Mole Percent FeS Research Articles

Phase relations in the Fe-Zn-S system to 5 kbars and temperatures between 325 degrees and 150 degrees C

Sphalerite, pyrite, and pyrrhotite were recrystallized together in a saturated ammonium iodide solution at 150 degrees to 325 degrees C and 0.4 to 5.1 kbars. The data were obtained from ten sets of experiments comprising gold capsules stacked consecutively within the bores of cold seal pressure vessels which were heated for periods of between 300 and 750 days. Five capsules contained magnetite as an additional reactant.A P-T-composition map has been constructed for sphalerite buffered by pyrite and hexagonal pyrrhotite. The upper P-T range for the present study includes the lower temperature boundary for the sphalerite geobarometer which has been located near 285 degrees C at 0.5 kbars and 300 degrees C at 5 kbars. The provisional lower P-T range for the present data extends from approximately 180 degrees C at 0.5 kbars to 190 degrees C at 5 kbars. Within the investigated P-T interval, isobaric composition curves for sphalerite show a progressive decrease in iron content with increasing pressure and decreasing temperature.Hexagonal pyrrhotite was the only iron monosulfide identified in the experimental reaction products and was found to increase in iron content with increasing pressure. Its occurrence in these long-term experiments supports earlier predictions that monoclinic pyrrhotite may be a metastable phase down to low temperatures. Hexagonal pyrrhotite may stably invert to monoclinic pyrrhotite at temperatures below 150 degrees C under isotropic stress conditions; however, at higher temperatures monoclinic pyrrhotite appears to be favored by anisotropic stress conditions which produce lattice strain. Under these conditions it may form and persist metastably as the preferred phase below the sphalerite geobarometer boundary. The presence of magnetite does not affect the compositions of sphalerite or pyrrhotite under the conditions of study.The present calibration may be applied to hydrothermal vein deposits provided codeposition of phases can be proven; however, it is expected to have limited application for massive sulfide ores that have prograded or retrograded to temperatures around or below 300 degrees C. This is because the calibration requires buffering by pyrite and hexagonal pyrrhotite, whereas most lower temperature assemblages appear to have been buffered by pyrite and monoclinic pyrrhotite over a range of temperatures, pressures, and anisotropic stress conditions. This latter buffer has restricted sphalerite compositions between around 11 and 12 mole percent FeS.

Internal zonation in a carbonate-hosted Zn-Pb-Ag deposit, Nanisivik, Baffin Island, Canada

Zn-Pb-Ag ores at Nanisivik, northwest Baffin Island, are hosted by upper Proterozoic laminated dolostone of the Society Cliffs Formation. Sulfide orebodies containing pyrite, sphalerite, galena, sparry dolomite, and pyrite pseudomorphous after marcasite are characterized by well-banded ore textures considered to have resulted from the progressive replacement of carbonate wall rock. Textural and mineralogical variations enable the Main orebody to be divided into three vertical and four horizontal ore zones. The upper lens of the Main orebody may be further subdivided into six texturally distinct, laterally extensive mine units.In the eastern upper lens, where textural variations are best developed, fluid inclusions indicate that most ore mineralization took place over the temperature range of 165 degrees to 210 degrees C and involved a brine containing 20 to 38 wt percent NaCl equiv. The sulfur isotope compositions of main- and late-stage pyrite crystals range from (delta 34 S = 27.4 to 28.0 per mil. Iron contents of sphalerite vary from 14 to 0 mole percent FeS, corresponding to well-developed color zoning, and constrain the oxygen activity of the ore fluid to 10 (super -46) to 10 (super -41) at 200 degrees C during sphalerite precipitation. Interbanded primary marcasite and sparry dolomite gangue indicates that the ore fluid fluctuated around pH = 5.Ore formation is modeled based on processes involving in situ reduction of sulfate by hydrocarbons and minor H 2 S encountered at the site of ore deposition. The location and configuration of the orebodies was probably controlled in part by the former position of a horizontal fluid interface. Banding is likely the result of repetitive sulfate reduction, metal precipitation, wall-rock dissolution, and dolomite recrystallization in response to the pulsatory influx of ore fluid. Textural and mineralogic variations probably resulted from slight variations in the availability of reduced sulfur.

