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2 EXPERIMENTAL METHODS 2 Autoclave and Vessel Preparation3 Reaction Solution Preparation3 Sulfate series3 Chloride series8 Calcite series8 Heating, Reaction Times, and Cooling8 ANALYTICAL METHODS9 Extraction and Purification of Iron Bisulfides9 Sulfate series9 Chloride series! 1 Calcite series! 1 Mineral Identification and Estimated FeS2 Abundances using X-Ray Diffraction13 Mineral grain mount preparation! 5 Solution Analysis! 5 Flame atomic absorption spectrophotometry! 5 Iron analysis! 7 Calcium analysis! 7 Sulfate analysis by BaSC>4 gravimetry1 7 RESULTS^ Morphology and Texture of Solids! 8 Sulfate Series Subgroups! 9 Heat-Cool subgroups 19A-G, 20A-G, 27A-E, and 28A-FII1 9 Other subgroups20 FeS2 Yield and Marcasite Fraction at 150° C2 1 FeSi Yield and Marcasite Fraction at 200° C2 1 Marcasite fraction vs molarity of H2SO4 or HC12 1 FeS2 yield and marcasite fraction in calcite series experiments2 3 SUMMARY REFERENCES CITED ABSTRACT Pyrite and marcasite (FeSi) precipitation was studied experimentally from 0 to 200° C under conditions that may have existed during the formation of carbonate-hosted Pb-Zn (Mississippi Valley Type) deposits. Dissolved ferrous iron (from FeSO4 or FeCla), elemental sulfur (S°), and hydrogen sulfide (H^S) were the reactants, with sulfuric acid (H2SO4) or hydrochloric acid (HCI) added to adjust the initial pH to 3.2 or less. A separate series of experiments using added CaCOs was run to examine the acid-neutralizing effect of CaCOs on FeS2 formation. Reaction times were from 1 to 110 hours with most reactions conducted for 24 hours. On the time scale of these experiments, iron disulfides did not form or formed in small amounts at temperatures of 150° C or less, whereas larger yields of disulfides were obtained at 200° C. In many instances, very pure disulfide phases were produced (>.95 weight percent FeS2 as marcasite or pyrite). Marcasite was usually the dominant disulfide mineral produced when both t^S and S° were initially present in acid-sulfate solutions (5095 weight percent of FeS2(totai)» where FeS2(totai)=pyrite + marcasite). Nearly pure marcasite (90 to >95 weight percent of FeS2(totai)) formed in acid-sulfate solutions when only S° (not H^S) was added. Over the range of 0.005-0.20 M H2SO4, increasing acidity of the solution promoted marcasite formation over pyrite. Beyond a total H2SO4 concentration of 0.20 M, however, the marcasite fraction of FeS2(totai) decreased. At an H2SO4 concentration of 0.24 M, only a trace of FeS2 formed (pyrite) and marcasite was not detected. A similar trend was found in acid-chloride solutions over the HCI concentration range of 0.005-0.03 M, where the marcasite fraction was 57-92 weight percent of FeS2(totai)« However, FeS2(totai) and weight percent marcasite maxima were reached at 0.01 M HCI, and then decreased as the HCI molarity increased to 0.03 M. Pyrite and marcasite were produced from ferrous chloride or ferrous sulfate solutions in the presence of CaCOs (without added acid or H2S), but pyrite was usually the dominant disulfide (greater than 50 weight percent of FeS2(totai))« Minerals formed in these experiments in addition to disulfides include iron monosulfide, anhydrite, gypsum, siderite, magnetite, and hematite. Marcasite comprised 50 weight percent or more of FeS2(totai) in only 2 of the 15 CaCOs experiments. In the other 13 experiments, marcasite did not exceed 33 weight percent in 7 experiments and was not detected in the other 6 experiments. Thus, the presence of CaCOs apparently decreases the relative amount of marcasite produced. The results are consistent with the fact that marcasite usually forms at pH values less than 5 and that pyrite tends to be the dominant disulfide formed at higher pH values.Pyrite and marcasite (FeSi) precipitation was studied experimentally from 0 to 200° C under conditions that may have existed during the formation of carbonate-hosted Pb-Zn (Mississippi Valley Type) deposits. Dissolved