Stratiform copper deposits hosted by low-energy sediments; IV, Aspects of sulfide precipitation

Abstract

Theoretical modeling of the sequences and amounts of Ag, Co, Cu, Fe, Mn, Pb, and Zn precipitated from 25 degrees C, 3-molal chloride waters saturated with dolomite or calcite, quartz, talc, gypsum, and initially, hematite or goethite was performed assuming parameters of constant chloride molality, pH, and concentrations of major dissolved species. Precipitation sequences were modeled by progressively increasing H 2 S fugacities from a minimum defined by the cuprite-native copper join to a maximum defined by the rhodochrosite-alabandite join in the respective waters. The initial contents of Ag, Co, Cu, Fe, Mn, Pb, and Zn in the waters were defined by source and transport constraints outlined in Haynes and Bloom (1987). Successive modeling runs were made at pH values of 7.6, 6.6, and 6.1, with pH controlled by the CO 2 -calcite (or dolomite) buffer in all waters. The choice of parameters, as well as their variation during the runs, was constrained by a hypothesis that most sulfide deposition in stratiform copper deposits hosted by low-energy sediments occurs within 50 cm of the sediment-water interface during bacterial sulfate reduction (Haynes, 1986a). Initial water compositions approximated continental, closed-basin waters derived from basaltic and granitic provenances, respectively, so that the likely total compositional range of probable source rocks for stratiform copper deposits was represented in the modeling.A generalized precipitation sequence is native silver-chalcocite-bornite-galena-linnaeite-sphalerite-cattierite-pyrite. Native copper appears prior to chalcocite at a pH above 6.6 if the initial copper concentrations in the waters are 800 ppm or at a pH above 7.2 if copper concentrations are 100 ppm. Linnaeite does not precipitate at pH values less than 6.3. Precipitation of CoCO 3 and rhodochrosite occurs in basalt-derived waters upon acidification from an initial pH of 7.6. Chalcocite, galena, linnaeite, sphalerite, and alabandite are the most abundant phases precipitated, and all, with the exception of alabandite, precipitate at H 2 S fugacities above the hematite-pyrite join. The amounts of bornite, covellite, and pyrite precipitated are small and chalcopyrite precipitation does not occur.Precipitation of all lead, cobalt, and zinc, as sulfides, requires H 2 S fugacities above 10 (super -12) bars at a pH of 7.6 or above 10 (super -9) bars at a pH of 6.6. Chalcocite is the only sulfide precipitated if H 2 S fugacities do not exceed 10 (super -14) bars at a pH of 7.6 or 10 (super -12) bars at a pH of 6.6.Modeling predicts a sulfide zonation sequence and metal ratios similar to those observed in stratiform copper deposits hosted by low-energy sediments. Modeling demonstrates that H 2 S fugacity in host rocks exerts an important control on metal ratios. If H 2 S fugacities do not exceed 10 (super -14) bars, deposits will consist of copper and silver, with no associated lead, zinc, and cobalt; if H 2 S fugacities exceed 10 (super -12) bars, deposits will have associated trace or minor lead, minor or significant cobalt, and minor or major zinc, depending on the source rock and transport constraints detailed in Haynes and Bloom (1987).Modeling suggests that chalcocite precipitation resulting from pyrite replacement is unlikely because the amount of copper removed from the water by such a process will be less than 2 ppm. However, precipitation of hematite or copper-iron sulfides will accompany pyrite replacement if iron saturation limits are locally exceeded. Local precipitation of hematite or copper-iron sulfides as a result of pyrite replacement is consistent with the occurrence of hematite-chalcocite intergrowths at White Pine and the occurrence of associated leucoxenebornite in the Kamoto Principal deposit.Modeling fails to predict the small cobalt contents in stratiform copper deposits with associated lead and zinc, and it fails to predict the occurrence of abundant chalcopyrite and bornite in most large stratiform copper deposits hosted by low-energy sediments. Small cobalt contents of deposits such as those within the German Kupferschiefer are probably caused by limited cobalt availability in source rocks during the metal-leaching episode. Relatively large amounts of bornite and chalcopyrite possibly originated by a postburial alteration of chalcocite-pyrite composite grains deposited during the bacterial sulfate reduction episode. Evidence for such postburial alteration is currently not diagnostic and clearly requires further research.Modeling was performed using parameters constrained by the hypothesis previously described by Haynes (1986a). Because predicted zonal sequences and predicted metal ratios, with the explicable exceptions of chalcopyrite, most bornite, and cobalt, are similar to those observed in stratiform copper deposits, the hypothesis could be a realistic description of the cause and timing of native copper, native silver, chalcocite, some bornite, galena, sphalerite, linnaeite, and cattierite precipitation in such deposits.