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.

Interdiffusion rate of zinc and iron in natural sphalerite

The interdiffusion coefficients of zinc and iron in natural iron-poor and iron-rich sphalerites were determined experimentally using electron microprobe analysis. The diffusion couples were heated in sealed silica tubes for predetermined lengths of time together with f (sub S 2 ) buffering materials such as iron and/or synthetic pyrrhotite at temperatures between 950 degrees and 500 degrees C. The interdiffusion rates were determined from measured compositional profiles across natural sphalerite crystal pairs.The penetration profiles of the diffusing Zn and Fe were clearly observed not only at the contact planes of crystals but also at both free end surfaces of the diffusion column and the side surfaces normal to the diffusion interface. Vapor transport probably played an important role in the Zn-Fe interdiffusion. The Zn-Fe interdiffusion coefficient increases slightly with increasing Fe content of sphalerite as expected from the enlargement of the unit cell with Fe concentration. The Arrhenius relation for interdiffusion rates, D (cm 2 /sec), in sphalerite of N mole percent FeS (2 2 /sec.The interdiffusion rate is not dependent on crystallographic orientation for directions normal to (110) and parallel to (110). A high-pressure run at 5 kbars and 800 degrees C shows that interdiffusion rates slightly decrease with increasing total pressure (log D = -10.5 + or - 0.1 cm 2 /sec at 1 bar pressure to log D = -10.8 + or - 0.2 cm 2 /sec at 5 kbars). A dependency of interdiffusion rates on f (sub S 2 ) was also observed in the 950 degrees and 900 degrees C experiments. D values are slower with increasing fugacity of sulfur (e.g., for 900 degrees C, log D = -9.8 + or - 0.1 cm 2 /sec at f (sub S 2 ) = 10 (super -9.6) bars vs. log D = 10.1 + or - 0.2 cm 2 /sec at f (sub S 2 ) = 10 (super -8.0) bars).

Kuroko-type deposits in the Middle-Cretaceous marginal basin of central Peru

Barite, massive sulfide, and siliceous stockwork deposits of Kuroko type in the Lima region are associated with the Casma Group, a sequence of submarine volcanic rocks of Middle Cretaceous age. These deposits were formed in an ensialic marginal basin with predominantly basaltic to andesitic fill. Volcanic-hosted deposits occur in the entire region; sediment-hosted deposits are restricted to the eastern Casma volcanic facies, which intercalate with limestones and shales deposited on a shelf platform adjacent to the marginal basin. In most cases, mineralization is spatially associated with dacitic domes and tuff breccias with zones of quartz-serieite alteration; the latter have locally been dated at 116 to 106 m.y. by the K/Ar method. Strata-bound deposits of bedded barite, pyrite, sphalerite, and pyrrhotite overlie the feeder zones.The most important deposits of this kind are Leonila-Graciela and Juanita, with 4 million tons of produced barite and 2.5 million tons of production plus reserves of massive sulfide ore. They are located in a roof pendant of folded strata intruded by two plutons of the Coastal batholith. Contact metamorphism of hornblende-hornfels facies affects both ore deposits and wall rocks. K-Ar ages on hornblende-biotite pairs from the granitic rocks indicate that they were emplaced 82 and 65 m.y. ago. Whole-rock ages on postmetamorphic dikes vary between 31 and 39 m.y.P-T conditions for contact metamorphism of hornblende-hornfels facies at Leonila-Graciela are estimated at 2.1 to 2.6 kb and 300 degrees to 500 degrees C on the basis of sphalerite geobarometry, stratigraphic reconstruction, metamorphic mineralogy, and interpretation of discordant K/Ar age patterns. Mole percent FeS in sphalerites increases in a prograde sense from the actinolite zone at Juanita to the biotite-muscovite zone at Gracicla. In massive sulfide specimens it varies correspondingly from 15.4 + or - 0.2 to 17.6 + or - 0.7. Sphalerites from siliceous stockworks show the same trend with 14.7 + or - 0.4 mole percent FeS and 17.6 + or - 1.1 mole percent FeS. Metamorphic equilibration was reached only in the biotite-muscovite zone at Graciela. This is demonstrated by the homogeneity of high mole percent FeS values detected in sphalerite, which coexists in mutual contact with pyrite and hexagonal pyrrhotite.

