FeS Content Of Sphalerite Research Articles

A Late Jurassic Epithermal Pb-Zn Deposit: Insights from Rb-Sr Dating of Quartz-Hosted Fluid Inclusions and Sphalerite Chemical Composition

The Kangjiawan Pb-Zn deposit, situated within the Shuikoushan polymetallic ore field in Changning, Hunan Province, China, is a large-scale Pb-Zn deposit unearthed in 1976. Based on detailed geological field investigations, this study presents the results of the Rb-Sr isotopic dating, electron probe microanalyses (EPMAs), and LA-ICP-MS analyses of the Kangjiawan Pb-Zn deposit in order to determine the ore-forming age and the occurrence of trace elements in sphalerite and thereby constrain the genesis of the deposit. The Rb-Sr dating of quartz-hosted fluid inclusions yielded an Rb-Sr isochron age of 150 ± 4 Ma, with an initial 87Sr/86Sr ratio of 0.71101 ± 0.00008 (MSWD = 1.1), suggesting that the Pb-Zn mineralization of the Kangjiawan deposit took place during the Late Jurassic, coeval with the magmatic activities within the ore field. EPMA and LA-ICP-MS analyses showed that Fe, Mn, and Cd were primarily incorporated into the sphalerite lattice through isomorphous substitution. Specifically, Fe and Mn substituted for Zn, whereas Cd replaced both Fe and Zn. Other elements such as Cu, Sb, and Sn occurred within the sphalerite lattice through mineral micro-inclusions or isomorphic substitution. EPMAs and LA-ICP-MS results showed that the FeS contents in sphalerite were less than 14.33%, with corresponding ore-forming temperatures below 259 °C. The LA-ICP-MS results showed that sphalerites from the Kangjiawan Pb-Zn deposit had relatively high Ga/In ratios ranging from 0.01 to 144, providing further support for medium-to-low-temperature mineralization. The trace element compositions of sphalerites from the Kangjiawan Pb-Zn deposit exhibit skarn-type characteristics, suggesting a potential association with contemporary magmatic activities within the Shuikoushan ore field. During the Late Jurassic, extensive granitic magmatic activities occurred in the study area. At the late stage of magma crystallization, hydrothermal fluid containing Pb and Zn precipitated at medium-to-low temperatures and generated the Kangjiawan Pb-Zn deposit.

Geology, Alteration, and Mineralization of the Kay Tanda Epithermal Gold Deposit, Lobo, Batangas, Philippines

