Experimental studies of the synthesis of pyrite and marcasite (FeS₂) from 0° to 200° C and summary of results

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 (50-95 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.

Chemical reaction path modeling of ore deposition in mississippi valley-type Pb-Zn deposits of the Ozark region, U.S. Midcontinent

“…Calculated pH values for calcite-saturated fluids flowing through limestone rather than through dolostone host rocks are in the same general range as those of dolomite-saturated fluids. Such low pH values are consistent with the presence of abundant hydrothermal marcasite which requires pH values of less than 4.5 to 5.0 to form (Murowchick and Barnes, 1986; Goldhaber and Stanton, 1987;Stanton and Goldhaber, 1991). Low pH values are also consistent with observed aluminosilicate alteration mineralogies (Sverjensky, 1984).…”

Arsenic-bearing pyrite and marcasite in the Fire Clay coal bed, Middle Pennsylvanian Breathitt Formation, eastern Kentucky

“…See Table 2 for complete microprobe analyses. sible for the enriched arsenian pyrite observed in Alabama coals may be the source (Stanton and Goldhaber, 1991;Goldhaber et al, 2001).…”

“…Experimental studies (see Richard et al, 1995) have shown that the formation of marcasite and pyrite is controlled by sulfur speciation and pH. Marcasite is the dominant Fe-disulfide phase formed at the low pH (b 5) when neutral polysulfides (H 2 S 2 ) are present Shoonen and Barnes, 1991;Stanton and Goldhaber, 1991). Polysulfides are, however, metastable , espe- cially in acidic environments (Giggenbach, 1974) typical of coal deposits undergoing early-to middiagenesis.…”

Comparison of the Cracow-Silesian mississippi valley-type district, southern Poland, with mississippi valley-type districts in North America

“…Perhaps the best examples of detrital sulfides in North American MVT deposits are the Polaris mine and Pine Point district, where there are abundant iron sulfides. The presence of abundant marcasite is indicative of low pH (< 4-5; Stanton and Goldhaber, 1991;Goldhaber and Stanton, 1987;Murowchick and Barnes, 1986), consistent with ore-stage carbonate dissolution. Deposition of iron sulfides is a highly effective acid generation process that could produce localized host-rock dissolution.…”