Chemical heterogeneity of sphalerite in a base metal sulfide deposit

The sphalerite in the Broken Hill orebody at Aggeneys, Northwest Cape Province, South Africa, comprises a primary suite which equilibrated at approximately 650 degrees C and 5.9 X 10 8 Pa (5.9 kb), and a secondary suite which formed during a process of supergene alteration. The primary sphalerite that equilibrated with pyrrhotite contains 14 to 22 mole percent FeS, that with pyrrhotite and pyrite contains 13.7 mole percent FeS, and that with chalcopyrite less than 4 mole percent FeS. All the primary varieties are low in copper (less than c.a. 1.6 mole % CuS). The secondary varieties include a chemically-altered population, here referred to as copper-rich sphalerite, and a neoformed population (mole % FeS less than 4.5, and mole % CuS between 1.6 and 2.4). The copper-rich sphalerite is characterized by minute intergrowths of a chalcopyrite-like sulfide (similar to chalcopyrite disease of Barton, 1978), is most frequently developed close to the water table, and is interpreted as the result of copper enrichment during the process of supergene enrichment. The reaction envisaged is: yCu (super +n) + Zn x Fe (sub 1-x) S Zn x Fe (sub 1-x-2y) S (sub 1-2y) + yCuFeS 2 + yFe (super +2) + (2-n)ye (n = 1 or 2). The net effect of this reaction is that the primary sphalerite is gradually depleted in iron as the process of supergene alteration proceeds, and this effect may have serious implications for the sphalerite geobarometer. The wide variation in the iron content of the sphalerite should also be taken into account in flotation operations.

Evolution of the sulfide ores of Saladipura, Rajasthan, India

At Saladipura stratiform sulfide ores occur in the Ajabgarh group of the middle Proterozoic Delhi Supergroup. Two coaxial buckle folds and another transverse one have deformed the rocks in the area, but no pronounced fault or shear zone is developed. The rocks evolved through a low-pressure intermediate facies series of metamorphism to a stage where the condition for the coexistence of the triple alumino-silicate polymorphs was closely approached, at about 600 degrees C and 5.5 kb. The origin of the amphibolitic rocks associated with the ores is indeterminate. They may represent a metamorphosed pile of volcanic and avolcanic rocks. Granite in the area, anatectic in origin, was emplaced at a late kinematic stage.Several orebodies occur in the area, of which the largest, defining a plunging synform (syncline), is traced out over a strike about 7 km long. The orebodies have been affected by all the deformation episodes. Different small structures in the ores are discussed. Pyrite and pyrrhotite are the dominant sulfides followed by sphalerite. Coexisting assemblages of ores and their metamorphic fabrics are critically analyzed. The wall rocks of the orebodies are composed of either cordierite + orthoamphibole(s) + quartz + or - scapolite, or muscovite + biotite + quartz + or - andalusite. The following features inter alia suggest the ores to be of sedimentary-diagenetic origin: very thin lamination in the ores, parallelism of the orebodies with the primary laminations in the associated (meta-) sediments, slump structures in ores, buckle-folding in the ore layers with the development of pyritic axial planar schistosity, virtual absence of replacement features between the ores and the silicate, and close association with carbonaceous matter. Sedimentation took place in a near-shore anoxic environment.The ores are metamorphosed. The invariant assemblage, pyrite + pyrrhotite + arsenopyrite (+ (As-S) l + V), and mole percent FeS in sphalerite coexisting with pyrrhotite suggest that the temperature and pressure of ore metamorphism were about 600 degrees C and 5.5 kb, respectively. These data agree well with those obtained from the metamorphosed host rocks. The growing idea that sulfur is internally controlled during metamorphism is upheld and it is concluded that, in ores rich in pyrite, a pyrite [rightharpoons] pyrrhotite (+ S 2 ) break-down should be negligible even in high-grade metamorphism. In the equilibration of Fe-silicates coexistent with sulfides, sulfidation was unimportant. Pegmatoid ores result from the circulation of volatiles emanated during granitic-pegmatitic activity.

