Argentite

Abstract: This paper illustrates the potential and also the complexity of using Au-Ag alloys (the mineral electrum) as petrogenetic indicators of ore-forming processes. Thermodynamic calculations indicate that the solubility of gold is highly dependent on the composition, or fineness, of naturally occurring Au-Ag alloys. Alloy composition, in turn, is a complex function of the temperature, a (sub S 2 , a (sub O 2 , Cl (super -) concentration, pH, and total Au/Ag content of the system in question. Hydrothermal solutions with low salinity, near-neutral pH, and high dissolved sulfide concentration have relatively high gold solubilities and Au/Ag solubility ratios. In contrast, more saline brines have lower gold solubilities and very low Au/Ag solubility ratios. Alloys formed from either type of fluid may be Au rich or Au poor, depending on the temperature, chemical environment, and mechanism of ore deposition. The compositions of both solid solution and aqueous solution may change drastically during reaction progress, resulting in deposits which are mineralogically zoned, either on a microscopic (individual electrum grain) or mesoscopic (ore deposit) scale. Dynamic modeling calculations are presented to illustrate these effects, using either open-system (Rayleigh fractionation) or closed-system boundary conditions. Coprecipitation with other Au- or Ag-bearing minerals (e.g., argentite, chlorargyrite, sulfosalts, tel- lurides, or selenides) places additional thermodynamic constraints on electrum composition. For example, the composition of electrum in equilibrium with argentite may be used as a geothermometer, provided an independent estimate of a (sub S 2 is available. Thermodynamic analysis indicates that Au-rich argentite may be a very important primary ore mineral, especially in deposits formed at high temperature, high a (sub S 2 , and/or high ratios of dissolved Ag/Au. Textural and compositional information recorded in parageneses containing electrum and other precious metal minerals must be interpreted with caution, especially if there is reason to suspect that ore deposition was rapid, or if there is evidence of postmineralization weathering, metamorphism, or other forms of reequilibration.

Abstract: The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns. Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor. Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively. Analyses of phase equilibria and related thermodynamic calculations indicate that the log f O2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH. We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower f O2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone.