Low Sulfur Fugacity Research Articles

Mineralogy and Conditions of Formation Genesis of Aggregates of Natural and Sulfide Minerals of the Poldnevskoe Demantoid Deposit (Middle Urals)

Polymineral aggregates of rounded shapes (“nodules”) composed of native and sulfide minerals of Cu, Ni, Fe, Ag, and other elements from vein magnetite–calcite–chrysotile rocks with jewelry demantoid in the Korkodinskoe hypermafic massif are described. A common feature of the six identified types of native sulfide nodules, composed of native copper, heazlewoodite, pentlandite, cuprite, and other native sulfide minerals, is their spheroidal shape, which makes them similar to individual grains of other gangue minerals (calcite, magnetite, etc.). In heazlewoodite–pentlandite nodules, specific symplectites of mercuric silver and nickel copper in heazlewoodite, as well as awaruite in Co–pentlandite, were found. The matching set of ore minerals in the host serpentinite vein mass (native copper, mercuric silver, heazlewoodite, pentlandite, awaruite) and nodules from the vein material indicates their genetic connection and the conjugation of demantoid mineralization with the evolving processes of serpentinization. It was established that the nodules formed at temperatures below 380°C under reducing conditions at very low sulfur fugacity values (10–17–10–27 bar) and oxygen (10–30 bar at 200°C to 10–21 bar at 350°C). For heazlewoodite–pentlandite nodules, such conditions persisted throughout the entire time of their formation, while, for other nodules, the reducing conditions of early parageneses were replaced by oxidative conditions in late parageneses, which is recorded by the replacement of native copper with cuprite. It is assumed that the features of the morphology and structure of native sulfide nodules and the presence of symplectite intergrowths of ore minerals in them are associated with specific conditions created during the decompression of the crust-mantle mixture rising to the surface in the fault zone. The source of the metals was a deep, high-temperature fluid interacting with mafic and ultramafic rocks under reducing conditions at a low water-to-rock ratio.

Sphalerite and Pyrite Geochemistry from the Pusangguo Co-Rich Cu–Zn–Pb Skarn Deposit, Tibet: Implications for Element Occurrence and Mineralization

The Pusangguo deposit (1.42 Mt @ 1.42% Cu, 0.14 Mt @ 1.82% Zn, 0.08 Mt @ 1.01% Pb, and 285.8 t Co @ 140 g/t Co) is the first Co-rich Cu-Zn-Pb skarn deposit discovered in the Gangdese metallogenic belt. However, the trace and minor element geochemistry of the sulfides in this deposit has not been studied, limiting further understanding of elements’ occurrence and mineralization. Here, we identified four ore stages, and two types of sphalerites (SpI and SpII) and pyrites (PyI and PyII), in this deposit. In this study, LA-ICP-MS in-situ trace element analyses were conducted on sphalerite and pyrite, to obtain their chemical compositions, elemental substitution mechanisms, and mineralization physicochemical conditions. The results indicate that two types of sphalerites are generally more enriched with Co than pyrite. SpI has higher concentrations of Co, Cr, Cu, Ag, and As compared to SpII. Both types of sphalerite have very low contents of Sn, Ge, and Ga. PyII has higher contents of most trace elements, such as Co, Ni, Mn, Zn, Cu, As, Sn, Se, Pb, Ag, and Bi, compared to PyI. Both types of pyrite are poor in Mn, Ga, Ge, and Cd, but enriched in As, Co, and Ni. The Mn, Fe, Co, and Cd in sphalerite, and Co, Ni, and Mn in pyrite are generally lattice-bound, while Cu, As, Ag, and Sb are usually present in both micro-inclusions and coupled substitution. Significant elemental correlations in sphalerite indicate the possible substitution mechanisms 2Fe2+ + Ga2+ ↔ 3Zn2+, 2Fe2+ + Ge4+ ↔ 4Zn2+, and (Sb3+, Sn3+) + (Cu+, Ag+) ↔ 2Zn2+. The correlation trends between trace elements in pyrite suggest the coupled substitution mechanisms of (Tl+ + Cu+ + Ag+) + (As3+ + Sb3+) ↔ 2Zn2+ and As3+ + Cu+ ↔ 2Zn2+. The mineralization temperature at Pusangguo, as determined by the GGIMFis sphalerite geothermometer, is 237–345 °C (avg. 307 °C), consistent with the high Zn/Cd ratio (avg. 203), low Ga/In (avg. 0.06), and high In/Ge (avg. 15.9) in sphalerite, and high Co/Ni ratio (avg. 24) in pyrite. These results indicate that the ore-forming fluid was high-temperature, with a low sulfur fugacity (fS2) (10−13.4 to 10−8.3) and low oxygen fugacity (fO2). The high temperature, and low sulfur fugacity and oxygen fugacity of the ore-forming fluid, and the fluid-mixing process, jointly controlled the sulfide precipitation, which caused the formation of the Pusangguo deposit.

In-situ trace element and S isotope compositions of sulfides from the Tonggou Cu deposit, Eastern Tianshan Orogen: Implication for ore-forming process

