| The examination of ore mineralogy, bulk metal compositions, and phase relations between the silver bearing minerals fahlore [(Cu,Ag) 10(Zn,Fe)2(Sb,As)4S13], polybasite-pearceite [(Ag,Cu)16Sb3S11-(Ag,Cu)16As 3S11], pyrargyrite [Ag3SbS3], and (Ag,Cu) 2S solid solutions at the epthermal La Colorada Ag-deposit in Zacatecas Mexico led to several important conclusions. The bulk metal content of the currently mined vein system is near the stability of miargyrite (AgSbS 2) in some assemblages. Deviation from miargyrite saturation is consistent with a net loss of Sb2S3 component, which is known to be mobile at other epithermal deposits. The use of sulfide assemblages and existing thermodynamic databases confirm fluid inclusion data that infer temperature of formation near 325°C for much of this system. At this temperature, significant silver may have been accommodated by galena (PbS). An asymmetric solution model is developed to quantify the limits of silver solubility in galena at temperatures representative of ore formation. Important terms that address non-ideality in the thermodynamic solution model developed for the ternary system Pb2S2-AgSbS2-AgBiS2 are 45.3 kJ for the ternary expression (W10), 8.5 kJ (W1) for the symmetric and 18.6 kJ (W4) for the significant asymmetric expression on the Pb2S2-AgSbS2 binary, and 11.7 kJ (W2) for the symmetric expression on the Pb2S 2-AgBiS2 binary. This model successfully reproduces the results of dry sinter experiments performed at 300°C, 350°C, 375°C, and 400°C with ternary compositions. The calculated miscibility gap conservatively predicts that a majority of the silver recovered from many epithermal adularia-sericite type deposits, sedex (sedimentary exhalative) deposits, MVT (Mississippi Valley type) deposits, some VMS (volcanogenic massive sulfide) deposits, and the Coeur d'Alene district can be accommodated in galena at temperatures of formation. If galena is the primary silver mineral, then the exsolution of silver minerals from galena must be considered in models of ore genesis. Specifically, solid-state reactions involving galena probably play a heretofore, underappreciated role in the genesis of silver ore.*; *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office. |
