Assemblage Of Pyrrhotite Research Articles

Mineralogy and mineral chemistry of the ABM replacement-style volcanogenic massive sulfide deposit, Finlayson Lake district, Yukon, Canada

The ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfide deposit (VMS) that is hosted by back-arc affinity rocks of the Yukon–Tanana terrane in the Finlayson Lake VMS district, Yukon, Canada. Massive sulfide zones occur as stacked and stratabound lenses subparallel to the volcanic stratigraphy, surrounded by pervasive white mica and/or chlorite alteration. Remnant clasts of volcanic rocks and preserved bedding occur locally within the massive sulfide lenses and indicate that mineralization formed through subseafloor replacement of pre-existing strata. Three mineral assemblages occur at the ABM deposit: (1) a pyrite–chalcopyrite–magnetite–pyrrhotite assemblage that is associated with Cu–Bi–Se–Co-enrichment and occurs at the center of the massive sulfide lenses; (2) a pyrite–sphalerite assemblage, which occurs more commonly towards lens margins and is enriched in Zn–Pb–Ag–Au–Hg–As–Sb–Ba; and (3) a minor assemblage comprising chalcopyrite–pyrrhotite–pyrite stringers associated with pervasive chlorite alteration, which occurs mostly at the sulfide lens margins. Petrographic observations of preserved primary, zone refining, and metamorphic textures in combination with in situ geochemistry show that the pyrite–sphalerite assemblage formed at lower temperatures (< 270 °C) than the other two mineral assemblages (~ 270–350 °C), and that mineral chemistry in all mineral assemblages was affected by greenschist facies metamorphism, although the effects are limited to recrystallization, small-scale remobilization (< 1 m) and trace element redistribution.

The effects of pressure, fO2, fS2 and melt composition on the fluid–melt partitioning of Mo: Implications for the Mo-mineralization potential of upper crustal granitic magmas

In order to better constrain the partitioning behavior of Mo during the crystallization of upper crustal granitic magmas, partition coefficients of Mo between aqueous fluids and granitic melts were experimentally determined. The experiments were conducted at 750–850 °C, 80–300 MPa, and fO2 + fS2 conditions being controlled either by the magnetite–pyrrhotite assemblage and an external NiNiO buffer, or fully internally by the magnetite–pyrrhotite–pyrite assemblage. During the experiments, fluids that were equilibrated with silicate melt, molybdenite and the internal buffer minerals were trapped in the form of synthetic fluid inclusions in quartz. Through analyzing the recovered synthetic fluid inclusions and run product glasses by LA-ICP-MS, the abundances of Mo in the fluids and the melts were determined, and corresponding fluid–melt partition coefficients (DMofluid/melt) were calculated. The DMofluid/melt values obtained in our experiments with supercritical fluids (DMosf/melt; ±1σ) range from 9 ± 2 to 54 ± 9 and vary as function of pressure, melt composition, and fO2 + fS2. In slightly peraluminous, magnetite–pyrrhotite-buffered melts at 750 °C, DMosf/melt increases from 15 ± 5 at 160 MPa to 34 ± 6 at 300 MPa. In contrast, over the range of 750 °C to 850 °C, no significant temperature effect was observed. Vapor–melt (DMovapor/melt; ±1σ) and brine–melt (DMobrine/melt; ±1σ) partition coefficients determined in two experiments with subcritical fluids are 4 ± 1 vs 58 ± 8, and 9 ± 2 vs 117 ± 15, respectively, suggesting that Mo favors both fluid phases relative to the silicate melt and partitions more strongly into brine relative to vapor. Most of our experimentally determined DMofluid/melt values agree well with those derived from coexisting fluid and melt inclusions in natural granitic samples and those found in a previous experimental study conducted on sulfur-bearing systems, but they are generally higher than those reported in previous experimental studies conducted on sulfur-free systems. Based on the observed DMofluid/melt values a quantitative Rayleigh fractionation model for the behavior of Mo during the crystallization of upper crustal granitic magma chambers was developed. The results suggest that the confining pressure exerts a strong effect on the evolution of Mo concentration in the residual melt during fractional crystallization. Specifically, magmas that crystallize at deeper levels (i.e., higher confining pressure) become more strongly depleted in Mo during fluid-saturated crystallization, which means that the Mo extraction efficiency of deeply exsolving fluids is higher. We conclude that Mo-mineralization potential of upper crustal granitic magmas may be critically affected by the magma emplacement depth.

Alteration and mineralization patterns in orogenic gold deposits: Constraints from deposit observation and thermodynamic modeling

