Abstract

Stoichiometric reaction coefficients of minerals and aqueous species in overall heterogeneous chemical reactions have been lithologically determined expressing intense late-stage modification of early low-grade copper-molybdenum mineralization during the growth of the large vein systems at Butte, Montana. Application of the principles of irreversible thermodynamics and conservation of mass in open chemical systems has provided the basis of a rock analysis method offering a quantitative deduction of the reaction chemistry, mass transfer, and magnitude of progressive late Main Stage hydrothermal alteration of the pre-existing mineralized wall rock. The critical information is mineral mass data derived from a systematic analysis of diamond drill core and crosscut samples from the western edge of the zone of superposition of hydrothermal events. The stoichiometric coefficient of each mineral is determined from the slope of its molar variation with Main Stage reaction progress variable, xi , related directly to the number of moles of an index mineral used for reference. Relict chalcopyrite has been chosen in this regard as it was formed during the pre-Main Stage hydrothermal event in the area studied and has undergone progressive destruction during Main Stage activity. Stoichiometric coefficients of aqueous species are determined by charge balance and conservation of mass within the Stoichiometric equations. By a conceptual removal of all severe, modifying effects caused by the large and relatively high grade Main Stage veins controlling wide and intense alteration envelopes, the nature of the early pre-Main Stage hydrothermal events have been reconstructed.The early high-temperature (600 degrees -700 degrees C) mineralization consisting of fracture-controlled molybdenite and disseminated pyrite and chalcopyrite is found to be distinctly zoned in its unmodified state at the elevation of this study, approximately 3,400 feet below the surface. The early zonation in plan, from outside toward the center of the pattern, is a peripheral pyritic zone with approximately 5 weight percent pyrite, an oxide zone consisting of 1 to 2 percent magnetite plus hematite with a copper grade contributed by early chalcopyrite of 0 to 0.2 percent, and an axially symmetric maximum early chalcopyrite zone with over 0.5 percent copper surrounding a lower grade chalcopyrite interior zone with approximately 0.2 percent copper. Molybdenum grades increase continuously from essentially zero at the outer edge of the maximum early chalcopyrite zone to over 0.05 percent inside the interior low-grade early copper zone. No pre-Main Stage bornite has been recognized.Superposition of late, relatively low temperature (200 degrees -350 degrees C), continuous high-grade Main Stage veins and ubiquitously related alteration envelopes upon the early disseminated sulfides and associated potassium silicate alteration has resulted in profound mineralogic effects. The earliest Main Stage modification of mineralized wall rock involves very little effect on the pre-existing pyrite and chalcopyrite, but plagioclase is converted in an outer argillic facies first to montmorillonite which in turn is replaced by kaolinite. Magnetite is partially oxidized to hematite during the early clay-forming event.Following these incipient, Main Stage effects, more intense effects are monitored continuously by the reaction progress variable and include continuous chalcopyrite destruction and hypogene leaching of early disseminated chalcopyrite from the pre-Main Stage assemblage in the near-vein sericitic facies of alteration. Molybdenite is apparently unaffected by strong sericitization. During the early part of intense Main Stage events, total copper decreases in the rock. As the reaction continues, Main Stage fluids eventually reach saturation with respect to bornite, which precipitates in vein structures. Finally chalcocite, digenite, covellite, and enargite form in narrow high-grade veins resulting in a net copper addition or hypogene enrichment. Throughout the entire span of Main Stage reaction, biotite, hematite, magnetite, orthoclase, and kaolinite after montmorillonite after plagioclase are progressively altered to an assemblage of sulfides, sericite, and quartz. Sphalerite of pre-Main Stage or early Main Stage age is slightly leached during Main Stage events.Interpretation of the Main Stage reaction chemistry in terms of calculated phase equilibria at 250 degrees C implies a strong pre-Main Stage mineralized wall-rock buffering effect on the hydrothermal fluids during the early stages of Main Stage reaction. With the progressive destruction of the early potassium silicate assemblage of chalcopyrite, magnetite, biotite, and orthoclase, oxidation effects proceeded, with a reduction in a (sub Fe (super +2) ) /a 2 (sub H (super +) ) by three orders of magnitude, essentially at constant a (sub Cu (super +) ) /a (sub H (super +) ) . First, saturation with respect to pyrite and bornite was reached, and ultimately high-grade chalcocite, covellite, enargite assemblages formed during the most intense sericitization of the wall rock at values of xi greater than 0.5. The presence of early pre-Main Stage chalcopyrite is interpreted to be of primary importance in buffering the initial Main Stage fluid composition at a relatively high value of log a (sub Cu (super +) ) /a (sub H (super +) ) near -4.5 causing subsequent hypogene sulfide saturation in the high-grade veins. Although the approach taken is admittedly macroscopic in nature in that the physical details of diffusion and infiltration are not considered, it has been possible to analyze exceedingly complex rock masses and to understand the coupling of mineralization and alteration, revealing the importance of element recycling to the vein-forming process. It is possible that much of the element content, i.e., base metals and sulfur, of high-grade Main Stage Butte veins originated as disseminated sulfides in the early hydrothermal protore mass which underwent a progressive hypogene redistribution and enrichment during a late-stage fracture-fluid circulation event possibly related to cooling porphyry intrusives that were emplaced after and geometrically offset from the earlier pre-Main Stage intrusives and hydrothermal events.

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