The Tonggou deposit is located in the western Bogda Orogen, the northern margin of Eastern Tianshan. Porphyry Cu and vein-type Cu-Zn mineralization are both developed in the Tonggou orefield. This well-preserved porphyry-vein system, therefore offers an excellent opportunity to evaluate the genetic relationship and ore-forming process between porphyry Cu and vein-type Cu-Zn mineralization. Porphyry mineralization consists of magnetite-quartz (P1) and chalcopyrite-quartz (P2) stages, whilst vein-type mineralization includes pyrite-quartz (V1), polymetallic sulfide-quartz (V2), and pyrite-calcite (V3) stages. Four pyrite generations are identified, including porphyry Cu ore-related (Py1) and vein-type Cu-Zn ore-related (Py2, Py3, and Py4) ones. In most cases, Py1 (stage P2) occurs as anhedral grains, Py2 (stage V1) as fine anhedral grains, Py3 (stage V2) as euhedral-subhedral grains, and Py4 (stage V3) as anhedral aggregates. The Sb content first decreases from Py1 to Py2, then increases from Py2 to Py3, and finally decreases from Py3 to Py4. Cobalt and Ni contents show opposite trend to that of Sb. The relatively high Sb but low Co-Ni contents in Py1 and Py3 indicate that the main ore stage (P2 and V2) environment was high oxygen fugacity and low sulfur fugacity. Moreover, the contents of Cu (Py1: avg. 4108 ppm, Py2: avg. 735 ppm, Py3: avg. 284 ppm, Py4: avg. 149 ppm) and Ag (Py1: avg. 204 ppm, Py2: avg. 11.1 ppm, Py3: avg. 6.29 ppm, Py4: avg. 2.98 ppm) gradually decrease from Py1 to Py4, indicating that the initial ore fluid was Cu-Ag rich. The δ34S values obtained are 4.6–6.8 ‰ (avg. 5.9 ‰; Py2), 5.7–7.4 ‰ (avg. 6.6 ‰; Py3), and 7.3–8.8 ‰ (avg. 8.1 ‰; Py4), which indicate that the sulfur of vein-type mineralization was initially derived from felsic magma, and subsequently mixed with strata sulfur. Furthermore, the similar δ34S range of Ccp1 (4.7–5.3 ‰) from porphyry ore and Ccp2 (3.3–4.8 ‰) from vein-type ore indicates that they have a similar felsic-magma-derived sulfur source. Combining with principal component analysis (PCA) of pyrite trace elements and REE patterns, the different pyrite generations can be divided into porphyry pyrite and vein-type pyrite, albeit with similar fluid source.Consequently, we suggest that the Tonggou porphyry and vein-type ores have similar ore-fluid source, and may belong to the same porphyry mineralization system with different ore-forming processes. Redox change may have played an important role in the metal enrichment and precipitation at Tonggou.

Chromitites from the Vavdos ophiolite (Chalkidiki, Greece): Petrogenesis and geotectonic settings; constrains from spinel, olivine composition, PGE mineralogy and geochemistry

Podiform chromitites of the Vavdos ophiolite are associated with dunite bodies hosted within residual mantle harzburgite. The mantle source is highly depleted and apparently developed at fore-arc settings as inferred from the presence of boninitic melts. The ascendance of these melts through dunite pathways and their subsequent interaction with the depleted harzburgite instigated chromitite mineralization within dunite. Spinel occurs in a) massive accumulations (Mg# = 0.56–0.68 and Cr# = 0.69–0.77), b) in non-massive disseminated textures (Mg# = 0.52–0.61 and Cr# = 0.80–0.81) and c) in schlieren chromitite deposits within dunite bodies (Mg# = 0.43–0.48 and Cr# = 0.79–0.82). Olivine is the main silicate inclusion in chromitites, whereas the absence of hydrous mineral phases is linked with crystallization at relatively deeper parts of the mantle.Olivine inclusions in spinel are enriched in Fo, Ni and Ca compared to the interstitial olivine grains. No compositional gaps were noted between the mineral analyses of chromitites and dunites; the calculated parental melts are also homogeneous, reinforcing the view for production of melt batches by similar processes. Platinum-group elements (PGE) concentrations and platinum-group minerals (PGM) demonstrate high (Os + Ir + Ru)/ (Rh + Pt + Pd) ratios, typical of many Tethyan ophiolitic chromitites. The main PGM phase is laurite that forms an extended solid solution series with substitution of Ru for Os + Ir, linked to the cooling of the system. Massive chromitites contain Ru-rich laurite and Os-Ir alloys whereas non-massive ores contain Ru-poor laurite. The PGM were included in fresh magnesiochromite favoring their magmatic origin and initial entrapment at ~ 1200 °C and low sulfur fugacity.

Mineralogical and geochemical characteristics and ore-forming mechanism of hydrothermal sediments in the middle and southern Okinawa Trough

In this study, analysis of mineralogy and geochemistry of fifteen surface sediment samples from four hydrothermal fields in the middle and southern Okinawa Trough is presented. Sediment samples can be divided into two types: (1) Metalliferous group (MS group) containing large amounts of sulfides and sulfate are enriched in Pb, Zn, Cu, Fe, Au, Ag, Ba, As, Hg, Se, Sb, Mo, slightly enriched in U, V, and have rare earth elements (REE) patterns similar to those of hydrothermal vent fluids and sulfides, indicating formation by a complex combination of wasting and erosion primary massive sulfide deposits and plume fallout of sulfides and sulfate. The occurrence of sphalerite with high contents of Fe and of co-existing pyrrhotite as major Fe-sulfide indicate that these minerals are formed at high temperature with low-sulfur fugacity in the Clam and Jade hydrothermal sites. The enrichment of Pb, Zn, Cu, Au, Ag, Ba, As, Hg, Se, Sb, and Mo is related to the presence of galena, sphalerite, Cu-sulfides, stibnite, and realgar in the sediments. (2) Non-metalliferous group (non-MS group) containing small amounts of pyrite are slightly enriched in Pb, Zn, Cu, Au, Ag, As, Hg, Se, Sb, and Mo relative to the East China Sea (ECS) continental shelf sediments, with REE exhibiting a “shale-like flat” patterns and close to Taiwan sediments in a REE discrimination plot, indicating formation by a large amount of terrigenous materials originated from Taiwan Island and a small amount of particles of sulfides in the hydrothermal nonbuoyant plume. The slight enrichment of Pb, Zn, Cu, Au, Ag, As, Hg, Se, Sb, and Mo maybe caused by co-precipitation with the fine-grained sulfides settling from the nonbuoyant plume. The positive relationships between the HREE fractionation index (Gd/Yb)N, heavy mineral proportion, and δEu suggest that (Gd/Yb)N can be used as a sensitive indicator for reflecting the effect of hydrothermal activity on the sediments.