Hydrothermal alteration and mineralization types of orogenic gold deposits display many variations depending on formation depths, types of host rock, compositions of ore fluids, spatial positions, fluid/rock ratios, and repetitive modifications during multiple fluid infiltrations in the complicated processes of fluid-rock interaction. Therefore, establishing holistic reference patterns of hydrothermal alteration with associated mineralization for the orogenic gold system is necessary. Based on the observation of typical orogenic gold deposits and thermodynamic simulation of fluid-rock reaction, the general features of proposed patterns include:(1)Orogenic ore fluids are capable of transporting sufficient gold from deep (5 kbar and 600°C) to shallow (1 kbar and 200°C) crustal levels, resulting in alteration and mineralization occurred in a much-varied depth. Only considering the effects of fluid-rock reaction and sulfidation, the formation of pyrrhotite and pyrite at relatively high and low P-T conditions, respectively, is the most important mechanism driving gold precipitation. In addition, the changes of fluid fO2 and pH can also trigger a decrease in the gold solubility in fluids.(2)Multiple waves of fluid infiltration are capable of dissolving the early-formedulfides and gold, and reprecipitate them in the distal alteration zones, which can explain the irregular gold concentration distribution patterns in wide alteration zones. The fluids under higher P-T conditions (5 kbar and 600°C and 3 kbar and 400°C) are more capable of dissolving early-formed sulfides and gold. Additionally, the later infiltrated fluids can transform early-formed pyrrhotite into pyrite.(3)The model of interactions between ore fluids and aluminosilicate rocks at varied P-T conditions indicates that the alteration mineral assemblages in hypozonal deposits (>12 km) mainly include biotite, amphibole, anorthite, K-feldspar, quartz, and pyrrhotite assemblages; that those in mesozonal deposits (6 to 12 km) are characterized by chlorite, calcite, quartz, muscovite, pyrite or pyrrhotite. The epizonal deposits (<6 km) commonly develop more abundant Fe-Mg bearing carbonates (e.g., dolomite, ankerite, and siderite), quartz, muscovite, and pyrite. The compositional variations of ore fluids and host rocks have important controls on the development of iron-oxides and carbonaceous material.(4) The alteration halo is relatively restricted under hypozonal to lower mesozonal conditions. However, broad carbonation develops under upper mesozonal to epizonal conditions, with Fe-Mg bearing carbonates and calcite distributed in proximal and distal zones, respectively.

Sulfide Mineralization in the Carbonatites and Phoscorites of the Guli Massif, Polar Siberia, and Their Noble-Metal Potential

We report the first combined investigation (neutron activation, X-ray fluorescence, and electron microprobe analysis) of mineral forms of Au and Ag and noble metal distribution in the sulfide-bearing phoscorites and carbonatites of the Guli alkaline ultrabasic massif (Polar Siberia) and magnetite and sulfide separates from these rocks. The highest noble metal contents were observed in the sulfide separates from the carbonatites: up to 2.93 Pt, 61.6 Au, and 3.61 ppm Ag. Pyrrhotite, djerfisherite, chalcopyrite, and pyrite are the most abundant sulfides and the main hosts for Au and Ag. The latest assemblage of chalcopyrite, Ag-rich djerfisherite, lenaite, sternbergite, and native silver shows significant Ag concentrations. The wide occurrence of K sulfides and presence of multiphase inclusions in pyrrhotite consisting of rasvumite, K‒Na–Ca carbonate, carbocernaite, strontianite, galena, chalcopyrite, sternbergite, lenaite, and native silver suggest that the sulfides were formed at high activities of K, Na, Sr, LREE, F, Cl, and S. Chlorine shows high complex-forming capacity to Ag and could be an agent of noble metal transport in the carbonatites. Crystallization of the early djerfisherite–pyrrhotite assemblages of the phoscorites and carbonatites began at a temperature not lower than 500°C and continued up to the formation of late Ag-bearing sulfides at temperatures not higher than 150°C. The carbonatite-series rocks could be enriched in Au and Ag during late low-temperature stages and serve as a source for Au placers.

Sulfide Formation as a Result of Sulfate Subduction into Silicate Mantle (Experimental Modeling under High P,T-Parameters)

Ca,Mg-sulfates are subduction-related sources of oxidized S-rich fluid under lithospheric mantle P,T-parameters. Experimental study, aimed at the modeling of scenarios of S-rich fluid generation as a result of desulfation and subsequent sulfide formation, was performed using a multi-anvil high-pressure apparatus. Experiments were carried out in the Fe,Ni-olivine–anhydrite–C and Fe,Ni-olivine–Mg-sulfate–C systems (P = 6.3 GPa, T of 1050 and 1450 °C, t = 23–60 h). At 1050 °C, the interaction in the olivine–anhydrite–C system leads to the formation of olivine + diopside + pyrrhotite assemblage and at 1450 °C leads to the generation of immiscible silicate-oxide and sulfide melts. Desulfation of this system results in the formation of S-rich reduced fluid via the reaction olivine + anhydrite + C → diopside + S + CO2. This fluid is found to be a medium for the recrystallization of olivine, extraction of Fe and Ni, and subsequent crystallization of Fe,Ni-sulfides (i.e., olivine sulfidation). At 1450 °C in the Ca-free system, the generation of carbonate-silicate and Fe,Ni-sulfide melts occurs. Formation of the carbonate component of the melt occurs via the reaction Mg-sulfate + C → magnesite + S. It is experimentally shown that the olivine-sulfate interaction can result in mantle sulfide formation and generation of potential mantle metasomatic agents—S- and CO2-dominated fluids, silicate-oxide melt, or carbonate-silicate melt.

Geology and ore genesis of the late Paleozoic Heijianshan Fe oxide–Cu (–Au) deposit in the Eastern Tianshan, NW China