AbstractThe Kay Tanda epithermal Au deposit in Lobo, Batangas is one of the Au deposits situated in the Batangas Mineral District in southern Luzon, Philippines. This study aims to document the geological, alteration, and mineralization characteristics and to determine the age of the mineralization, the mechanism of ore deposition, and the hydrothermal fluid characteristics of the Kay Tanda deposit. The geology of Kay Tanda consists of (i) the Talahib Volcanic Sequence, a Middle Miocene dacitic to andesitic volcaniclastic sequence that served as the host rock of the mineralization; (ii) the Balibago Diorite Complex, a cogenetic intrusive complex intruding the Talahib Volcanic Sequence; (iii) the Calatagan Formation, a Late Miocene to Early Pliocene volcanosedimentary formation unconformably overlying the Talahib Volcanic Sequence; (iv) the Dacite Porphyry Intrusives, which intruded the older lithological units; and (v) the Balibago Andesite, a Pliocene postmineralization volcaniclastic unit. K‐Ar dating on illite collected from the alteration haloes around quartz veins demonstrated that the age of mineralization is around 5.9 ± 0.2 to 5.5 ± 0.2 Ma (Late Miocene). Two main styles of mineralization are identified in Kay Tanda. The first style is an early‐stage extensive epithermal mineralization characterized by stratabound Au‐Ag‐bearing quartz stockworks hosted at the shallower levels of the Talahib Volcanic Sequence. The second style is a late‐stage base metal (Zn, Pb, and Cu) epithermal mineralization with local bonanza‐grade Au mineralization hosted in veins and hydrothermal breccias that are intersected at deeper levels of the Talahib Volcanic Sequence and at the shallower levels of the Balibago Intrusive Complex. Paragenetic studies on the mineralization in Kay Tanda defined six stages of mineralization; the first two belong to the first mineralization style, while the last four belong to the second mineralization style. Stage 1 is composed of quartz ± pyrophyllite ± dickite/kaolinite ± diaspore alteration, which is cut by quartz veins. Stage 2 is composed of Au‐Ag‐bearing quartz stockworks associated with pervasive illite ± quartz ± smectite ± kaolinite alteration. Stage 3 is composed of carbonate veins with minor base metal sulfides. Stage 4 is composed of quartz ± adularia ± calcite veins and hydrothermal breccias, hosting the main base metal and bonanza‐grade Au mineralization, and is associated with chlorite‐illite‐quartz alteration. Stage 5 is composed of epidote‐carbonate veins associated with epidote‐calcite‐chlorite alteration. Stage 6 is composed of anhydrite‐gypsum veins with minor base metal mineralization. The alteration assemblage of the deposit evolved from an acidic mineral assemblage caused by the condensation of magmatic volatiles from the Balibago Intrusive Complex into the groundwater to a slightly acidic mineral assemblage caused by the interaction of the host rocks and the circulating hydrothermal waters being heated up by the Dacite Porphyry Intrusives to a near‐neutral pH toward the later parts of the mineralization. Fluid inclusion microthermometry indicates that the temperature of the system started to increase during Stage 1 (T = 220–250°C) and remained at high temperatures (T = 250–290°C) toward Stage 6 due to the continuous intrusion of Dacite Porphyry Intrusives at depth. Salinity slightly decreased toward the later stages due to the contribution of more meteoric waters into the hydrothermal system. Boiling is considered the main mechanism of ore deposition based on the occurrence of rhombic adularia, the heterogeneous trapping of fluid inclusions of variable liquid–vapor ratios, the distribution of homogenization temperatures, and the gas ratios obtained from the quantitative fluid inclusion gas analysis of quartz. Ore mineral assemblage and sulfur fugacity determined from the FeS content of sphalerite at temperatures estimated by fluid inclusion microthermometry indicate that the base metal mineralization at Kay Tanda evolved from a high sulfidation to an intermediate sulfidation condition.

ORE MINERALS FROM KUROKO TYPE DEPOSIT OF TOYA-TAKARADA MINE, HOKKAIDO, JAPAN

Toya-Takarada mine is Au- and Ag-rich Kuroko-type deposit located in Takarada, Toya-mura, southwest Hokkaido, Japan. The deposits were hosted in rhyolitic tuff and mudstone of Middle Miocene age. Ore samples of fine-grained black ore, vuggy black-yellow ore, granular vuggy black ore, quartz-sulfide ore and massive quartz-barite ore were studied to identify the ore minerals association in the Toya-Takarada mine. The ore minerals are dominated by sphalerite, galena, chalcopyrite and pyrite with fewer amounts of electrum, tetrahedrite-tennantite, and other sulfosalt minerals with secondary mineral of covellite.The quantitative chemical analysis of ore minerals by EPMA indicated that FeS contents in sphalerite is low (0.3-1.2 mol.%) in all kinds of ore samples. Small grains of electrum as inclusions in pyrite are identified in vuggy black-yellow ore with Ag content around 32-33 atm %.In general, the silver minerals in Kuroko-type deposits occurred mainly in the black and yellow ores zone dominantly composed of sphalerite, galena, pyrite, chalcopyrite and barite as a form of electrum and/or argentian tetrahedrite-tennantite series. Thus, the massive quartz-barite ore sample of Toya-Takarada mine are also contain some rare silver sulfosalt minerals such as proustite, Cu-rich pearceite, geocronite-jordanite and fizelyite. Those minerals were found together in association with sphalerite. It seems that sphalerite was crystallized first followed by proustite and Cu-rich pearceite, then geocronite-jordanite and fizelyite are crystallized later.Sphalerites from quartz-sulfide ore of Toya-Takarada contain some fluid inclusions and measured homogenization temperatures are in the range of 164-247°C (av. 208°C) with salinity ranging from 1.9 to 4.7 wt.% NaClequiv. (av. 3.9 wt.% NaClequiv.). The mineral assemblage, iron content in sphalerite and silver content in electrum were indicated that sulfur fugacity was slightly higher during ore mineralization in Toya-Takarada mine.