Mineralogy, geochemistry, and Sr–Pb and in situ S isotopic compositions of hydrothermal precipitates from the Tangyin hydrothermal field, southern Okinawa Trough: Evaluation of the contribution of magmatic fluids and sediments to hydrothermal systems

The Okinawa Trough (OT), is a back-arc basin where rifting is in its initial stages, and where hydrothermal fields (HTFs) have developed above terrigenous sediments and volcaniclastics layers of variable thickness. The source of ore-forming materials within the hydrothermal systems of the OT is complex, and may include seawater, magmatic rock, magmatic fluid, and sediments. The contribution of magmatic fluids and sediments to the hydrothermal systems of the OT is still uncertain. At least 10 modern seafloor HTFs have been reported from the OT, including the Jade, Hakurei, Iheya North Knoll, CLAM, Tangyin, Yonaguni Knoll IV, Hatoma, and Irabu. Here, we report results of mineralogical, geochemical, and whole-rock Sr–Pb and in situ S isotopic analyses of hydrothermal precipitates from the Tangyin HTF in the southern OT. This field was discovered during the HOBAB3 cruise in 2014 and contains four types of hydrothermal precipitates: Fe–Zn- and Zn–Pb-rich massive sulfides, silica-rich precipitates, and native sulfur-rich samples. Sulfates in Zn–Pb-, Fe–Zn-, and silica-rich samples have 87Sr/86Sr ratios of 0.71068–0.71750, which are higher than those of modern seawater and magmatic rocks, and indicate that the Sr is derived mainly from seawater and sediments. The Zn–Pb-, Fe–Zn-, and silica-rich samples have 206Pb/204Pb (18.5129–18.5307), 207Pb/204Pb (15.6374–15.6489), and 208Pb/204Pb (38.8759–38.9113) ratios that are intermediate between sediments and magmatic rocks, suggesting the Pb is a mixture of sediment- and magmatic-rock-derived Pb. The in situ S isotopic data indicate that the ore-forming fluids associated with all sample types had low δ34S values, with the Fe–Zn- (mean δ34S = 1.27‰ ± 1.38‰; SD, n = 12) and Zn–Pb-rich (mean δ34S = 0.93‰ ± 0.32‰; n = 36) massive sulfides, and silica-rich precipitates (mean δ34S = 2.85‰ ± 1.14‰; n = 58) having lower δ34S values than those of native sulfur-rich samples (mean δ34S = 4.23‰ ± 0.72‰; n = 5). The Zn–Pb- and Fe–Zn-rich massive sulfides, and silica-rich precipitates represent the medium–high temperature (>120 °C) early–middle stages of mineralization, formed under reducing conditions at low sulfur fugacity (fS2), based on the occurrence of low-fS2 mineral assemblages (e.g. high-Fe sphalerite ± isocubanite ± pyrrhotite). The low δ34S values of these three types of samples, and of the Hakurei and CLAM HTFs with low-fS2 mineral assemblages and low concentrations of Cu, Bi, and Au, most likely reflect the incorporation of biological sulfur previously formed by MSR within the sediments. The native sulfur-rich samples of the Tangyin HTF formed at low temperatures (<120 °C), and have slightly higher δ34S values than the Fe–Zn-, Zn–Pb-, and silica-rich samples, may reflect the incorporation of biological sulfur formed mainly by MSR during hydrothermal flow in a seawater recharge zone. Some sulfates in the Jade and Iheya North Knoll HTFs have δ34S values lower than that of modern seawater; this, in combination with high-fS2 oxidizing hydrothermal fluids, and high concentrations of Cu and Au in sulfide samples, indicates a magmatic fluid contribution to these HTFs.

Mineralogy and mineral chemistry related to the Au mineralization in the Dunde Fe-Zn deposit, western Tianshan

The Dunde deposit, which is located in the Awulale Fe metallogenic belt, Xinjiang, China, is a uniquely large Fe-Zn deposit and also contains considerable Au resources. We present the results of a comprehensive study of this deposit based on fieldwork, mineralogy, and mineral geochemistry to constrain the Au mineralization. Our study indicates that the mineralization of Fe, Zn, and at least some Au precipitated in the same hydrothermal system. The Au mineralization, which occurs in the sulfide stage, is characterized by invisible Au at high temperatures and by electrum at low temperatures. Unusual sulfide is recognized in the deposit, such as cobaltite and bismuth minerals. Pyrrhotite compositions indicate that the early precipitation of sulfide generated in high temperatures (410–570 ℃) and high sulfur fugacities (−2.47 to −6.68). Arsenopyrite compositions indicate that the late precipitation of sulfide formed in low temperatures (326–393 ℃) and low sulfur fugacities (−7.41 to −12.00). Bismuth minerals exhibit a close relationship with the Au mineralization. In high temperatures and high oxygen fugacities, bismuth dissolved in the hydrothermal fluid as Bi3+ and precipitated as galenobismuthite and cosalite accompanying precipitation of the invisible Au. Accompanying the decrease of oxygen fugacities and temperatures, the Bi melt directly separated from the hydrothermal fluid and scavenged Au. The reduced condition further enriched Au in Bi melts through the decomposition of the invisible Au-bearing Bi-Pb sulfide. The decrease of temperatures caused the decomposition of the Bi-Au melt and the formation of native bismuth, bismuthinite, and electrum. In addition to the mineralization of Fe, Zn and Au, considerable Cu-, Co-, and Mn-bearing sulfide precipitated in the Zn ores. Integrated utilization of the Zn ores may get greater economic values.