ferrous iron (from FeSO4 or FeCla), elemental sulfur (S°), and hydrogen sulfide (H^S) were the reactants, with sulfuric acid (H2SO4) or hydrochloric acid (HCI) added to adjust the initial pH to 3.2 or less. A separate series of experiments using added CaCOs was run to examine the acid-neutralizing effect of CaCOs on FeS2 formation. Reaction times were from 1 to 110 hours with most reactions conducted for 24 hours. On the time scale of these experiments, iron disulfides did not form or formed in small amounts at temperatures of 150° C or less, whereas larger yields of disulfides were obtained at 200° C. In many instances, very pure disulfide phases were produced (>.95 weight percent FeS2 as marcasite or pyrite). Marcasite was usually the dominant disulfide mineral produced when both t^S and S° were initially present in acid-sulfate solutions (5095 weight percent of FeS2(totai)» where FeS2(totai)=pyrite + marcasite). Nearly pure marcasite (90 to >95 weight percent of FeS2(totai)) formed in acid-sulfate solutions when only S° (not H^S) was added. Over the range of 0.005-0.20 M H2SO4, increasing acidity of the solution promoted marcasite formation over pyrite. Beyond a total H2SO4 concentration of 0.20 M, however, the marcasite fraction of FeS2(totai) decreased. At an H2SO4 concentration of 0.24 M, only a trace of FeS2 formed (pyrite) and marcasite was not detected. A similar trend was found in acid-chloride solutions over the HCI concentration range of 0.005-0.03 M, where the marcasite fraction was 57-92 weight percent of FeS2(totai)« However, FeS2(totai) and weight percent marcasite maxima were reached at 0.01 M HCI, and then decreased as the HCI molarity increased to 0.03 M. Pyrite and marcasite were produced from ferrous chloride or ferrous sulfate solutions in the presence of CaCOs (without added acid or H2S), but pyrite was usually the dominant disulfide (greater than 50 weight percent of FeS2(totai))« Minerals formed in these experiments in addition to disulfides include iron monosulfide, anhydrite, gypsum, siderite, magnetite, and hematite. Marcasite comprised 50 weight percent or more of FeS2(totai) in only 2 of the 15 CaCOs experiments. In the other 13 experiments, marcasite did not exceed 33 weight percent in 7 experiments and was not detected in the other 6 experiments. Thus, the presence of CaCOs apparently decreases the relative amount of marcasite produced. The results are consistent with the fact that marcasite usually forms at pH values less than 5 and that pyrite tends to be the dominant disulfide formed at higher pH values. INTRODUCTION Pyrite and marcasite (iron disulfide, FeS2) coexist in Mississippi Valley Type (MVT) carbonate-hosted Pb-Zn deposits. Much interest centers on sulfide minerals as indicators of processes that result in formation of MVT and other sulfide ore deposits (Cathles and Smith, 1983). It is of particular interest to explain how marcasite can form and persist in a carbonate host rock because experimental evidence indicates that marcasite requires a pH less than 5 to form (Goldhaber and Stanton, 1987; Murowchick, 1984). Different generations of iron disulfides formed and persisted during emplacement of MVT deposits (Heyl and others, 1959). Pyrite formation may have formed voids isolated from carbonate host-rock where marcasite formation could take place. Petrographic studies from different MVT ore districts bear out this sequence of pyrite formation followed by marcasite formation (Heyl and others, 1959). An additional interest in pyrite and marcasite is that the geochemical conditions under which both FeS2 dimorphs form and coexist are not completely understood. Pyrite is found in nearly every geologic environment and is therefore an important mineral in the global sulfur cycle (Berner, 1970). Marcasite, on the other hand, is much rarer and limited to a few geochemically extreme e

Experimental studies of the synthesis of pyrite and marcasite (FeS<sub>2</sub>) from 0° to 200° C and summary of results