The Heijianshan Fe–Cu (–Au) deposit, located in the Aqishan-Yamansu belt of the Eastern Tianshan (NW China), is hosted in the mafic–intermediate volcanic and mafic–felsic volcaniclastic rocks of the Upper Carboniferous Matoutan Formation. Based on the pervasive alteration, mineral assemblages and crosscutting relationships of veins, six magmatic–hydrothermal stages have been established, including epidote alteration (Stage I), magnetite mineralization (Stage II), pyrite alteration (Stage III), Cu (–Au) mineralization (Stage IV), late veins (Stage V) and supergene alteration (Stage VI). The Stage I epidote–calcite–tourmaline–sericite alteration assemblage indicates a pre-mineralization Ca–Mg alteration event. Stage II Fe and Stage IV Cu (–Au) mineralization stages at Heijianshan can be clearly distinguished from alteration, mineral assemblages, and nature and sources of ore-forming fluids.Homogenization temperatures of primary fluid inclusions in quartz and calcite from Stage I (189–370 °C), II (301–536 °C), III (119–262 °C) and V (46–198 °C) suggest that fluid incursion and mixing probably occurred during Stage I to II and Stage V, respectively. The Stage II magmatic–hydrothermal-derived Fe mineralization fluids were characterized by high temperature (>300 °C), medium–high salinity (21.2–56.0 wt% NaCl equiv.) and being Na–Ca–Mg–Fe-dominated. These fluids were overprinted by the external low temperature (<300 °C), medium–high salinity (19.0–34.7 wt% NaCl equiv.) and Ca–Mg-dominated basinal brines that were responsible for the subsequent pyrite alteration and Cu (–Au) mineralization, as supported by quartz CL images and H–O isotopes. Furthermore, in-situ sulfur isotopes also indicate that the sulfur sources vary in different stages, viz., Stage II (magmatic–hydrothermal), III (basinal brine-related) and IV (magmatic–hydrothermal). Stage II disseminated pyrite has δ34Sfluid values of 1.7–4.3‰, comparable with sulfur from magmatic reservoirs. δ34Sfluid values (24.3–29.3‰) of Stage III Type A pyrite (coexists with hematite) probably indicate external basinal brine involvement, consistent with the analytical results of fluid inclusions. With the basinal brines further interacting with volcanic/volcaniclastic rocks of the Carboniferous Matoutan Formation, Stage III Type B pyrite–chalcopyrite–pyrrhotite assemblage (with low δ34Sfluid values of 4.6–10.0‰) may have formed at low fO2 and temperature (119–262 °C). The continuous basinal brine–volcanic/volcaniclastic rock interactions during the basin inversion (∼325–300 Ma) may have leached sulfur and copper from the rocks, yielding magmatic-like δ34Sfluid values (1.5–4.1‰). Such fluids may have altered pyrite and precipitated chalcopyrite with minor Au in Stage IV. Eventually, the Stage V low temperature (∼160 °C) and low salinity meteoric water may have percolated into the ore-forming fluid system and formed late-hydrothermal veins.The similar alteration and mineralization paragenetic sequences, ore-forming fluid sources and evolution, and tectonic settings of the Heijianshan deposit to the Mesozoic Central Andean IOCG deposits indicate that the former is probably the first identified Paleozoic IOCG-like deposit in the Central Asian Orogenic Belt.

Mineralogy and genetic characteristics of the Rudnik Pb-Zn/Cu,Ag,Bi,W polymetallic deposit (Central Serbia) - New occurrence of Pb(Ag)Bi sulfosalts

The mineral aschamalmite, of heyrovskyite-like composition, but different in structure, from the Rudnik Pb–Zn/Cu,Ag,Bi,W polymetallic deposit in the central part of Serbia has been investigated. This polymetallic deposit includes over 90 hydrothermal and skarn-replacement orebody types, primarily hosted by Cretaceous sediments and occassionally by Oligocene dykes and sills of dacitic composition, and contact-metamorphic-metasomatic rocks. These rocks are host to an assemblage of pyrrhotite, colloform pyrite, chalcopyrite, galena, arsenopyrite, native bismuth and scheelite as well as minor pyrite, sphalerite, bismuthinite, argentopentlandite, and native silver. The chemical composition of the ore is very complex, where weight contents of valuable metals range as follows (%): Zn 0.49–4.49; Pb 0.90–5.66; Cu 0.08–2.18; WO 3 0.05–1.18; Ag 0.005–0.030; Bi 0.005–0.081; and Cd 0.002–0.016. In reflected light, aschamalmite is optically anisotropic, with moderately high bireflectance ranging from slightly greenish to slightly gray. Electron-microprobe analysis gave an average crystallochemical formulae of (Pb 5.82 Ag 0.20 ) Σ6.02 Bi 2.03 (S 8.93 Te 0.02 Se 0.01 ) Σ8.96 . The strongest diffraction maximums of the X-ray powder pattern [ d (in A)(I)] are 3.419(100), 3.382(92), and 3.334(66). Monoclinic unit cell parameters are a =13.727(7); b =4.122(3); c =31.32(2) A; β =90.72(5) o ; and V =1771.8(1) A 3 . Mineral assemblages and genesis of the Rudnik polymetallic deposit are discussed in detail and the sulfobismuthite mineralization has been compared with similar well-known global deposits.

Constraints of mineralogical characterization of gold ore: Implication for genesis, controls and evolution of gold from Kundarkocha gold deposit, eastern India

