pb-Ag-Cu mineralisation formed during several discrete magmatic-hydrothermal pulses in the same district.

Different carbon reservoirs of auriferous fluids in African Archean and Proterozoic gold deposits? Constraints from stable carbon isotopic compositions of quartz-hosted CO 2 -rich fluid inclusions

Abstract: Stable carbon (and when present, nitrogen) isotope ratios of fluid inclusions in quartz from selected gold deposits in Ghana and Zimbabwe have been analyzed using a crushing device interfaced to an isotopic ratio mass spectrometer (IRMS) in order to constrain possible sources of the auriferous fluids. The study revealed a striking difference in stable carbon isotopic compositions of CO2 in quartz-hosted fluid inclusions from Archean and Paleoproterozoic orogenic gold deposits and points to diverse sources of CO2 in the studied deposits. Whether this finding can be generalized for other Archean and Proterozoic orogenic gold deposits worldwide remains open. However, a significant CO2 contribution by mantle degassing can be ruled out for every deposit studied. Devolatilization of greenstone belt rocks is the most likely source for CO2 in some Archean Au deposits in Zimbabwe, whereas CO2 in Proterozoic vein-type Au deposits in the West African Craton is most likely derived from Corg-bearing metasedimentary rocks. The δ13CCO2 values of high-density CO2-rich, water-poor inclusions hosted in quartz pebbles from the world-class Au-bearing conglomerate deposits at Tarkwa (Ghana) differ considerably from the δ13CCO2 values of similar high-density CO2-rich inclusions in vein quartz from the giant Ashanti deposit (Ghana) and disprove the idea of derivation of the Tarkwaian quartz (and gold?) from an older equivalent to the Ashanti vein-type gold deposit.

Characteristics of Cu isotopes from chalcopyrite-rich black smoker chimneys at Brothers volcano, Kermadec arc, and Niuatahi volcano, Lau basin

Abstract: We analysed primary chalcopyrite from modern seafloor ‘black smoker’ chimneys to investigate high-temperature hydrothermal Cu isotope fractionation unaffected by metamorphism. Samples came from nine chimneys collected from Brothers volcano, Kermadec arc, and Niuatahi volcano, Lau backarc basin. This is the first known study of Cu isotopes from submarine intraoceanic arc/backarc volcanoes, with both volcanoes discharging significant amounts of magmatic volatiles. Our results (n = 22) range from δ65Cu = −0.03 to 1.44 ± 0.18 ‰ (2 sd), with the majority of samples between ∼0.00 and 0.50 ‰. We interpret this cluster (n = 17) of lower δ65Cu values as representing a mantle source for the chimney Cu, in agreement with δ65Cu values for mantle rocks. The few higher δ65Cu values (>0.90 ‰) occur (1) within the same chimneys as lower values, (2) randomly distributed within the chimneys (i.e. near the top and bottom, interior and exterior), and (3) within chalcopyrite of approximately the same age (<1 year). This suggests the higher δ65Cu values are not related to oxidation by mixing with ambient seawater, but to isotopic variation within the vent fluids over a relatively short time. Theoretical studies demonstrate significant isotopic fractionation can occur between aqueous and vapourous complexing species. When combined with evidence for periodic release of magmatic volatiles at Brothers, we believe vapour transport of Cu is responsible for the observed isotopic fractionation. When compared to global δ65Cu data for primary chalcopyrite, volcanic arc chimneys are most similar to porphyry copper deposits that also form from magmatic-hydrothermal processes in convergent tectonic settings.

Liquid immiscibility between arsenide and sulfide melts: evidence from a LA-ICP-MS study in magmatic deposits at Serranía de Ronda (Spain)

Abstract: The chromite-Ni arsenide (Cr-Ni-As) and sulfide-graphite (S-G) deposits from the Serrania de Ronda (Malaga, South Spain) contain an arsenide assemblage (nickeline, maucherite and nickeliferous lollingite) that has been interpreted to represent an arsenide melt and a sulfide-graphite assemblage (pyrrhotite, pentlandite, chalcopyrite and graphite) that has been interpreted to represent a sulfide melt, both of which have been interpreted to have segregated as immiscible liquids from an arsenic-rich sulfide melt We have determined the platinum-group element (PGE), Au, Ag, Se, Sb, Bi and Te contents of the arsenide and sulfide assemblages using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to establish their partitioning behaviour during the immiscibility of an arsenide melt from a sulfide melt Previous experimental work has shown that PGE partition more strongly into arsenide melts than into sulfide melts and our results fit with this observation Arsenide minerals are enriched in all PGE, but especially in elements with the strongest affinity for the arsenide melt, including Ir, Rh and Pt In contrast and also in agreement with previous studies, Se and Ag partition preferentially into the sulfide assemblage The PGE-depleted nature of sulfides in the S-G deposits along with the discordant morphologies of the bodies suggest that these sulfides are not mantle sulfides, but that they represent the crystallization product of a PGE-depleted sulfide melt due to the sequestering of PGE by an arsenide melt