Carbonate hosted intermetallic compounds in Paleoproterozoic Salumber Ghatol metallogenic belt, Aravalli Craton, Rajasthan, India

Carbonate hosted intermetallic compound in the Umarvaniyan area is localized within the intensively sheared (mylonitised) dolomite in a NW–SE shear zone (~15 km), belongs to Salumber Ghatol metallogenic belt, in Debari Group of Aravalli Craton, Rajasthan, India. It is characterized by extensive silicification and ferruginisation with hematite, goethite, magnetite and native gold specks. The intermetallic compound within the dolomite is composed of varying proportion of Cu–Zn–Ni–Os–Fe which has been detected by electron probe microanalysis (EPMA) study. The EMPA (WDS) results of the intermetallic compounds also reveal occurrences of intermetallic compounds of Cu–Zn–Ni–Os–Fe and native Au. The occurrence of these non-separable compounds is probably because these metals were formed at very high temperatures and in reducing condition during the evolving shear with low oxygen and low sulfur fugacity. The fast cooling effect thereafter probably made the geochemical environment least conducive for reaction between Cu/Zn/Ni and sulphur or oxygen.

Organic matters and acid‐sulfate alteration in Itomuka mercury mine, Hokkaido, Japan: Implications for the transportation and deposition mechanisms of Hg

AbstractIn order to examine the transportation and deposition mechanisms of Hg, we investigated the ore and hydrothermal alteration minerals and solid organic matters from Itomuka mercury mine located in the eastern part of central Hokkaido. In addition to the ore minerals, native mercury and cinnabar, quartz, marcasite, alunite, kaolinite, and minor amounts of pyrite and smectite were identified in the Hg ore by powder X‐ray diffraction (XRD) analysis. This mineral assemblage of acid sulfate alteration was likely developed under the conditions of low temperature (≤100°C) and low pH (≤2) in the steam‐heated environment. The H2SO4 was produced above the water table by the oxidation of H2S separated from deep, near‐neutral fluids by boiling. The dominance of native mercury over cinnabar in Hg ore indicates that the greater part of mineralized Hg was transported as Hg0 in aqueous solution and vapor with low sulfur fugacity. The solid organic matters found in the Hg ore were analyzed with SEM‐EDS, micro‐XRD, and micro‐Fourier transform infrared (FTIR) spectroscopy, and these results suggest that the organic matters contributed to keeping the low fO2 of the Hg‐bearing fluid and transportation of Hg as Hg0 in S‐poor condition. Because the solubility of Hg in acidic fluid is low, neutral to alkaline fluid seems to have leached Hg from the basement sedimentary rocks of Hidaka Group which also supplied the organic matters to the fluid. The oxidation and cooling of Hg‐bearing solution and vapor triggered the deposition of liquid Hg as a primary phase.

Carbonation of ophiolitic ultramafic rocks: Listvenite formation in the Late Cretaceous ophiolites of eastern Iran

Late Cretaceous mantle peridotite of the Birjand ophiolite (eastern Iran) contains variably serpentinized and carbonated/listvenitized rocks. Transformation from harzburgite protolith to final listvenite (quartz + magnesite/± dolomite + relict Cr-spinel) reflects successive fluid-driven reactions, the products of which are preserved in outcrop. Transformation of harzburgite to listvenite starts with lizardite serpentinization, followed by contemporaneous carbonation and antigorite serpentinization, antigorite-talc-magnesite alteration, finally producing listvenite where alteration is most pervasive. The spectrum of listvenitic assemblages includes silica-carbonate, carbonate and silica listvenites with the latter (also known as birbirite) being the youngest, based on crosscutting relationships. The petrological observations and mineral assemblages suggest hydrothermal fluids responsible for the lizardite serpentinization had low aCO2, oxygen and sulfur fugacities, distinct from those causing antigorite serpentinization and carbonation/listvenitization, which had higher aCO2, aSiO2, and oxygen and sulfur fugacities. The carbonate and silica listvenite end-members indicate variations in aSiO2 and aCO2 of the percolating hydrothermal fluids, most likely driven by local variations in pH and temperature.Beyond the addition of H2O, serpentinization did not significantly redistribute major elements. Progressive infiltration of CO2-rich fluids and consequent carbonation segregated Mg into carbonate and Si into silica listvenites. Trace element mobility resulted in different enrichments of fluid-mobile, high field strength, and light rare earth elements in listvenites, indicating a “listvenite mobility sequence”.The δ13C, δ18O and 87Sr/86Sr values of magnesite and dolomite in carbonated lithologies and veins point to sedimentary carbonate as the main C source. Fluid-mobile element (e.g., As and Sb) patterns in carbonated lithologies are consistent with contribution of subducted sediments in a forearc setting, suggesting sediment-derived fluids. Such fluids were produced by expulsion of pore fluids and release of structurally bound fluid from carbonate-bearing sediments in the Sistan Suture Zone (SsSZ) accretionary complex at shallow parts of mantle wedge. The CO2-bearing fluids migrated up along the slab-mantle interface and circulated through the suture zone faults to be sequestered in mantle peridotites with marked element mobility signatures.

Origin of Platinum Group Minerals (PGM) Inclusions in Chromite Deposits of the Urals