Gold mineralization in Kundarkocha gold deposit occurs in the eastern Indian Craton that is hosted by sheared quartz–carbonate–sulfide veins emplaced within the graphitic schist, carbonaceous phyllite and talc–chlorite–serpentine schist belongs to Gorumahisani–Badampahar schist belt of Iron Ore Group. Gold mineralization exhibits both lithological and structural controls in the study area, albeit the stratigraphic control is more ubiquitously observed. Detailed mineralogical characterization coupled with electron probe microanalysis of the sulfide phases reveal the occurrences of gold in three distinct forms (i) as lattice-bound form within sulfides especially enriched in arsenopyrite, loellingite, pyrite, pyrrhotite and chalcopyrite in decreasing order of abundance; (ii) as micro inclusions or nano-scale gold inclusions within pyrite and arsenopyrite especially along the growth zones and micro-fractures as substrates and (iii) as free milling nugget gold grains either along the grain boundaries of sulfides or within the host rocks. Three generations of pyrite (Py-I, Py-II and Py-III) and arsenopyrite (Asp-I, Asp-II, Asp-III) have been identified based on textural, morphological characteristics and mineral chemistry. The lattice-bound gold content in pyrite and arsenopyrite varies from 600 to 2700ppm and 900 to 3600ppm respectively and increase in concentration of such refractory gold is seen in the order of chalcopyrite>pyrrhotite>pyrite>loellingite/arsenopyrite.The evolutionary stages of different forms of gold include remobilization of the lattice-bound grains in pyrite and arsenopyrite (Py-I and Asp-I) and re-concentration along the zoned-pyrite and arsenopyrite (Py-II and Asp-II) and ultimately as native gold/nuggets surrounding the sulfides as well as within the main mineralized zone. Lattice-bound gold distribution could have resulted due to metamorphic devolatilization reactions which are further aided by the influx of hydrothermal fluids. These reactions along with additional input of hydrothermal fluid paved the way for expulsion of lattice-bound gold from sulfides to concentrate as nuggets/main lode within shear fractures channelizing the fluid flow. Thermometry results of the arsenopyrite–pyrite–pyrrhotite assemblage yielded a temperature range from 375 to 390°C which is the ideal condition for gold precipitation. Native silver, gersdorffite and arsenian-ullmannite are being reported for the first time from this deposit indicating the complexity and wide variety of mineral phases associated with gold implying magmatic-hydrothermal input of the source fluid.

Gold Solubilities in Bornite, Intermediate Solid Solution, and Pyrrhotite at 500 to 700 C and 100 MPa

Experiments were conducted at 100 MPa with an oxygen fugacity buffered by Ni-NiO, and at temperatures of 500°, 600°, and 700°C, to determine the solubility of Au within bornite, high-temperature chalcopyrite (intermediate solid solution [ISS]), and pyrrhotite over a range of sulfur activities likely to be found in the porphyry environment. The activity of sulfur in the system, ![Formula][1] , was buffered by ISS (Cu = Fe), ISS + bornite, or ISS + pyrrhotite mineral assemblages, which induced a ![Formula][2] between −11 ± 1 and 0.4 ± 0.8 (1σ). The solubility of Au in bornite increased from 1,000 μ g/g at 500°C to 1,800 μ g/g at 700°C and with an increase in the ![Formula][3] of −11 to −6.0. The solubility of Au in ISS was evaluated as both a function of temperature and the ![Formula][4] and increased from 300 μ g/g at 500°C and low ![Formula][5] to 4,000 μ g/g at 700°C and high ![Formula][6] . The ![Formula][7] was found to exert a control on Au solubility as, at 700°C, Au solubility in ISS increased from 1,100 to 4,000 μ g/g as the ![Formula][8] increased from −6.0 ± 0.1 to 0.4 ± 0.8 (1σ). The solubility of Au in ISS at 600°C increased from 800 to 1,800 μ g/g as the ![Formula][9] increased from −8.3 ± 0.7 to −2 ± 1 (1σ). Gold in pyrrhotite ranged from 300–500 μ g/g and did not vary appreciably over the entire range of temperature and ![Formula][10] studied. Nernst-style partition coefficients for gold between ISS and pyrrhotite, ![Formula][11] , were calculated at 600°C to be 3.6 ± 1.9 (1σ) for a ![Formula][12] of −2 ± 1 (1σ) and at 700°C to be 7.3 ± 4.7, 7.8 ± 2.9, and 10 ± 4 (1σ) at ![Formula][13] of −0.2 ± 0.4, 0.3 ± 0.4, and 0.4 ± 0.8 (1σ), respectively. Partition coefficients for gold between bornite and ISS, ![Formula][14] , were calculated to be 2.5 ± 1.8, 1.9 ± 0.7, and 1.6 ± 0.7 (1σ) for 500°, 600°, and 700°C at ![Formula][15] of −11 ± 1, −8.3 ± 0.7, and −6.0 ± 0.1 (1σ), respectively. In summary, the solubility of Au in bornite and ISS was found to increase with increasing temperature and the ![Formula][16] , whereas Au in pyrrhotite did not vary over the range of temperature and ![Formula][17] studied. Further, Au will partition preferentially into ISS relative to pyrrhotite in porphyry systems with an ISS + pyrrhotite assemblage and into bornite for the bornite + ISS assemblage. These data illustrate the importance of temperature and the ![Formula][18] in controlling the solubility of Au in sulfide minerals and provide important constraints for exploration. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif [5]: /embed/mml-math-5.gif [6]: /embed/mml-math-6.gif [7]: /embed/mml-math-7.gif [8]: /embed/mml-math-8.gif [9]: /embed/mml-math-9.gif [10]: /embed/mml-math-10.gif [11]: /embed/mml-math-11.gif [12]: /embed/mml-math-12.gif [13]: /embed/mml-math-13.gif [14]: /embed/mml-math-14.gif [15]: /embed/mml-math-15.gif [16]: /embed/mml-math-16.gif [17]: /embed/mml-math-17.gif [18]: /embed/mml-math-18.gif

The Characteristics and Origin of the Big Lake Mafic-Ultramafic-Hosted Volcanogenic Massive Sulfide Occurrence, Marathon, Ontario, Canada