Occurence of Zn-Pb Deposits in Danjang-Myeon, Milyang Area

New occurrences of large-scaled Zn-Pb deposits are recently found in the Danjang-myeon, Milyang. They are skarn-type deposits which replaced the intercalated limestone beds in the Jeonggaksan Formation. This study aims at characterizing occurrences, mineralogy, and chemistry of Zn-Pb ores and skarn minerals. Skarn orebodies are mainly found in 3 areas, named Gukjeon-ri, Gorye-ri, and Gucheon-ri orebodies, where sphalerite found as main ore mineral in 200-300 m in height and amount of galena increases as altitude does. Ores are dark grey to dark green in color and closely related with clinopyroxene zone. They occur with hedenbergite, grossular, actinolite, epidote, and small amounts of axinite, calcite, and quartz. Main ore mineral is sphalerite which includes tiny spotted grains of galena and chalcopyrite and becomes rich in grade in association with clinopyroxene and epidote. FeS contents in sphalerite show relatively wide range between 1.53 and 23.07 mole%, whose contents intend to increase towards biotite granite known as ore-related igneous rocks. CdS contents are in the range of 0.22-0.93 mole%, showing decrease tendency from southwest (Gukjeon-ri) to northeast (Gucheon-ri). Zn-Pb deposits developed in Danjang-myeon reveal decrease in temperature with increase of altitude, leading to gradual changes in compositions of ore and skarn minerals.

Petrology, Geochemistry, and Fluid Inclusion Microthermometry of Sphalerite from the Laloki and Federal Flag Strata‐Bound Massive Sulfide Deposits, Papua New Guinea: Implications for Gold Mineralization

AbstractThe Laloki and Federal Flag deposits are two of the many (over 45) polymetallic massive sulfide deposits that occur in the Astrolabe Mineral Field, Papua New Guinea. New data of the mineralogical compositions, mineral textures, and fluid inclusion studies on sphalerite from Laloki and Federal Flag deposits were investigated to clarify physiochemical conditions of the mineralization at both deposits. The two deposits are located about 2 km apart and they are stratigraphically hosted by siliceous to carbonaceous claystone and rare gray chert of Paleocene–Eocene age. Massive sulfide ore and host rock samples were collected from each deposit for mineralogical, geochemical, and fluid inclusion studies. Mineralization at the Laloki deposit consists of early‐stage massive sulfide mineralization (sphalerite‐barite, chalcopyrite, and pyrite–marcasite) and late‐stage brecciation and remobilization of early‐stage massive sulfides that was accompanied by late‐stage sphalerite mineralization. Occurrence of native gold blebs in early‐stage massive pyrite–marcasite‐chalcopyrite ore with the association of pyrrhotite‐hematite and abundant planktonic foraminifera remnants was due to reduction of hydrothermal fluids by the reaction with organic‐rich sediments and seawater mixing. Precipitation of fine‐grained gold blebs in late‐stage Fe‐rich sphalerite resulted from low temperature and higher salinity ore fluids in sulfur reducing conditions. In contrast, the massive sulfide ores from the Federal Flag deposit contain Fe‐rich sphalerite and subordinate sulfarsenides. Native gold blebs occur as inclusions in Fe‐rich sphalerite, along sphalerite grain boundaries, and in the siliceous‐hematitic matrix. Such occurrences of native gold suggest that gold was initially precipitated from high‐temperature, moderate to highly reduced, low‐sulfur ore fluids. Concentrations of Au and Ag from both Laloki and Federal Flag deposits were within the range (<10 ppm Au and <100 ppm Ag) of massive sulfides at a mid‐ocean ridge setting rather than typical arc‐type massive sulfides. The complex relationship between FeS contents in sphalerite and gold grades of both deposits is probably due to the initial deposition of gold on the seafloor that may have been controlled by factors such as Au complexes, pH, and fO2 in combination with temperature and sulfur fugacity.