Sphalerite geobarometry of deformed sulphide ores from the C. S. A. Mine, Cobar, Australia

The C. S. A. Mine is located near Cobar, central New South Wales. The copper-zinc-lead ores occur in Early Devonian rocks of the Cobar Super-Group. Lower greenschist (“slate-grade”) metamorphism has developed elongate lenticular ore systems parallel to the extension (down-dip) lineation in cleavage. FeS contents of sphalerites coexisting with pyrite and pyrrhotite outside and inside pressure shadows indicate much higher pressures (7.7 to 9.0 kbar) than those inferred from stratigraphic reasoning and the low metamorphic grade. The homogeneous distribution of Fe in sphalerites suggests equilibration with pyrite-pyrrhotite; and concentrations of Co and Ni in iron sulphides, and Mn, Cd and Cu in sphalerite are too low to have influenced phase relations in the FeS-ZnS pseudobinary system. The anomalously high pressures are therefore ascribed to reequilibration of sphalerite compositions with a monoclinic pyrrhotite-pyrite buffer. The FeS contents of the reequilibrated sphalerites apparently reflect the differing mean stress domains that exist outside and inside pressure shadows. This suggests that reequilibration occurred under the same stress distribution as produced the original pressure shadows, and implies FeS dissolution during the decay of the cleavage-producing structuro-metamorphic event. The commonly observed scatter of sphalerite compositions in low grade assemblages appears to record micro-scale mean stress domains, and thereby testifies to the pressure sensitivity of the mole percent FeS contents.

Multistage vein ores of the Lake City District, western San Juan Mountains, Colorado

Multistage vein ores of the Lake City District, western San Juan Mountains, Colorado

Genesis and metamorphism of two stratiform massive sulfide deposits at Ambaji and Deri in the Precambrian of western India

Massive, stratiform, zinc-lead-copper ores occur at Ambaji and Deri, located 8 km apart, in Gujarat and Rajasthan, respectively. The ores are concentrated in lensoid bodies of metamorphosed magnesian and calc-magnesian rocks, such as cordierite-anthophyllite-chlorite rocks, quartz-chlorite-tremolite schists, and diopside-forsterite marble, enclosed within argillaceous and arenaceous metasediments or orthoamphibolites. The rocks belong to the lower part of the Ajabgarh series of Precambrian Delhi system. The host rocks and ores were subjected to an earlier low-grade regional metamorphism under greenschist facies conditions and a superimposed hornblende hornfels facies thermal metamorphism, apparently related to the intrusion of granite and alkali syenite plutons at Ambaji and Deri, respectively.The P-T conditions of the last metamorphic episode have been estimated by comparing the mineral assemblages having a hornfelsic fabric with experimentally determined mineral reactions. Based on the stability relations of Mg-Al chlorites, anthophyllite, magnesian cordierite, and the assemblage diopside-tremolite-calcite-quartz, it is inferred that thermal metamorphism took place in the temperature range of 525 degrees to 625 degrees C at 2 to 4 kb pressure, with a mean condition of 575 degrees C/3 kb. The average mole percent FeS in sphalerite from the buffered assemblage sphalerite-pyrrhotite-pyrite, in well-annealed Deri ores, is 16.3 + or - 0.2, which for the temperature range mentioned corresponds to a pressure range of 3.8 to 3.1 kb, according to Scott9s (1973) geobarometer.The fugacities of O 2 and S 2 are determined by the mineral assemblage and vary considerably from rock to rock. A log f (sub O 2 ) -- log f (sub S 2 ) diagram at the mean P-T of thermal metamorphism (3.45 kb and 575 degrees C) has been prepared and contoured with the sphalerite composition in chlorite-anthophyllite rock from the Ambaji-Deri area.The compositions of coexisting sulfides and silicates from the magnesian assemblages were studied with the electron microprobe. The iron sulfide coexisting with Mg chlorite, anthophyllite, and Zn-rich pleonaste is pyrite and the Mg enrichment appears to be due to the original bulk composition of these rocks rather than the iron extracted during metamorphism.From a number of geological features it is inferred that the host rocks of sulfide ores were formed essentially by isochemical metamorphism of sedimentary calc-magnesian and magnesian concentration accumulated in a transitional shelf environment. The metals were derived from basalt, which was subsequently metamorphosed to amphibolite, and were concentrated syngenetically and/or diagenetically with the magnesian sediments. A model of consanguinous origin is presented which invokes the derivation of the metals, the concentration of the magnesia, the nonbacteriogenic reduction of seawater sulfate, and the concentration of stratiform sulfides by sea-water-basalt interactions under near-shore peritidal conditions.