The Big Lake volcanogenic massive sulfide (VMS) occurrence, located in the Schreiber-Hemlo belt in the Wawa subprovince of the Superior province, is hosted in an Archean mafic-ultramafic metavolcanic sequence lacking felsic volcanic or volcaniclastic rocks. Discovered in 2006, the occurrence is a subeconomic assemblage of pyrrhotite, chalcopyrite, sphalerite, cobaltite, and galena currently defined over an area of ~0.5 × 0.5 km in plan view. The mineralized zone is less than 10 m thick, with the highest grade intersection containing 4.0 m of 7.5% Cu, 2.2% Zn, 138.0 gpt Ag, and 9.2 gpt Au. Mineralization is hosted in metavolcanic rocks of basaltic to komatiitic basalt compositions. The sulfides and spatially associated hydrothermal alteration (regionally metamorphosed to chlorite, actinolite, hornblende, biotite-phlogopite, and anthophyllite) are located 5 to 50 m structurally below a thick ultramafic cumulate complex. The host sequence to the occurrence is overturned, with VMS sulfides stratigraphically overlying the ultramafic cumulates. Rocks stratigraphically below the VMS mineralization are transitional to alkaline basalts and komatiitic basalts, with convex-upward LREE and fractionated HREE when normalized to primitive mantle. Basalts stratigraphically overlying the VMS mineralization have flat normalized REE patterns (La/Ybcn = 1–2) typical of tholeiitic plateau basalts found elsewhere in the Wawa belt. The lithostratigraphic assemblage at Big Lake is consistent with an oceanic plateau setting and is interpreted to have formed by eruption of plateau basalts and thick ultramafic flows from a mantle plume. Unlike VMS systems that formed above subvolcanic intrusions >0.5 km below sea floor, the Big Lake VMS occurrence may have formed as a result of cooling of the stratigraphically underlying ultramafic cumulates and demonstrates that similar styles of ponded flow-driven VMS systems, albeit small ones, may be preserved elsewhere in the Superior province or in other Archean cratons.

Sulfide inclusions in diamonds: not monosulfide solid solution

Sulfide inclusions in diamonds, the most common of all inclusions, contain critical evidence about the timing and physical/chemical conditions prevailing during diamond formation. Typically, sulfide inclusions are encapsulated as a monosulfide solid solution (Mss) in the Fe-Ni-S system, with a minor amount of Cu. This Mss and the enclosing diamond have sufficiently different thermal expansion properties, so that, after encapsulation, the Mss creates a series of cracks in the diamond radiating from the sulfide. On cooling, this increase in volume permits the Mss to undergo exsolution to an assemblage of pyrrhotite + pentlandite + chalcopyrite + pyrite. The kinetics of this exsolution is so rapid that practically no Mss remains in nature. Instead, in recovered diamonds, all sulfides that originally were Mss now consist of this fine-grained assemblage. Chalcopyrite prefers to form around the edges of the inclusions and also migrates into the minute cracks in the diamonds. It is the bulk composition of the Mss as encapsulated that is important for interpretation of diamond petrogenesis (P- versus E-type diamonds) and to the commonly used Re-Os dating technique. However, this bulk composition is definitely not attainable with polished sections cut through the inclusions. The assumption that the kernel of the sulfide inclusion for Re-Os age dating represents the entire original Mss may also be incorrect, depending what has been lost, mostly chalcopyrite, which has migrated into the surrounding cracks within the diamond host.

Mine drainage from the weathering of sulfide minerals and magnetite

Pyrite and pyrrhotite are the principal minerals that generate acid drainage in mine wastes. Low-pH conditions derived from Fe-sulfide oxidation result in the mobilization of contaminant metals (such as Zn, Cd, Ni and Cr) and metalloids (such as As) which are of environmental concern. This paper uses data from detailed mineralogical and geochemical studies conducted at two Canadian tailings impoundments to examine the mineralogical changes that pyrite, pyrrhotite, sphalerite and magnetite undergo during and after sulfide oxidation, and the subsequent release and attenuation of associated trace elements. The stability of sphalerite in tailings impoundments generally is greater than that of pyrrhotite, but less than pyrite. Dissolved Ni and Co derived from Fe sulfides, and to a lesser extent, dissolved Zn and Cd from sphalerite, are commonly attenuated by early-formed Fe oxyhydroxides. As oxidation progresses, a recycling occurs due to continued leaching from low-pH pore waters and because the crystallinity of Fe oxyhydroxides gradually increases which decreases their sorptive capacity. Unlike many other elements, such as Cu, Pb and Cr, which form secondary minerals or remain incorporated into mature Fe oxyhydroxides, Zn and Ni become mobile. Magnetite, which is a potential source of Cr, is relatively stable except under extremely low-pH conditions. A conceptual model for the sequence of events that typically occurs in an oxidizing tailings impoundment is developed outlining the progressive oxidation of a unit of mine waste containing a mixed assemblage of pyrrhotite and pyrite.