This paper reviews a database of about 1500 published and 1000 unpublished microprobe analyses of platinum-group minerals (PGM) from chromite deposits associated with ophiolites and Alaskan-type complexes of the Urals. Composition, texture, and paragenesis of unaltered PGM enclosed in fresh chromitite of the ophiolites indicate that the PGM formed by a sequence of crystallization events before, during, and probably after primary chromite precipitation. The most important controlling factors are sulfur fugacity and temperature. Laurite and Os–Ir–Ru alloys are pristine liquidus phases crystallized at high temperature and low sulfur fugacity: they were trapped in the chromite as solid particles. Oxygen thermobarometry supports that several chromitites underwent compositional equilibration down to 700 °C involving increase of the Fe3/Fe2 ratio. These chromitites contain a great number of PGM including—besides laurite and alloys—erlichmanite, Ir–Ni–sulfides, and Ir–Ru sulfarsenides formed by increasing sulfur fugacity. Correlation with chromite composition suggests that the latest stage of PGM crystallization might have occurred in the subsolidus. If platinum-group elements (PGE) were still present in solid chromite as dispersed atomic clusters, they could easily convert into discrete PGM inclusions splitting off the chromite during its re-crystallization under slow cooling-rate. The presence of primary PGM inclusions in fresh chromitite of the Alaskan-type complexes is restricted to ore bodies crystallized in equilibrium with the host dunite. The predominance of Pt–Fe alloys over sulfides is a strong indication for low sulfur fugacity, thereby early crystallization of laurite is observed only in one deposit. In most cases, Pt–Fe alloys crystallized and were trapped in chromite between 1300 and 1050 °C. On-cooling equilibration to ~900 °C may produce lamellar unmixing of different Pt–Fe phases and osmium. Precipitation of the Pt–Fe alloys locally is followed by an increase of sulfur fugacity leading to crystallize erlichmanite and Ir–Rh–Ni–Cu sulfides, occurring as epitaxic overgrowth on the alloy. There is evidence that the system moved quickly into the stabilization field of Pt–Fe alloys by an increase of the oxygen fugacity marked by an increase of the magnetite component in the chromite. In summary, the data support that most of the primary PGM inclusions in the chromitites of the Urals formed in situ, as part of the chromite precipitation event. However, in certain ophiolitic chromitites undergoing annealing conditions, there is evidence for subsolidus crystallization of discrete PGM from PGE atomic-clusters occurring in the chromite. This mechanism of formation does not require a true solid solution of PGE in the chromite structure.

The hydrothermal Waterberg platinum deposit, Mookgophong (Naboomspruit), South Africa. Part 1: Geochemistry and ore mineralogy

ABSTRACTThe Waterberg platinum deposit is an extraordinary example of a vein-type hydrothermal quartz-hematite-PGE (platinum-group element) mineralization. This study concentrates on the geochemical character of the ores and the platinum-group mineral (PGM) assemblage by application of reflected-light and scanning electron microscopy followed by electron probe microanalysis.The PGM-bearing quartz veins show multiple banding indicating numerous pulses of fluid infiltration. Mineralization was introduced contemporaneously with the earliest generation of vein quartz and hematite. High oxygen and low sulfur fugacities of the mineralizing fluids are indicated by hematite as the predominant opaque mineral and the lack of sulfides.The ‘Waterberg type’ mineralization is characterized by unique metal proportions, namely Pt&gt;Pd&gt;Au, interpreted as a fingerprint to the cradle of the metals, namely rocks and ores of the Bushveld Complex, or reflecting metal fractionation during ascent of an oxidized, evolving fluid. The PGM assemblage signifies three main depositional and alteration events. (1) Deposition of native Pt and Pt–Pd alloys (&gt;90% of the PGM assemblage) and Pd–Sb–As compounds (Pt-rich isomertieite and mertieite II) from hydrothermal fluids. (2) Hydrothermal alteration of Pt by Cu-rich fluids and formation of Pt–Cu alloys and hongshiite [PtCu]. (3) Weathering/oxidation of the ores producing Pd/Pt-oxides/hydroxides.Platinum-group element transport was probably by chloride complexes in moderately acidic and strongly oxidizing fluids of relatively low salinity, and depositional temperatures were in the range 400–200°C. Alternatively, quartz and ore textures may hint to noble metal transport in a colloidal form and deposition as gels.The source of the PGE is probably in platiniferous rocks or ores of the Bushveld Complex which were leached by hydrothermal solutions. If so, further Waterberg-type deposits may be present, and a prime target area would be along the corridor of the Thabazimbi-Murchison-Lineament where geothermal springs are presently still active.

Compounds of Ru–Se–S, alloys of Os–Ir, framboidal Ru nanophases, and laurite–clinochlore intergrowths in the Pados-Tundra complex, Kola Peninsula, Russia

This is the first report of occurrences of platinum-group minerals (PGM) and unnamed phases of platinum-group elements (PGE) from the Pados-Tundra ultramafic complex of Paleoproterozoic age, Kola Peninsula. The PGM occur as individual inclusions or intergrowths (≤1–10 μm) hosted by cores of zoned grains of chromite–magnesiochromite (Chr) in chromitite of the Dunite block in the eastern portion of the complex. Osmium-poor laurite is abundant, commonly as intimate intergrowths with clinochlore (>80% laurite grains), followed by Os- and Ir-dominant alloys (i.e., the minerals osmium and iridium, respectively); unnamed phases of ruthenium selenide [RuSe 2 ] and rhodium telluride [RhTe] are rare. Selenium-rich laurite, not reported previously, likely belongs to an inferred RuS 2 –RuSe 2 series. Native Ru forms micro-(nano)-spherules in framboid-type aggregates, in association with relict laurite, which are surrounded by a skeletal grain of clinochlore; the host is Chr. The framboidal texture of nanophases of ruthenium, hitherto unreported in PGE alloys, likely formed as a result of deposition from a H 2 -bearing fluid, involving a reaction of desulfurisation and reduction of laurite. A highly S-deficient environment is indicated; a low sulfur fugacity, presumably below the Os–OsS 2 buffer, caused the observed coexistence of a low-Os variety of laurite with Os-dominant alloy. An ultimate loss of total S is implied to have decreased the S/Se ratio, which resulted in the precipitation of Ru-dominant sulfoselenide–selenide phases, likely from an oxidizing fluid at a late stage of mineralization. We also infer that the laurite–clinochlore intergrowths crystallized relatively late, from microvolumes of an H 2 O-bearing fluid of contrasting composition, enriched in Ru, S, and lithophile elements.