Ag‐Cu‐Pb‐Bi‐S Minerals Newly Discovered from the Ohori Base Metal Deposit, Yamagata Prefecture, NE Japan: Implications for Bi‐metallogenesis in the Green‐Tuff Region

AbstractThe Ohori deposit, one of the base metal deposits in the Green‐Tuff region, NE Japan, is composed of two types of mineralization; a skarn‐type (Kaninomata orebody) made by the replacement of the Miocene calcareous layer, and a vein‐type (Nakanomata orebody). While the ore mineral assemblage of the deposit (chalcopyrite, pyrite, sphalerite and galena) has been known for being rather simple, some Pb‐Bi‐S minerals have been discovered for the first time in the present study. The minerals mainly occur in the chalcopyrite‐rich ores of both orebodies. They essentially belong to the Pb‐Bi‐S system and contain Cu and Ag in minor amounts, which correspond to the lillianite–gustavite solid solution series (phases Z and X), cosalite, neyite, felbertalite, krupkaite and Bi‐bearing galena. The chalcopyrite‐rich (Bi‐bearing) ores from both orebodies are richer in chalcopyrite, pyrite and chlorite, and have higher homogenization temperatures (>300°C) of fluid inclusions, and higher FeS contents in sphalerite compared to the Bi‐free ores. In the Green‐Tuff region, Bi‐minerals have been reported from many base metal deposits. Most of these Bi‐bearing ore deposits are referred to as xenothermal‐type deposits, and are characterized by the following common features; composite mineralization of high‐ and low‐temperatures in the shallower environments, and close relationships with the Tertiary granitic rocks. The whole mineralization at the Ohori deposit also has a similar xenothermal character because of the coexistence of high‐temperature chalcopyrite‐rich ores with Pb‐Bi‐S minerals, which were formed by the influence of the Tertiary granitic rocks at a shallow depth.

Petrology, Geochemistry, and Genesis of the Copper zone at the Brunswick No. 6 Volcanogenic Massive Sulfide Deposit, Bathurst Mining Camp, New Brunswick, Canada

A Cu-rich pyrrhotite-pyrite zone that occurs at the base of the Brunswick No. 6 Pb-Zn massive-sulfide lens is part of a south-plunging synclinal sheath fold. To the north of the unmined open pit, this Cu zone’s preliminary ore-reserve calculations indicate >1.7 Mt grading 0.9% Cu. Pyrite, pyrrhotite, chalcopyrite, sphalerite, and magnetite are the major opaque minerals, and are accompanied by trace amounts of arsenopyrite-cobaltite, bismuthinite, and cassiterite. Most of the chalcopyrite and pyrite is fine grained, but cataclastically deformed pyritic porphyroblasts, porphyroclasts, and boudins of pyritic massive sulfide are hosted by a matrix of remobilized and recrystallized chalcopyrite-bearing pyrrhotite. Eleven 1.6 m-long intervals were sampled near the mid point of massive sulfide intersections from 10 diamond-drill holes (DDH) intersecting the Cu zone. Re-assaying of these samples yielded averages of 0.96% Cu, 0.10% Zn, 0.06% Pb, 12.2 g/t Ag, 0.04% Bi, 0.08 g/t Au, 0.03% As, 0.01% Sb, and Sn values below the detection limit of 50 ppm. Six 1.6 m core intervals in the exhalative Pb-Zn zone (DDH B-259) were also re-assayed, yielding averages of 0.79% Cu, 1.08% Pb, 3.46% Zn, 0.051% Bi, 58.6 g/t Ag, 0.50 g/t Au, 0.311% As, 0.063% Sb, and Sn values of 80 to 670 ppm. The concentrations of Zn, Cd, Pb, Ag, As, Sb, Mo, Ca, and Sr decrease with increased depth into the sheath-shaped basal Cu zone, which has notably higher Ba, Se, and Te contents. In contrast to the chemical differences, the bulk δ 34 S values for both zones range from 13‰ to 15‰, which are similar to the values for other deposits within the Tetagouche Group. The contrasting distribution of major and trace elements suggests that the zoning is a syngenetic feature, modified by D 1 deformation and related metamorphism. Geobarometry of sphalerite shielded within pyrite indicates peak D 1 pressures of >7 kb, similar to those at the nearby Brunswick No. 12 deposit. Late re-equilibration in the presence of pyrrhotite resulted in very high mole % FeS contents in sphalerite. The high Cu and low Pb-Zn contents within the Cu zone compared with those in the overlying, contiguous Zn-Pb-Ag exhalative massive sulfide zone, is a pattern commonly observed in proximal VMS deposits. The metals’ distribution is interpreted to reflect a higher temperature zone-refining within the massive sulfides, which are located above a stockwork feeder zone that has been transposed to the north. The interpreted zone refining is consistent with: (1) the relatively high pyrrhotite-to-pyrite abundance and the higher abundance of chalcopyrite; (2) lower sphalerite, galena, tetrahedrite-tennantite, arsenopyrite, and cassiterite abundances in the Cu zone; and (3) the low S/Se ratio typical of other Cu-rich zones. This interpretation is consistent with the similarity of δ 34 S values for the Cu and Pb-Zn zones.