A shear zone related greenstone belt hosted gold mineralization in the Archean of West Greenland. A petrographic and combined Pb–Pb and Rb–Sr geochronological study

Widespread gold showings have been found in an Archean greenstone belt on Storø in Godthåbsfjord, West Greenland. The Storø Greenstone Belt (SGB) comprises metasedimentary and metavolcanic rocks with minor magnetite-rich bands. Gold mineralization is hosted by amphibolites, mica schists and garnetite rocks. Gold occurs in various mineral associations, but its presence in löllingite–arsenopyrite–pyrrhotite parageneses is prevalent. To a lesser extent, native gold occurs within seriticized plagioclase, in sheeted quartz veins, and as inclusions in garnet. Based on petrographic observations and comparison with similar ore mineral textures from other deposits we propose the following model for the deposition of the gold-bearing löllingite–arsenopyrite–pyrrhotite assemblage on Storø: In the first stage, arsenopyrite likely formed with gold in solid solution. In the second stage, gold-bearing arsenopyrite was replaced by löllingite and pyrrhotite. This took place during regional prograde metamorphism. Gold accumulated within löllingite as discrete grains. During subsequent retrograde metamorphic conditions, löllingite and pyrrhotite reacted and formed arsenopyrite, and gold was subsequently exsolved and preferably deposited as discrete grains or films along arsenopyrite–löllingite grain boundaries. During a later event a garnet overgrowth partly affected the gold-bearing löllingite–arsenopyrite–pyrrhotite assemblage. The isotopic composition of Pb released from these garnets by Pb stepwise leaching experiments (PbSL) and the bulk Pb isotopic compositions of arsenopyrite and pyrrhotite indicate a common source of Pb in the fluid from which first the sulfides and subsequently garnet were formed. Bulk Pb analyses and PbSL experiments of arsenopyrite and PbSL experiments of garnet yield ages of 2863 ± 24 Ma (2 σ; MSWD = 0.2) for arsenopyrite and 2748 ± 62 Ma (2 σ; MSWD = 220) for garnet. The 2863 ± 24 Ma age is thus a minimum age for the mineralization. Rb–Sr isotopic analyses of cogenetic biotite from within strongly mineralized zones reveal Middle Proterozoic (ca. 1.65 Ga) mineral ages, well in accordance with a regional reheating event known to have affected the area, and are thus interpreted as cooling ages thereof. This regional reheating event has apparently not affected the Pb–Pb isotopic system of arsenopyrite, implying that the blocking temperature of the U–Pb system in arsenopyrite is likely to be above ∼ 300 °C. The Late Archean ages for arsenopyrite and garnet are consistent with the regional amphibolite facies metamorphism related to major terrane assemblies in the study area occurring at that time. The ore textures preserved today are interpreted as those formed under this regional metamorphic event.

The partitioning behavior of As and Au in S-free and S-bearing magmatic assemblages

The partitioning of As and Au between rhyolite melt and low-salinity vapor (2 wt% NaCl eq.) in a melt–vapor–Au metal ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 120 MPa and f O 2 = NNO . The S-bearing runs have calculated values for the fugacities of H 2S, SO 2 and S 2 of log f H 2 S = 1.1 , log f SO 2 = - 1.5 , and log f S 2 = - 3.0 . The ratio of H 2S to SO 2 is on the order of 400. The experiments constrain the effect of S on the partitioning behavior of As and Au at magmatic conditions. Calculated average Nernst-type partition coefficients (±1 σ) for As between vapor and melt, D As v / m , are 1.0 ± 0.1 and 2.5 ± 0.3 in the S-free and S-bearing assemblages, respectively. These results suggest that sulfur has a small, but statistically meaningful, effect on the mass transfer of As between silicate melt and low-salinity vapor at the experimental conditions. Efficiencies of removal, calculated following Candela and Holland (1986), suggest that the S-free and S-bearing low-salinity vapor can scavenge approximately 41% and 63% As from water-saturated rhyolite melt, respectively, during devolatilization assuming that As is partitioned into magnetite and pyrrhotite during second boiling. The S-free data are consistent with the presence of arsenous acid, As(OH) 3 in the vapor phase. However, the S-bearing data suggest the presence of both arsenous acid and a As–S complex in S-bearing magmatic vapor. Apparent equilibrium constants, log K As ′ ( ± 1 σ ) , describing the partitioning of As between melt and vapor are −1.3 (0.1) and −1.1 (0.1) for the S-free and S-bearing runs, respectively. The increase in the value of K As ′ with the addition of S suggests a role for S in complexing and scavenging As from the melt during degassing. The calculated vapor/melt partition coefficients (±1 σ) for Au between vapor and melt, D Au v / m , in S-free and S-bearing assemblages are 15 ± 2.5 and 12 ± 0.3, respectively. Efficiencies of removal ( Candela and Holland, 1986) for the S-free melt, calculated assuming that magnetite is the dominant Au-sequestering solid phase during crystallization ( Simon et al., 2003), suggest that magmatic vapor may scavenge on the order of 72% Au from a water-saturated melt. Efficiencies of removal calculated for the S-bearing assemblage, assuming pyrrhotite and magnetite are the dominant Au-sequestering solid phases, indicate that vapor may scavenge on the order of 60% Au from the melt. These model calculations suggest that the loss of pyrrhotite and magnetite from a melt, owing to punctuated differentiation during ascent and emplacement, does not prohibit the ability of a rhyolite melt to generate a large-tonnage Au deposit. Apparent equilibrium constants describing the partitioning of Au between melt and vapor were calculated using the mean D Au v / m values for the S-free and S-bearing assemblages; only S-bearing data from runs longer than 400 h were used as shorter runs may not have reached equilibrium with respect only to vapor/melt partitioning of Au. The values for log K Au ′ ( ± 1 σ ) are −4.4 (0.1) and −4.2 (0.2) for the S-free and S-bearing runs, respectively. These data suggest that the presence of S does not affect the mass transfer of Au from degassing silicate melt to an exsolved, low-salinity vapor in a low- f S 2 assemblage (i.e., pyrrhotite–magnetite at NNO) at the experimental conditions reported here. Efficiencies of removal are calculated and used to model the mass transfer of Au from a crystallizing silicate melt to an exsolved, low-salinity vapor phase. The calculations suggest that the model, absolute tonnage of Au scavenged and transported by S-free and S-bearing vapors, from a crystallizing melt, would be comparable and that the time-integrated flux of low-salinity vapor could be responsible for a significant quantity of the Au in magmatic-hydrothermal ore deposits.