Mo isotope fractionation during hydrothermal evolution of porphyry Cu systems

Abstract: We present Mo isotope compositions of molybdenite types from three successive stages of ore deposition in several porphyry copper deposits of the Kerman region, Iran. The data provide new insights into controlling processes on Mo isotope fractionation during the hydrothermal evolution of porphyry systems. The Mo isotope compositions of 27 molybdenite samples show wide variations in δ97Mo ranging from −0.37 to +0.92 ‰. The data reveal that molybdenites in the early and transitional stages of mineralization (preferentially 2H polytypes; δ97Mo mean = 0.35 ‰) have higher δ97Mo values than late stage (mainly 3R polytypes; δ97Mo mean = 0.02 ‰) molybdenites. This trend suggests that fractionation of Mo isotopes occurred in high-temperature stages of mineralization and that hydrothermal systems generally evolve towards precipitation of molybdenite with lower δ97Mo values. Taking into account the genetic models proposed for porphyry Cu deposits along with the temperature-dependent fractionation of Mo isotope ratios, it is proposed that large variations of Mo isotopes in the early and the transitional stages of ore deposition could be controlled by the separation of the immiscible ore-forming fluid phases with different density, pH, and ƒO2 properties (i.e., brine and vapor). The fractionation of Mo isotopes during fluid boiling and Rayleigh distillation processes likely dominates the Mo isotope budget of the remaining ore-forming fluids for the late stage of mineralization. The lower δ97Mo values in the late stage of mineralization can be explained by depletion of the late ore-forming hydrothermal solutions in 97Mo, as these fluids have moved to considerable distance from the source. Finally, the relationship observed between MoS2 polytypes (2H and 3R) and their Mo isotopic compositions can be explained by the molecular vibration theory, in which heavier isotopes are preferentially partitioned into denser primary 2H MoS2 crystals.

Methane origin and oxygen-fugacity evolution of the Baogutu reduced porphyry Cu deposit in the West Junggar terrain, China

Abstract: Most porphyry copper deposits worldwide contain magnetite, hematite, and anhydrite in equilibrium with hypogene copper-iron sulfides (chalcopyrite, bornite) and have fluid inclusions with CO2 >> CH4 that are indicative of high fO2 In contrast, the Baogutu porphyry Cu deposit in the West Junggar terrain (Xinjiang, China) lacks hematite and anhydrite, contains abundant pyrrhotite and ilmenite in equilibrium with copper-iron sulfides (chalcopyrite), and has fluid inclusions with CH4 >> CO2 that are indicative of low fO2 The mineralized intrusive phases at Baogutu include the main-stage diorite stock and minor late-stage diorite porphyry dikes The main-stage stock underwent fractional crystallization and country-rock assimilation-contamination, and consists of dominant diorite and minor gabbro and tonalite porphyry The country rocks contain organic carbons (021–079 wt%) The δ13CvPDB values of the whole rocks (−231 to −258 ‰) in the wall rocks suggest a sedimentary organic carbon source The δ13CvPDB values of CH4 (−282 to −360 ‰) and CO2 (−68 to −200 ‰) in fluid inclusions require an organic source of external carbon and equilibration of their Δ13CCO2-CH4 values (82–250 ‰) at elevated temperatures (294–830 °C) suggesting a significant contribution of thermogenic CH4 Mineral composition data on the main-stage intrusions, such as clinopyroxene, hornblende, biotite, magnetite, ilmenite, sphene, apatite, and pyrrhotite, suggest that the primary magma at Baogutu was oxidized and became reduced after emplacement by contamination with country rocks Mineral compositions and fluid inclusion gas compositions suggest that the redox state of the system evolved from logfO2 > FMQ + 1 in the magma stage, to logfO2 FMQ in the hydrothermal stage Though oxidized magma was emplaced initially, assimilation-contamination of carbonaceous country rocks decreased its fO2 such that exsolved fluids contained abundant CH4 and deposited a reduced assemblage of minerals