Origin of EL3 chondrites: Evidence for variable C/O ratios during their course of formation—A state of the art scrutiny

AbstractMineral inventories of enstatite chondrites; (EH and EL) are strictly dictated by combined parameters mainly very low dual oxygen (fO2) and sulfur (fS2) fugacities. They are best preserved in the Almahata Sitta MS‐17, MS‐177 fragments, and the ALHA 77295 and MAC 88136 Antarctic meteorites. These conditions induce a stark change of the geochemical behavior of nominally lithophile elements to chalcophile or even siderophile and changes in the elemental partitioning thus leading to formation of unusual mineral assemblages with high abundance of exotic sulfide species and enrichment in the metallic alloys, for example, silicides and phosphides. Origin and mode of formation of these exotic chondrites, and their parental source regions could be best scrutinized by multitask research experiments of the most primitive members covering mineralogical, petrological, cosmochemical, and indispensably short‐lived isotopic chronology. The magnitude of temperature and pressure prevailed during their formation in their source regions could eventually be reasonably estimated: pre‐ and postaccretionary could eventually be deduced. The dual low fugacities are regulated by the carbon to oxygen ratios estimated to be &gt;0.83 and &lt;1.03. These parameters not only induce unusual geochemical behavior of the elements inverting many nominally lithophile elements to chalcophile or even siderophile or anthracophile. Structure and mineral inventories in EL3 and EH3 chondrites are fundamentally different. Yet EH3 and EL3 members store crucial information relevant to eventual source regions and importantly possible variation in C/O ratio in the course of their evolution. EL3 and EH3 chondrites contain trichotomous lithologies (1) chondrules and their fragments, (2) polygonal enstatite‐dominated objects, and (3) multiphase metal‐rich nodules. Mineralogical and cosmochemical inventories of lithologies in the same EL3 indicate not only similarities (REE inventory and anomalies in oldhamite) but also distinct differences (sinoite‐enstatite‐graphite relationship). Oldhamite in chondrules and polygonal fragments in EL3 depict negative Eu anomaly attesting a common cosmochemical source. Metal‐dominated nodules in both EL3 and EH3 are conglomerates of metal clasts and sulfide fragments in EH3 and concentrically zoned C‐bearing metal micropebbles (≥25 μm ≤50 μm) in EL3 thus manifesting a frozen in unique primordial accretionary metal texture and composition. Sinoite‐enstatite‐diopside‐graphite textures reveal a nucleation and growth strongly suggestive of fluctuating C/O ratio during their nucleation and growth in the source regions. Mineral inventories, sulfide phase relations, sinoite‐enstatite‐graphite intergrowth, carbon and nitrogen isotopic compositions of graphite, spatial nitrogen abundance in graphite in metal nodules, and last but not least 129I/129Xe and 53Mn/53Cr systematics negate any previously suggested melting episode, pre‐accretionary or dynamic, in parental asteroids.

THE Pd–Ni–Fe–s Phase System At 550 and 400 °C

Abstract The condensed Pd–Ni–Fe–S phase system was studied at 550 and 400 °C by synthesis in evacuated silica glass tubes and a combination of textural and electron microprobe studies. The monosulfide Fe 1– x S–Ni 1– x S solid solution persists at both temperatures. Maximum solubility of Pd, exhibited at elevated x values in the mss formula, is 0.7 at.% at 550 °C and up to 0.1 at.% at 400 °C. The field of sulfide melt is reduced to a narrow Ni-Pd rich stripe at 550 °C and is absent at 400 °C. Sulfur contents of the melt are limited to the interval 30–40 at.% S at 550 °C, i.e ., it is metal-rich. Retreat of the melt field at 550 °C results in a plethora of phase associations in its place. Compositional fields of alloys associated with melt and of those associated with ∼(Ni,Fe) 3± x S 2 extend to high Pd contents. Several sulfur-poor associations contain alloys with only low nickel contents, close in composition to Pd 3 Fe. Shrinkage of the ∼(Ni,Fe) 3± x S 2 phase field (the β 1 -β 2 pair of Kitakaze et al . 2011) and the appearance of pentlandite represent the most important developments in the phase system between 725 and 400 °C. The ∼(Ni,Fe) 3± x S 2 phase dissolves up to 2 at.% Pd at 550 °C. Pentlandite associated with Pd-rich phases dissolves up to 4 at.% Pd at 550 °C, and 5.4 at.% Pd at 400 °C. The latter value represents almost 92% Pd occupancy of the octahedral site. At 550 °C, Pd-rich pentlandite coexists with sulfide melt which contains between 17 and 47 at.% Pd. The rare occurrences of such Pd-rich pentlandite in nature could indicate the presence of Pd-rich residual melts. Other cases of Pd-rich pentlandite associate with Pd-rich alloy or phases like PdS and Pd 2.2 S. In the association with mss , Pd is concentrated in the sulfide melt, especially at a low temperature. At low temperatures or high S fugacities, nickel prefers mss but pentlandite and ∼(Ni,Fe) 3± x S 2 complicate this simple scheme. For mineral associations with elevated sulfur fugacities, pyrrhotite ( mss ) appears to be a principal solid-state collector of Pd at high temperatures, with its gradual release on cooling, making it available for subsequent reactions. At low sulfur fugacities Pd concentrates in its Fe-Ni alloys. Because of the inverse relationship between Ni solubility in PdS and the temperature of formation, the composition of natural vysotskite (and Pd-rich braggite) may serve as a temperature indicator.

Origin and K-Ar age of the phreatomagmatic breccia at the Trepča Pb-Zn-Ag skarn deposit, Kosovo: Implications for ore-forming processes