The composition of sphalerite and associated sulfides in reactions of the Cu–Fe–Zn–S, Fe–Zn–S and Cu–Fe–S systems at 1 bar and temperatures between 250 and 535 °C

Sphalerites and sulfides in selected reaction assemblages of the Cu–Fe–Zn–S, Cu–Fe–S, and Fe–Zn–S systems were crystallized in molten salt fluxes contained in evacuated capsules at temperatures between ∼250 and ∼535 °C. The combined compositional data for sphalerite indicate large increases in the mol% FeS content and the Fe/Cu ratio with decreasing sulfur fugacity at given temperatures. Pyrite and the associated copper-iron sulfides show parallel increases for Cu/Fe ratios in the Cu–Fe–S system, and increases for Fe/Cu and Fe/Zn ratios in the Cu–Fe–Zn–S system. These compositional changes occur over a range of sulfur fugacity–temperature conditions that include many sulfide deposits. A log fS 2–temperature (1000/K) composition map has been constructed for sphalerite using Lusk and Bray's (2002) data for a sequence of buffer reactions in the Cu–Fe–S and Fe–S systems. Accordingly, sphalerites with mean mol% FeS compositions of ∼1.7 (i.e. for the reaction: pyrite+idaite=bornite+S (vapor)), ∼4.4 (i.e. pyrite+bornite=chalcopyrite+S), ∼14.4 (i.e. pyrite+chalcopyrite=cubanite (iss)+S), 22.9 (i.e. pyrite+cubanite (iss)=pyrrhotite+cubanite (iss)+S), and ∼21.5 (i.e. pyrite=pyrrhotite+S) were obtained. A slight increase in the mol% FeS content of sphalerite with decreasing temperature was obtained for the pyrite=pyrrhotite+S buffer reaction at temperatures below ∼550 °C. The copper contents of sphalerite are generally low, ranging between ∼3 and ∼0.5 mol% CuS, and are highest in sphalerites associated with bornites. The equation: log 10 fS 2=11.01−9.49 (1000/K)+[0.187−0.252 (1000/K)] (mol% FeS)+[0.35−0.2 (1000/K)] (mol% CuS in sphalerite) can be applied to sphalerites that have equilibrated with buffer assemblages containing pyrite, or pyrite and pyrrhotite, in both the Fe–Zn–S and Cu–Fe–Zn–S systems at temperatures (K is in Kelvin) between 250 and 550 °C at 1 bar. This equation incorporates present data and a selection of published experimental data that is fitted to within 10%.

