BULK CHEMICAL AND MINERALOGICAL DATA ACROSS A BONANZA SILVER ZONE IN THE JULCANI, PERU, HYDROTHERMAL SOLUTION PATH

Bulk chemical, X-ray diffraction and electron microprobe data for a high density sampling array across a bonanza silver zone of epi-mesothermal, polymetallic vein deposits from the Julcani mining district, Peru, are reported. In this zone silver, antimony and bismuth have been dumped along a hydrothermal solution path characterized by monotonically increasing Pb/Cu values and this zone exhibits the maximum in Ag/Pb of the flow path. The bonanza zone has a measureable beginning and end, and is a distinct geochemical anomaly. The silver content of tetrahedrites is strongly correlated with the abundance of phases in the galena system and the highest-Ag tetrahedrites occur on the down-path side of the Ag/Pb maximum. Textural and other evidence suggests that these tetrahedrites were enriched in silver by a post depositional solid state reaction with coprecipitated Ag-Sb-Bi rich galena. Based on phase relations of Ag-Sb-Bi rich galena, the primary temperature of mineralization of T~ 320C was estimated. On cooling AgSbS₂ and AgBiS₂ components of primary galenas were depleted by the Ag-Cu exchange reaction with tetrahedrite

1/10 Cu₁₀(Zn,Fe)₂Sb₄S₁₃ + AgSbS₂ + PbS=CuPbSbS₃ + 1/10 Ag₁₀(Zn,Fe)₂Sb₄S₁₃

and by net-transport reactions which produced a variety of secondary phases depending on the Pb/Cu value along the flow path. In addition to bournonite, other secondary phases produced by net-transport reactions include pyrargyrite, bismuthian diaphorite, aramayoite diaphorite. Julcani thus provides constraints ideal for charactering the parameters of the fluid phase during the deposition of a bonanza Ag deposit and for calibrating modern methods of mineral exploration based on realistic thermochemical models for solid solutions of sulfides and sulfosalts.