The Trepca Pb-Zn-Ag skarn deposit in Kosovo is spatially and temporarily related to the phreatomagmatic breccia of Oligocene age (~23 Ma). The deposit shows features typical for skarn deposits worldwide, including a stage of isochemical metamorphism, a prograde stage of an anhydrous, low oxygen and low sulfur fugacity character and a retrograde stage characterized by an increase in the water activity as well as by an increase in oxygen and sulfur fugacities. The mineralization is hosted by the recrystallized Upper Triassic limestone. The prograde mineralization consists mainly of Ca-Fe-Mn±Mg pyroxenes. The host recrystallized limestone at the contact with the prograde (skarn) mineralization have the increased content of Fe, Mn, Mo, As, Au, Cs, Ga, REE and Y suggesting their transport by infiltrating magmatic fluids. The decreased d13C and d18O values reflect a contribution of magmatic CO2. The retrograde mineral assemblage comprises ilvaite, magnetite, arsenopyrite, pyrrhotite, marcasite, pyrite, Ca-Fe-Mn±Mg carbonates and quartz. Hydrothermal ore minerals, mostly galena, sphalerite and pyrite, were deposited contemporaneously with the retrograde stage of the skarn development. Syn-ore and post-ore carbonates reflect a diminishing influence of magmatic CO2. Syn-ore carbonates are enriched in Fe, Mg, Mn, many chalcophile elements, including Ag, As, Bi, Cd, Cu, Pb, Sb and Zn, as well as in Au, Y and REE. The post-ore stage accompanied the precipitation of significant amount of Ca-rich carbonates including the travertine deposits at the deposit surface. The phreatomagmatic breccia was developed along a NW dipping contact between the ore bearing recrystallized limestone and the overlying schist. It has an inverted cone shape with the vertical extension up to 800 m and diameter up to 150-m. The upper part of the diatreme (an underground segment of the phreatomagmatic breccia) is characterized by the presence of a hydrothermally altered rootless quartz-latite dike surrounded by an unsorted polymict breccia mantle. Despite the alteration processes, the dike has a preserved porphyritic texture. Partly preserved sanidine, accompanied with the mixture of muscovite and quartz, reflects a near-neutral to weakly acidic environment. The clasts of country rocks and skarn mineralization underwent intense milling and mixing due to repeated magmatic penetrations. Sericitization of the breccia matrix, locally accompanied with minor kaolinitization, point to an increased water activity under near-neutral to weakly acidic conditions. Large fragments originally composed of anhydrous skarn minerals (pyroxenes) are usually completely altered to a mixture of fibroradial magnetite, quartz and various amount of carbonates suggesting an increase in oxygen fugacity. Their pyrite rims reflect that the increase in oxygen fugacity was followed by an increase in sulfur fugacity. The clast predominantly composed of Fe-sulfides and minor Bi-sulfides point that the increase in sulfur fugacity was locally sufficient to complete sulfidation of hedenbergite to pyrrhotite and/or pyrite. Although the phreatomagmatic breccia at the Trepca Pb-Zn-Ag skarn deposit does not carry significant amounts of the ore mineralization, its formation was crucial for the ore deposition. Phreatomagmatic explosions and formation of the breccia turned the system from the lithostatic to hydrostatic regime and triggered the retrograde stage increasing the water activity and oxygen fugacity in the system. In addition, cooling and decompression of the system contributed to more effective degassing of magmatic sulfur increased the sulfur fugacity.

Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings

Research Article| June 01, 2014 Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings Peter Koděra; Peter Koděra 1Department of Geology of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia Search for other works by this author on: GSW Google Scholar Christoph A. Heinrich; Christoph A. Heinrich 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Markus Wälle; Markus Wälle 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Jaroslav Lexa Jaroslav Lexa 3Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia Search for other works by this author on: GSW Google Scholar Author and Article Information Peter Koděra 1Department of Geology of Mineral Deposits, Faculty of Natural Sciences, Comenius University, Mlynská dolina, 842 15 Bratislava, Slovakia Christoph A. Heinrich 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Markus Wälle 2Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland Jaroslav Lexa 3Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, 840 05 Bratislava, Slovakia Publisher: Geological Society of America Received: 06 Nov 2013 Revision Received: 05 Mar 2014 Accepted: 12 Mar 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (6): 495–498. https://doi.org/10.1130/G35270.1 Article history Received: 06 Nov 2013 Revision Received: 05 Mar 2014 Accepted: 12 Mar 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter Koděra, Christoph A. Heinrich, Markus Wälle, Jaroslav Lexa; Magmatic salt melt and vapor: Extreme fluids forming porphyry gold deposits in shallow subvolcanic settings. Geology 2014;; 42 (6): 495–498. doi: https://doi.org/10.1130/G35270.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The recently discovered Biely Vrch deposit in the Western Carpathian magmatic arc is the most extreme example of a porphyry gold deposit, being practically free of copper, molybdenum or any other sulfide minerals. Microanalytical data on fluid inclusions in quartz veinlets, including a characteristic type of banded veinlets, show that this deposit formed from nearly anhydrous Fe-K-Na-Cl salt melts containing ∼10 ppm Au, coexisting with hydrous vapor of very low density. This exceptional fluid evolution required an Fe-rich dioritic source magma that was emplaced at shallow subvolcanic depth (<3.5 km), directly exsolving a hypersaline liquid and magmatic vapor at high temperature (∼850 °C). During ascent to the level of the porphyry intrusion (0.5–1 km), fluid expansion at high temperature but low pressure led to halite precipitation and further water loss to the vapor, generating an increasingly Fe-K-rich salt melt that transported high concentrations of Au but negligible Cu into the fractured porphyry stock. The low sulfur fugacity resulting from fluid expansion suppressed precipitation of sulfide, explaining the gold-only enrichment in this globally recurring but rare type of gold ore. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

THE FIRST REPORT OF PLATINUM-GROUP MINERALS IN MAGNETITE-BEARING GABBRO, FREETOWN LAYERED COMPLEX, SIERRA LEONE: OCCURRENCES AND GENESIS