北海道光竜鉱山金銀石英脈の流体包有物

The Koryu gold-silver mine is located approximately at 141°19' of east longitude and 42°51' of north latitude in southwestern Hokkaido. The ore deposits are of epithermal gold-silver-quartz vein type, and they are composed of the No. 1-No. 7 veins hosted by the Miocene black mudstone. In this study, we have determined homogenization and ice melting temperatures of fluid inclusions in quartz, and chemical compositions of some ore minerals by microprobe analyses for the interpretation of mechanism of gold-silver deposition in the two veins (No. 1 and No. 3). Three stages of mineralization are discriminated on the basis of brecciation and cross-cutting relations in the vein, and gold-silver deposition occurred predominantly during the middle stage. The ores ordinarily exhibit banded structure composed of quartz, ±adularia, ±clay minerals, ±carbonates, and gold-silver and other metallic minerals (ginguro). The ore minerals are electrum (44-74 atom.%Ag), aguilarite (Se/(S+Se) atomic ratio : 0.25-0.51), pyrargyrite-proustite, pearceite-polybasite, miargyrite, Ag-bearing tetrahedrite (22-24 wt.%Ag) , galena, sphalerite (0.6-1.2 mole% FeS), chalcopyrite and pyrite etc. Ag content of electrum tends to decrease slightly toward the upper levels in the No. 3 vein, whereas FeS content of sphalerite is invariable between the upper and lower levels. Fluid inclusions observed in quartz of the middle stage are two phase and liquidrich type. Homogenization temperatures for the No. 1 vein are mainly 220°-270°C with high frequency in the range 250°-260°C. The temperatures for the No. 3 vein are about the same as those for the No. 1 vein, but show higher values (250°-280°C) at 30 mL. Ice melting temperatures of inclusion fluids for the No. 1 and No. 3 veins are from −0.4°C to −1.3°C, and from −0.5°C to −1.4°C, respectively. Spatial variations of these homogenization and ice melting temperatures, together with the data previously reported, would indicate that the relatively CO2-rich ore fluids had boiled on the way to form ore shoots.

Sulphide mineralogy, metamorphism and deformation in the Matchless massive sulphide deposit, Namibia

The sulphide mineralogy of the Matchless massive sulphide deposit has been reviewed and the morphology, mineral chemistry and paragenesis of the opaque minerals described and related to genetic processes. Amphibolite facies metamorphism at 510–540 °C and 6–6.5 kbar and accompanying polyphase penetrative deformation of the ores has resulted in prograde corrosion and replacement of pyrite, onto which is superimposed retrograde overgrowths and recrystallisation and annealment of porphyroblastic pyrite. Pyrite exhibits examples of both brittle and ductile deformation, although late retrograde cataclasis of recrystallised pyrite is the dominant preserved texture. Contrasts in competence between pyrite, the more ductile sulphides and silicates has resulted in the most extreme example of deformation: the ‘breccia ores’, in which clasts of silicate minerals have been enclosed in a matrix of chalcopyrite and pyrrhotite. The FeS content of sphalerite (spFeS) depends, to a large extent on the coexisting iron sulphides (spFeS: 7.22 ± 1.22% in samples where pyrite is dominant, 16.78 ± 0.93% where pyrrhotite is dominant). Only where the sphalerite has been buffered by pyrite and pyrrhotite is application of the sphalerite geobarometer possible. In such cases, the sphalerite compositions have apparently not re-equilibrated during retrograde metamorphism. In four samples, spFeS values of 10.5–13.5% give pressure estimates of 6–8 kbar. Such estimates are consistent with the tectonic history of the region.

Sulfidation equilibria as guides to gold mineralization in volcanogenic massive sulfides; evidence from sulfide mineralogy and the composition of sphalerite