Ore mineralogy of the Great Gossan Lead, Virginia

The Great Gossan Lead is a northeast-trending belt of massive to near-massive sulfide pods and lenses extending over 28 km through Carroll and Grayson Counties of southwestern Virginia. The closely spaced en echelon pods and lenses constitute ore zones which are essentially conformable to foliation in the quartz-muscovite and chlorite schist of the late Precambrian Ashe Formation and which range from less than one to more than 50 m in thickness and up to 7.5 km in length.Ore mineralogy is dominated by hexagonal pyrrhotite (>90% by volume) with lesser amounts of sphalerite, chalcopyrite, and galena; other phases--arsenopyrite, cubanite, mackinawite, tetrahedrite, stannite, and native bismuth--are minor but disseminated throughout the ores. Pyrite, except at the northeastern end of the district where it is present as euhedral porphyroblasts, occurs only very locally as highly corroded crystals.A strongly developed metamorphic fabric, the absence of mineral zoning, and the presence of a zone of iron depletion in the silicates surrounding the ores indicate that the ores and host rocks have been subject to the same metamorphism. At a proposed thermal maximum of 415 degrees to 455 degrees C, the ore assemblage would have developed a sulfur activity of approximately 10 (super -9) to 10 (super -7) and would have been subject to an oxygen activity below about 10 (super -25) .Sphalerite occurs as disseminated anhedral aggregates which commonly contain oriented lamellae and blebs of chalcopyrite and pyrrhotite. Sphalerite coexisting with pyrite and pyrrhotite contains as much as 14 mole percent FeS and thus indicates a metamorphic pressure of about 5.5 kb.The Great Gossan Lead is interpreted to have formed as massive, synsedimentary pyritic deposits possibly related to submarine volcanism. Metamorphism to the lower amphibolite facies, intense deformation, and subsequent retrograde effects resulted in the present mineral assemblage and produced the observed tectonic fabric.

Kuroko deposits of the Kosaka Mine, Northeast Honshu, Japan; products of submarine hot springs on Miocene sea floor

The Kosaka mine is a productive Cu-Pb-Zn-Au-Ag mine in Japan and is located at the eastern margin of the Hokuroku mining district in northeast Honshu Island. The ore deposits of the mine are divided into three main groups from north to south, namely, the Motoyama, Uchinotai, and Uwamuki ore deposits. Each ore deposit has both massive stratiform and underlying network or disseminated, siliceous orebodies. The geologic features of each deposit are described.The iron content of sphalerite coexisting with pyrite from the Uchinotai-nishi and Uwamuki No. 4 ore deposits never exceeds 5 mole percent FeS. The iron content of sphalerite from the latter and two other Kuroko deposits decreases gradually upward from stockwork to stratiform orebodies. This vertical zoning may be a common feature of the Kuroko deposits, although those from the Uchinotai-nishi ore deposit show a minimum iron content at the boundary between the black and yellow ore zones.Mixing of sea water with the ascending hydrothermal solution causes an increase in oxygen fugacity and a simultaneous temperature decrease in the ore-forming environment. They are the main causes of ore deposition. Deposition of barite is likely to be the result of an increase in activity of metastable sulfate ions which are added from sea water to the reducing ore-forming system.A magmatic origin of the ore-forming fluid is also proposed on the basis of fluid inclusion data and geologic evidence.