Copper partitioning in a melt–vapor–brine–magnetite–pyrrhotite assemblage

The effect of sulfur on the partitioning of Cu in a melt–vapor–brine ± magnetite ± pyrrhotite assemblage has been quantified at 800 °C, 140 MPa, f O 2 = nickel–nickel oxide (NNO), log f S 2 = - 3.0 (i.e., on the magnetite–pyrrhotite curve at NNO), log f H 2 S = - 1.3 and log f SO 2 = - 1 . All experiments were vapor + brine saturated. Vapor and brine fluid inclusions were trapped in silicate glass and self-healed quartz fractures. Vapor and brine are dominated by NaCl, KCl and HCl in the S-free runs and NaCl, KCl and FeCl 2 in S-bearing runs. Pyrrhotite served as the source of sulfur in S-bearing experiments. The composition of fluid inclusions, glass and crystals were quantified by laser-ablation inductively coupled plasma mass spectrometry. Major element, chlorine and sulfur concentrations in glass were quantified by using electron probe microanalysis. Calculated Nernst-type partition coefficients (±2 σ) for Cu between melt–vapor, melt–brine and vapor–brine are D Cu v / m = 63 ± 31 , D Cu b / m = 240 ± 80 , and D Cu v / b = 0.27 ± 0.10 , respectively, in the S-free system. The partition coefficients (±2 σ) for Cu between melt–vapor, melt–brine and vapor–brine are D Cu v / m = 316 ± 22 , D Cu b / m = 443 ± 68 , and D Cu v / b = 0.69 ± 0.16 , respectively, in the S-bearing system. Apparent equilibrium constants (±1 σ) describing Cu and Na exchange between vapor and melt and brine and melt were also calculated. The values of K Cu,Na v / m are 34 ± 21 and 128 ± 29 in the S-free and S-bearing runs, respectively. The values of K Cu,Na b/m are 33 ± 22 and60 ± 5 in the S-free and S-bearing runs, respectively. The data presented here indicate that the presence of sulfur increases the mass transfer of Cu into vapor from silicate melt. Further, the nearly threefold increase in D Cu v / b suggests that Cu may be transported as both a chloride and sulfide complex in magmatic vapor, in agreement with hypotheses based on data from natural systems. Most significantly, the data demonstrate that the presence of sulfur enhances the partitioning of Cu from melt into magmatic volatile phases.

Zr-bearing minerals as indicators for the polymetamorphic evolution of the eastern, lower Austroalpine nappes (Stubenberg Granite contact aureole, Styria, Eastern Alps, Austria)

Contact metamorphism during emplacement of the Permian Stubenberg Granite has led to the formation of the assemblage forsterite + calcite + titanian clinohumite ± phlogopite ± chlorite in the adjacent marbles. During intrusion of the granite, veins, rich in Ti, Zr, REE, and actinides (U + Th) formed. These veins show a distinct mineralogical zoning-sequence with four zones. Going from the center of the vein to the margin, these zones include the assemblages (1) geikielite + baddeleyite + zirconolite + apatite + calcite + chlorite ± magnetite ± pyrrhotite assemblage, (2) calcite + chlorite, (3) forsterite + titanian clinohumite + chlorite + calcite ± phlogopite and (4) calcite ± forsterite. Baddeleyite is always replaced by zirconolite, possibly via the model reaction baddeleyite + 2 geikielite + 3 calcite + CO 2 = zirconolite + 2 dolomite. Zirconolite (Zirc I) shows a strong internal oscillatory zoning and distinct overgrowths (Zirc II), which have a different chemical composition. The chemical variation between the cores (Zirc I) and the rims (Zirc II) can be explained by using the substitutions: Me 5+ + Me 2+ = Ti 4+ + Me 3+ and REE 3+ + Me 5+ + Me 2+ = Ca 2+ + 2Ti 4+. In contrast to zirconolite from metacarbonates associated with contact aureoles, these analyses show elevated Nb contents of up to 4.5 wt.% Nb 2O 5 and unusually high W contents of 1–2 wt.% WO 3. A strong Eo-Alpine metamorphic overprint led locally to the formation of the assemblage chlorite + dolomite + calcite + ilmenite ± zirconolite II ± geikielite + Fe-sulfides. Late zircon grew locally, presumably as the last Zr-bearing mineral in the carbonates which formed during Permian contact metamorphism. The Zr-mineral sequence baddeleyite–zirconolite–zircon implies an increasing a(SiO 2) and fCO 2 during the growth of these minerals. Electron-microprobe dating of zirconolite (Zirc I) yields a weighted average age of 263 ± 16 Ma and indicates that the HFSE-and REE-rich assemblages formed during the Permian emplacement of the Stubenberg granite. As a result of the subsequent high- P Eo-Alpine metamorphic overprint (111 ± 15 Ma), HFSE and REE were locally re-mobilized leading to dissolution of Zirc I and reprecipitation of the REE and Nb-rich overgrowths of Zirc II.