Hydrothermal fluids responsible for the formation of volcanogenic massive sulfides may transport gold in solution as Au(HS) 2 (super -) at low temperatures ( or =350 degrees C), low pH, and elevated salinities (> or =1 m NaCl). The documented fluid chemistry of active hydrothermal vents on the modern seafloor confirms that gold transport is mainly due to Au(HS) 2 (super -) , and the calculated solubility of gold as Au(HS) 2 (super -) is highest in low-temperature (150 degrees -250 degrees C) vent fluids at elevated oxygen and sulfur activities. Sulfides which coprecipitate with gold commonly have properties which can be related to the temperature and sulfidation state of the hydrothermal fluids and therefore reflect conditions which were favorable for the transport or deposition of gold as Au(HS) 2 (super -) .A comparison of gold grades with sulfide mineral equilibria and the FeS contents in sphalerite from a variety of deposits indicates that gold enrichment is closely related to the temperature-a (sub S 2 ) conditions in the ore-forming fluids. Sea-floor sulfides range from sulfur-rich, pyrite-marcasite assemblages to relatively sulfur-deficient, pyrrhotite-isocubanite assemblages, with coexisting sphalerite containing 0 to 55 mole percent FeS. Significant gold enrichment occurs exclusively in low-temperature ( 300 degrees C contain Ferich sphalerite (1-5 mole % FeS) and gold grades < 1 ppm Au. Archean Cu-Zn deposits at Noranda, Quebec, consist mainly ofpyrite-pyrrhotite ores containing 0.5 to 1 ppm Au together with Fe-rich sphalerite (10-12 mole % FeS). However, pyritic ores which occur stratigraphically up-section commonly contain 1 to 3 ppm Au and relatively Fe-poor sphalerite (5 mole % FeS). Different sulfide mineral equilibria and FeS contents in sphalerite are interpreted to reflect the same physical and chemical conditions which influence gold grades and suggest that petrologic indicators of the sulfidation state may be useful guides to gold mineralization in volcanogenic massive sulfides.

Oxide-sulfide-silicate equilibria associated with metamorphosed ore deposits; Part II, Pelitic and felsic volcanic terrains

Calibrated equilibria are presented which document the responses of silicate-oxide-sulfide assemblages to variations in temperature, pressure, f (sub O 2 ) , f (sub S 2 ) , and f (sub H 2 O) in and around metamorphosed ore deposits in pelitic and felsic volcanic terrains. Using equations developed in Part I and calibrations based largely on published experimental results, a FeO -a (sub Al 2 SiO 5 ) , f (sub O 2 ) -X Fe , and AFM diagrams were produced for temperature and pressure conditions of 500 degrees C, 4 kb; 600 degrees C, 2, 4, and 6 kb; and 700 degrees C, 4 kb with X (sub H 2 O) = 1.0. Results of the calculations indicate that with increasing f (sub O 2 ) and/or f (sub S 2 ) values near ore deposits there is a general trend for staurolite and garnet to disappear and chlorite, (sub Al 2 SiO 5 ) , and cordieritc to become more common. Superimposed on these changes in assemblages are steadily declining X Fe values for Fe-bearing silicates. Also correlated with these changes are various transitions in oxide-sulfide assemblages and declining X FeS contents in sphalerite and pyrrhotite. Variations in volatile equilibria due to the presence of ore deposits during metamorphism consist of decreasing fugacity values for H 2 and H 2 S in vicinity of the ore and increasing values for SO 2 . Comparisons with published field studies of metamorphosed deposits indicate good agreement between predicted and observed patterns in silicate-oxide-sulfide assemblages and mineral chemistries.

The formation of the volcanic-hosted massive sulfide ore deposit at Rosebery, Tasmania

