Comparison of metasomatic reactions between a common CO 2 -rich vein fluid and diverse wall rocks; intensive variables, mass transfers, and Au mineralization at Alleghany, California

Abstract

The gold deposits at Alleghany, California, are typical of many epigenetic gold-bearing hydrothermal vein systems in metamorphic terranes worldwide. Detailed analyses of alteration halos in serpentinite, mafic amphibolite, and granite wall rocks at Alleghany indicate that widely contrasting deposit types, ranging from fuchsite-carbonate schists to pyrite-albitites, resulted when different wall rocks interacted with the same externally derived CO 2 -rich hydrothermal vein fluid. Patterns of element redistribution within halos and among lithologic units suggest a complex process involving fluid flow along vein fractures and diffusion (+ or - infiltration) normal to the veins. Wall rocks locally controlled both the directions and magnitudes of chemical fluxes across vein walls.Vein fluids at constant temperature and pressure, with relatively constant ratios of mobile species (Na (super +) /K (super +) /H (super +) and CO 2 /H 2 S/H 2 ), caused contrasting (divergent) reactions in proximal alteration zones because the wall rocks controlled the ratios of relatively immobile species (Al/(Cr + Fe + Mg) and Fe/Mg), and the component activities, of hydrothermal mineral solid solutions. Thus, pyrite and albite were stable with respect to the relatively pure siderite and K mica that occur sparingly in proximal granite zones, but they were not stable with respect to the relatively impure siderite and K mica that characterize proximal serpentinite zones. Partly as a result of divergent reactions, altered rocks exhibit contrasting mobile element concentration gradients (e.g., Ca, Ba, Sr, Rb, Na/K, C/S) and imply local sources and sinks, and possibly short residence times, for some of these components along discordant fluid flow paths.Calculated reaction coefficients for H (super +) , H 2 , and H 2 S indicate that different wall rocks at different stages of alteration may have inhibited or promoted gold precipitation. The distribution of gold and its calculated solubility are consistent with its having been precipitated from Au(HS) 2 (super -) complexes in response to pyritization of granite (fluid desulfidation and reduction), and possibly in part, in response to dolomitization and sericitization of serpentinite (fluid acidification?). Strata-bound disseminated orebodies may be expected in a variety of other host rocks with higher than average Fe/Mg ratios, where pyrite was stable with respect to intermediate magnesite-siderite, as in the granite.It is proposed that the CO 2 -rich vein fluids approached partial equilibrium with alteration assemblages in the average, or dominant, rocks along their flow paths. Simpler, but radically different, proximal assemblages were then produced where the fluids encountered chemically extreme rock types. Fluid-rock interaction equilibria can account for much of the diversity in the mineralogy and chemistry of gold deposits in heterogeneous metamorphic terranes, while maintaining a simple genetic classification based on the distinctive chemistry of the hydrothermal fluids. Comparative studies of multicomponent phase equilibria and mass exchanges can provide both rationalizations and predictions of local wall-rock effects on geochemical anomalies and the locations of ore deposits in lithologically heterogeneous terranes. Simultaneous consideration of contrasting high-variance (phase-deficient) assemblages along discordant veins is suggested as a general method for quantifying fluid-intensive variables in metasomatic systems.