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Retrograde reactions involving galena and Ag-sulphosalts in a zoned ore deposit, Julcani, Peru

¹ Department of Earth & Space Sciences, Box 351310, University of Washington, Seattle, WA, 98195-1310, USA
² Department of Geological Sciences, University of Texas, El Paso, TX 79968, USA

^* E-mail: sack{at}geology.washington.edu

The sulphide ores from the Julcani mining district (Peru) display many retrograde reactions that may be attributed to solid-state processes accompanying cooling. Fahlores [~(Cu,Ag)₁₀(Zn,Fe)₂(Sb,As)₄S₁₃] from the Herminia mine exhibit pronounced downstream enrichments in molar Ag/(Ag+Cu) ratios that are strongly correlated with the abundance of PbS-AgSbS₂-AgBiS₂ phases. These correlations, discontinuous core to rim Sb/(Sb+As) enrichments in bournonites, and prominent reaction textures involving fahlores, bournonites and galenas provide strong evidence that the fahlores in these ores have been enriched in Ag by the Ag–Cu exchange reaction

which occurred during cooling following mineralization of a PbS-AgSbS₂-AgBiS₂ galena and has been documented elsewhere. Secondary PbS-AgSbS₂-AgBiS₂ minerals aramayoite, bismuthian diaphorite [Pb₂Ag₃(Bi,Sb)₃S₈], and diaphorite were produced from primary galenas with cooling of ores with high Pb/Cu and Bi/Sb; pyrargyrite formed from the breakdown of the Ag₁₀Zn₂Sb₄S₁₃ component in the most Ag-rich fahlores, as an exsolution product of galena, and from replacement of aramayoite and galena with the evolution of semimetal sulphides. Based on mineral compositions, phase equilibria, a thermochemical database for sulphides and sulphosalts, and the reintegrated composition for primary grains of Ag-rich PbS-AgSbS₂-AgBiS₂ phases, we estimate that the primary temperature of hydrothermal mineralization was >320±10°C, that these reactions ceased to affect fahlore Ag/(Ag+Cu) ratios and Bi/(Bi+Sb) ratios of aramayoite and miargyrite after cooling through 220±10°C. Galenas, however, appear to have continued to adjust their compositions to reflect even lower temperatures by continued exsolution of semimetals and production a diverse suite of sulphosalts that occur in fine intergrowths with galena.

KEYWORDS: galena, Ag-sulphosalts, fahlore, retrograde reactions, Julcani District, Peru

This article has been cited by other articles:

				R. O. Sack and D. S. Ebel Thermochemistry of Sulfide Mineral Solutions Reviews in Mineralogy and Geochemistry, January 1, 2006; 61(1): 265 - 364. [Full Text] [PDF]

				R. O. Sack, R. Fredericks, L. S. Hardy, and D. S. Ebel Origin of high-Ag fahlores from the Galena Mine, Wallace, Idaho, U.S.A. American Mineralogist, May 1, 2005; 90(5-6): 1000 - 1007. [Abstract] [Full Text] [PDF]

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				R. O. Sack, R. O. Sack, J. V. G. Lynch, and F. Foit Jr. Fahlore as a petrogenetic indicator: Keno Hill Ag-Pb-Zn District, Yukon, Canada Mineralogical Magazine, October 1, 2003; 67(5): 1023 - 1038. [Abstract] [Full Text] [PDF]

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