Platinum-group minerals (PGM) have been located in magnetite-bearing gabbroic rocks of the Freetown Layered Complex, Sierra Leone for the first time. The PGM occurrences described here are from four stratiform layers characterized by a local, more primitive, composition of the associated silicate minerals and elevated platinum-group elements (PGE) and base metal assays. The uppermost layer (Horizon B) is the best exposed; the three lower layers are currently known only from single outcrops. Platinum-group minerals are irregularly distributed through these layers. Altered PGE arsenides and antimonides, possibly derived from sperrylite (PtAs2) and “mertieite” [Pd8(Sb,As)3 to Pd11(Sb,As)4], occur in the lowest known PGE-enriched layer (Horizon M). Cooperite (PtS) and Pt-Fe alloys were observed in a higher layer (Horizon D). Pd-Cu alloys including nielsenite (PdCu3) are unique to the next stratigraphically higher PGM-bearing layer (Horizon C). The uppermost layer (Horizon B) contains the most PGM. Pt-Fe alloy (usually Pt3Fe) is by far the most abundant PGM and cooperite is a significant component of the assemblage. Bowieite [(Pt,Ir,Rh)2S3], tulameenite (Pt2CuFe), and laurite (RuS2) have also been found in this horizon. Cooperite occurs in a late magmatic, high Ca-amphibole (cannilloite) which has formed interstitially to plagioclase and between plagioclase and olivine at a depth of about 20 km. The cooperite is often attached to rounded magnetite grains within the amphibole. Where cooperite is located at the edge of the amphibole or in interstitial locations it has been altered to Pt-Fe alloy. Platinum-group minerals sometimes occur near the margin of sulfides, mainly chalcopyrite and minor pentlandite. The entire PGM assemblage is found mainly interstitial to olivine, pyroxene, and plagioclase, often at the edge of amphibole or in interstitial sites associated with amphibole, chlorite, or quartz. Many of the PGM grains are less than 1 micron in size, with a few in the 2–4 micron range; the largest example is a grain of Pt-Fe alloy that measures 9 by 6 microns. The PGM described here from the rocks are three orders of magnitude smaller than the eluvial and alluvial PGM of the Freetown complex, which are predominantly Pt-Fe alloys, laurite, erlichmanite (OsS2), and Os-Ir alloys. The large eluvial and alluvial PGM might be derived from a horizon containing coarse-grained mineralization, but no vestige of such a horizon has yet been discovered. Previous studies of this area have suggested that secondary remobilization of the PGE has occurred, leading to the growth of large secondary PGM. The primary PGE-mineralization associated with magnetite and Cu-rich sulfide mineralization maybe due to late sulfur saturation in the magma after Ni has been removed by being incorporated into earlier olivine. The presence of digenite, native copper, and nielsenite indicates a low sulfur fugacity, possibly due to oxidation. The four distinctive PGM-bearing horizons are the result of separate injections of magma. The silicate compositions, the minor element content, the PGM mineral assemblage, and the Cu/Pd ratios of the rocks of the PGM-bearing horizons demonstrate that these four magma pulses are compositionally distinct.

PGE mineralization of dunite-wehrlite massifs at the Gutara-Uda interfluve, Eastern Sayan

The Pt-Pd and Au-Ag mineralization hosted in both wehrlite without visible links to sulfide mineralization (dispersed assemblage of the Tartai massif) and disseminated Cu-Ni sulfide ore (ore assemblage of the Ognit massif) was found in dunite-wehrlite massifs localized in the fold framework of the Siberian Craton. The Pt minerals in both assemblages comprise sperrylite (PtAs2) and secondary Pt-Fe-Ni alloys in the Ognit massif and Pt-Fe-Cu and Pt-Cu alloys in the Tartai massif. The Pd minerals are widespread in the ore assemblages as compounds with Te, Sb, and Bi, whereas in the dispersed assemblage Pd is concentrated primarily in Pd-Cu-Sb compounds. Both assemblages are characterized by similar substitution of sperrylite with orcelite (Ni5 − xAs2) and then with secondary Pt-Fe-Ni or Pt-Fe-Cu and Pt-Cu alloys; the occurrence of Au-Ag alloys with prevalence of Ag over Au; and replacement of them with auricupride (Cu3Au) at the late stage. Sperrylite in both assemblages contains Ir impurities, while the Pd minerals contain Cu and Ni admixtures, which are typical of mineral assemblages related to the ultramafic intrusions with nickel specialization. PGM were formed under a low sulfur fugacity and high As, Bi, and Sb activities. The postmagmatic fluids affected the primary mineral assemblages under reductive conditions, and this effect resulted in replacement of sperrylite with Ni arsenide (orcelite) and Pt-Fe-Ni and Pt-Fe-Cu alloys; Ni and Cu sulfides were replaced with awaruite and native copper.

MAGMATIC AND POST-MAGMATIC SIGNIFICANCE OF CHROMITITE AND ASSOCIATED PLATINUM-GROUP MINERALS (PGM) IN THE EASTERN KHOY OPHIOLITIC COMPLEX (NW IRAN)

The Khoy ophiolites are one of the largest Iranian ophiolitic complexes and are characterized by a very complicate geology, consisting of several ophiolites with different ages and geodynamic settings of formation. Few small lenticular and irregular bodies of podiform and schlieren type chromitites have been recognized in the Khoy ophiolites, occurring in the serpentinized mantle peridotites. They have massive and nodular textures. According to their chromite composition, the Khoy chromitites can be classified as Al-rich and fall clearly in the field of podiform chromitites. The magmatic chromite composition of the podiform chromitites of the Khoy ophiolites suggests that they crystallized from a MORB type melt in an extensional regime. The Khoy chromitites were altered during ocean floor metamorphisms and no magmatic silicates were preserved. The composition of chlorite occurring in the silicate matrix or in contact with chromite has been used as a geothermometer. The estimated temperatures possibly reflect the effects of low-grade metamorphism. The Khoy chromitites have very low total platinum group elements (PGE) concentration with a predominance of Os + Ir + Ru over Rh + Pt + Pd. Consistently with the geochemical data, they contain few platinum group minerals (PGM) of Ru, Ir and Os. Laurite is the most common PGM along with a minor Ir-Cu sulfide, possibly cuproiridsite, osmium and erlichmanite. Most of the PGM are magmatic in origin. They crystallized in a narrow range of temperature, around 1000 o C, at relatively low sulfur fugacity and in the absence of fluids. This observation is consistent with formation of the chromitites in a mid-oceanic ridge (MOR) rather than in a supra-subduction zone (SSZ).