Geochemistry of tetrahedrite and mineral zoning at Casapalca, Peru

Abstract

The extensive Casapalca veins cut Tertiary volcanic rocks and red beds near the crest of the Peruvian Andes. The veins correspond to a fracture system which probably formed during one period of tectonic activity. Paragenetic studies identify four stages of mineralization without obvious time breaks characterized by (1) sphalerite-galena-pyrite, (2) chalcopyrite-pyrite-sphalerite, (3) tetrahedrite, and (4) quartz-carbonates. The tetrahedrite is the main source of silver, and its paragenetic position can be correlated from one part of the mine to another over great distances.Quantitative electron microprobe analyses show that tetrahedrite from Casapalca has a chemical composition approximately(Cu,Ag) (sub 10+x) (Fe,Zn) 2 (Sb,As) (sub 4+z) S 13 where 0 < x < 0.5, 0 < z < 0.15.Qualitative electron microprobe analyses performed in this study include numerous profiling and area scans, as well as more than 10,000 points of step-traverse analyses on about 60 selected samples from most of the accessible vein system. Silver and antimony in tetrahedrite are always positively correlated. The silver-antimony correlation may be mostly controlled by two factors, namely, the activity ratios of Ag (super +) and Cu (super +) in the hydrothermal solution from which tetrahedrite crystallized and a crystallographic effect by which tetrahedrite with higher antimony content could accommodate more silver.Correlation of the chemical composition of tetrahedrite throughout the Casapalca vein system outlines a zoning pattern compatible with the zonation of wall-rock alteration and other studies of mineral zoning and metal distribution. As-rich Ag-poor tetrahedrite predominates in the inner zone which is probably close to the center of the hydrothermal system at Casapalca. The outer Sb-rich Ag-rich tetrahedrite zone envelops the As-rich Ag-poor tetrahedrite zone. This zoning pattern differs from the one used at Casapalca for several decades. The newly proposed zoning of mineralization should be useful as a guide to mineral exploration at Casapalca.A similar zoning pattern of the Sb/As ratio and silver in tetrahedrite, where the antimony and silver in tetrahedrite increase from the central to the peripheral zone may exist in many ore deposits. The chemical variations of tetrahedrite may reveal the hydrology and the evolution of certain chemical parameters of ore fluids in a hydrothermal system.