Pyrrhotite pTRM acquisition in metamorphic limestones in the light of microscopic observations

The potential of pyrrhotite as a recorder of successive, independent pTRMs in graphite bearing metacarbonates depends on the amount and composition of metamorphic fluids. During its formation very low amounts of H 2S dominated fluids will favour an in situ epitaxy of pyrrhotite at the expense of iron bearing minerals in its surroundings. The resulting particle assemblage of pyrrhotite is within the single domain grain-size range and exhibits a broad blocking temperature spectrum. With increasing fluid content the desulfidation of pyrite controls the pyrrhotite formation and results in large crystals up to several millimeters, where independent pTRMs cannot be expected. As both types are present in regional and contact metamorphic rocks, the existence of pervasive fluids determines whether or not pyrrhotite will be able to record successive pTRMs, i.e. changes of the earth magnetic field (EMF) during metamorphic cooling.

An integrated thermodynamic mixing model for sphalerite geobarometry from 300 to 850°C and up to 1 GPa

An asymmetric, Margules-type, solid solution model was used to model the mixing behavior of Fe-Zn sphalerites. The model is based on an analysis of experimental results from fifteen independent data sources. After a careful, stepwise, analysis of the available runs, the solid solution model was constrained using a refined experimental database of 279 experiments which were simultaneously regressed to obtain the excess parameters and a general geobarometric equation. The model indicates that, when pressures are low, the value of γ FeS Sph, which is always greater than one, is higher at low FeS contents and an increase in temperature causes it to decline. However, for certain compositions γ FeS Sph values might be slightly less at low T than at high T. This behavior is corrected when pressure increases, regardless of the composition. The excess Gibbs free energy has positive values at any P&T while it is asymmetric. Pressure increases the value of the excess free energy. On the other hand, the Gibbs free energy of mixing is always negative, with a single minimum that tends to move towards Fe-poorer compositions as the pressure goes up. An increase in temperature leads to a more negative Gibbs free energy mixing function suggesting that increasingly Fe-poorer sphalerite would be expected at high temperatures and pressures. The application of the solid solution model to a selection of case-studies provided results which are consistent with independent pressure estimates. However, the pressure determinations for sphalerite + pyrite + pyrrhotite and sphalerite + pyrrhotite assemblages are very sensitive to uncertainties in the composition of the phases involved and, to a lesser extent, to temperature. The results of the application of the model to a field case (scheelite-mineralized Hercynian veins from the Central Pyrenees) were also consistent when compared with independent pressure-constraining silicate assemblages. Thus, the solid solution model described in this paper provides a workable framework with which to compute the pressures of the formation of rocks over a wide range of geological conditions.

Experimental determination of the hydrothermal solubility of ReS2 and the Re-ReO2 buffer assemblage and transport of rhenium under supercritical conditions.

To understand the aqueous species important for transport of rhenium under supercritical conditions, we conducted a series of solubility experiments on the Re–ReO2 buffer assemblage and ReS2. In these experiments, pH was buffered by the K–feldspar–muscovite–quartz assemblage; in sulfur-free systems was buffered by the Re–ReO2 assemblage; and and in sulfur-containing systems were buffered by the magnetite–pyrite–pyrrhotite assemblage. Our experimental studies indicate that the species ReCl40 is dominant at 400°C in slightly acidic to near-neutral, and chloride-rich (total chloride concentrations ranging from 0.5 to 1.0 M) environments, and ReCl3+ may predominate at 500°C in a solution with total chloride concentrations ranging from 0.5 to 1.5 M. The results also demonstrate that the solubility of ReS2 is about two orders of magnitude less than that of ReO2. This finding not only suggests that ReS2 (or a ReS2 component in molybdenite) is the solubility-controlling phase in sulfur-containing, reducing environments but also implies that a mixing process involving an oxidized, rhenium-containing solution and a solution with reduced sulfur is one of the most effective mechanisms for deposition of rhenium. In analogy with Re, TcS2 may be the stable Tc-bearing phase in deep geological repositories of radioactive wastes.

Petrography, mineralogy and geochemistry of the Zarshuran Carlin-like gold deposit, northwest Iran

Gold mineralisation at Zarshuran, northwestern Iran, is hosted by Precambrian carbonate and black shale formations which have been intruded by a weakly mineralised granitoid. Granitoid intrusion fractured the sedimentary rocks, thereby improving conditions for hydrothermal alteration and mineralisation. Silicification is the principal hydrothermal alteration along with decalcification and argillisation. Three hydrothermal sulphide mineral assemblages have been identified: an early assemblage of pyrrhotite, pyrite and chalcopyrite; then widespread base metal sulphides, lead-sulphosalts and zoned euhedral arsenical pyrite; and finally late network arsenical pyrite, massive and colloform arsenical pyrite, colloform sphalerite, coloradoite, and arsenic–antimony–mercury–thallium-bearing sulphides including orpiment, realgar, stibnite, getchellite, cinnabar, lorandite and a Tl-mineral, probably christite. Most of the gold at Zarshuran is detectable only by quantitative electron microprobe and bulk chemical analyses. Gold occurs mainly in arsenical pyrite and colloform sphalerite as solid solution or as nanometre-sized native gold. Metallic gold is found rarely in hydrothermal quartz and orpiment. Pure microcrystalline orpiment, carbon-rich shale, silicified shale with visible pyrite grains and arsenic minerals contain the highest concentrations of gold. In many ways Zarshuran appears to be similar to the classic Carlin-type sediment-hosted disseminated gold deposits. However, relatively high concentrations of tellurium at Zarshuran, evidenced by the occurrence of coloradoite (HgTe), imply a greater magmatic contribution in the mineralising hydrothermal solutions than is typical of Carlin-type gold deposits.