Environment of ore deposition at Cerro de Pasco, Peru

Environment of ore deposition at Cerro de Pasco, Peru

ChemInform Abstract: EQUILIBRIUM PHASES BETWEEN LITHIUM SULFIDE AND IRON SULFIDES

AbstractDas Phasendiagramm des Systems Li,S‐FeS wird durch Röntgenbeugungsanalyse und DTA bestimmt.

The FeS Content of Sphalerite Along the Chalcopyrite-Pyrite-Bornite Sulfur Fugacity Buffer

Concentrations of 0.19 + or - 0.02 and 0.55 + or - 0.08 mole percent FeS have been measured in sphalerite crystallized at 1 kbar in equilibrium with the chalcopyrite-pyrite-bornite sulfur fugacity buffer at temperatures of 271 degrees and 395 degrees C, respectively. These data yield a relationship of log C = --1353/T + 1.77, where C is measured in mole percent FeS, T is in degrees K, and the pressure is 1 kb. The effect of pressure on C is evaluated. Hence-forth, indications of equilibrium or disequilibrium crystallization and estimates of temperature of formation may be made for the assemblage sphalerite-chalcopyrite-pyrite-bornite at temperatures in the range 150 degrees to 450 degrees C.The composition of bornite in equilibrium with chalcopyrite and pyrite is found to be Cu (sub 5.24) Fe (sub 0.86) S (sub 4.00) at 271 degrees C and Cu (sub 5.02) Fe (sub 0.93) S (sub 4.00) at 395 degrees C. At these temperatures chalcopyrite and pyrite in the buffer assemblage do not deviate measurably from stoichiometry. Bornite and chalcopyrite are found to incorporate 0.03 to 0.18 and 0.00 to 0.43 atomic percent Zn in the range 197 degrees to 503 degrees C.

Sphalerite composition in relation to the stress distribution of a boudinage

The content of FeS in sphalerites from different parts of a boudinage is compared with the theoretical stress pattern. The calculated differences in stress correspond to less than one mole percent FeS and are often masked by the small variations in the fugacity of sulfur.

Composition of sphalerite from Quemont Mine, Quebec

Microprobe analyses of ore samples suggest equilibrium on the microscale of individual polished sections. Sphalerite with pyrite and pyrrhotite ranged from 8.9 to 13.3 mole percent FeS, with pronounced mode at 11-12 percent. Sphalerite with pyrite alone had larger range of 2.7-13.7 percent and same mode. This modal composition is in better accord with Barton and Toulmin9s (1966) extrapolation of the curve for the composition of sphalerite in equilibrium with pyrite-pyrrhotite than with the experimental results of others.

Subsolidus studies in the ZnS-FeS-FeS 2 system

The sluggish nature, measured in periods of up to twelve months, of solid state reactions in the Zn-Fe-S system hampered the low-temperature phase studies of earlier workers. New techniques involving the preparation of sulfide starting material by precipitation and fluxing of these sulfides with salts has made it possible to grow (Zn,Fe)S crystals in equilibrium, established in one direction only, with pyrrhotite and pyrite in less than a week at 395 degrees C. As a result it was possible to investigate the sphalerite solvus in the ZnS-FeS-FeS 2 system from 714 degrees C to a new low temperature of 303 degrees C.After reaching an FeS composition maximum at 580 degrees C, the sphalerite solvus does not reverse its slope but instead drops vertically at a constant composition of 20.8 + or - 0.5 mole percent FeS to the hexagonal-monoclinic pyrrhotite inversion temperature. At 303 degrees C hexagonal pyrrhotite, pyrite and sphalerite constitute the equilibrium assemblage on the sphalerite solvus. Monoclinic pyrrhotite was present in a run at 210 degrees C. Therefore, over the temperature range of geological interest, below 600 degrees C, the assemblage pyrite-hexagonal pyrrhotite-sphalerite is unsuitable for geothermometry.