The Rosebery pyritic zinc-lead-copper-silver-gold orebody occurs in dominantly felsic volcanic rocks of Cambrian age in western Tasmania. Ore formation occurred in a marine environment following eruption of a thick pyroclastic unit consisting mainly of welded, ashflow tuffs, and subsequent subsidence. The main orebody consists of a discontinuous massive sulfide horizon and in the southern part of the mine there is a generalized metal zonation with a lower and central Cu-Fe-rich zone surrounded and overlain by Zn-Pb-Ag-rich ore. Barite-sulfide ore occurs as separate lenses higher in the sequence separated from the sulfide orebody by unmineralized, cleaved siltstone.The orebody is underlain by an extensive zone of alteration marked by depletion of Na and Sr and enrichment in Rb, K, Mg, Mn, and H 2 O. The pyrite and chalcopyrite content of the altered footwall tuff and the Co content of the pyrite is highest beneath the Fe- and Cu-rich zones in the sulfide orebody and a feeder is inferred in this area.The delta 34 S values for sulfides in the sulfide orebody are higher in the richest Zn-Pb-Ag ore (>14.1ppm) than the Fe-Cu ore (>7.8ppm), a pattern paralleled by a decrease in the Fe/Fe + Mg + Mn ratio of chlorite and a decrease in the FeS content of sphalerite. The delta 34 S values for sulfides in the barite ore are even higher (14.5 to 19.8ppm) and the FeS content of sphalerite lower. Barite throughout the barite and sulfide orebodies is fairly constant between 34.6 and 41.2 per mil, but that in the sulfide ore appears to be epigenetic. The delta 34 S values for barite-sulfide pairs indicate a temperature range of 255 degrees to 298 degrees C for the barite orebody, but the temperature of barite ore formation was probably about 250 degrees C.The use of delta 34 S values for sulfides as stratigraphic markers indicates that the sulfide orebody is diachronous. It is possible to account for the metal and delta 34 S distribution by assuming that the ore solutions are buoyant on reaching the sea floor during the Fe- and Cu-rich phases but show reversing buoyancy during the Pb-Zn-rich phase.The coexisting arsenopyrite-pyrite-chlorite-quartz assemblage in the sulfide orebody defines a range of aO (sub 2 (super -) ) T conditions using the six-component solid solution chlorite model of Walshe and Solomon (1981). The maximum indicated temperature of 300 degrees C is consistent with the minimum temperature required by iron and copper solubilities. However, the chlorite compositions may be postdepositional, as seems to be the case for the FeS contents of sphalerite. The delta 34 S (sub Sigma S) for the sulfide orebody is probably derived by almost complete reduction of Cambrian seawater sulfate mixed with sulfur leached from underlying rocks. The delta 34 S values for the barite orebody may represent partial reduction of seawater sulfate.The ore fluid is supposed to have been generated by convective circulation of seawater in a rock pile heated by a Cambrian granitoid pluton.

Pressures of formation of iron meteorites from sphalerite compositions

The FeS content of sphalerite, a minor phase in some meteorites, is strongly dependent on pressure when the sphalerite is in equilibrium with troilite. We have determined FeS contents for sphalerite in Bogou, Gladstone, Sardis and Odessa ; these, together with published data on Odessa and Campo del Cielo, have been used to calculate pressures of formation of meteorites, assuming that FeS-diffusion in sphalerite ceases at 350°C. Calculated pressures range from 0.2 to 3.1 kbar, corresponding to formation at centres of chondritic objects from 140 to 410 km in radius, or metallic objects of from 50 to 200 km radius. Formation at shallower depths would require the objects to have been correspondingly larger. All meteorites in this study are members of Ga-Ge group I. Inverse correlation between Ge content and pressure of formation suggests formation at various depths in a compositionally zoned (fractionated?) object. Comparison between our pressure estimates and radii estimated from cooling rates ( Fricker et al., 1970, Geochim. Cosmochim. Acta 34, 475–492) suggests that Odessa, Bogou and possibly other Group I meteorites formed in a single object with a radius between 400 and 180 km and an overall composition richer in metal than average chondrites.

Changes in iron content in sphalerites in relation to structural conditions and localization of ores in Kholst deposit

A consistent, relatively rapid decrease in FeS molecular content of sphalerites at decreasing depth is recognizable in upper parts of this structurally controlled Pb-Zn ore deposit, as an expression of tectonic calm and continuity of the productive stages of the mineralization. Elsewhere, in the depths, abrupt changes in quantity of isomorphous iron in sphalerites, within a relatively short depth-span, signify intensive movements during deposition of the ore. Wherever the structures are bent and healed by early quartz sufficiently to retard circulation of the solutions and to promote their cooling, variations in the FeS content of sphalerite at different depths may be related to thermal “knickpoints” of the ascending solutions. Nonetheless, FeS molecular content of sphalerites is no clue to the temperature of their crystallization. There is no relationship between the depth at which sphalerite was formed and the isomorphous Cd or Mn in crystal structures of the mineral. — V.P. Sokoloff.