Drill-core samples from the Miocene epithermal Ag-Zn-Sn-rich Cortaderas Zone (CZ) at the Pirquitas mine, Jujuy, Argentina, were studied using optical petrographic techniques, SEM-EDS, LA-ICP-MS, and EMPA to: (1) interpret the origin of color-banded sphalerite and compositionally zoned pyrite using their major-, minor-, and trace-elements; (2) resolve the origin of complex sulfosalt and sulfide intergrowth textures; and (3) estimate the temperature of Ag mineralization by sulfosalt geothermometry.
The CZ containing the Cortaderas breccia is dominated by colloform sphalerite which displays rhythmic color-banding progressing from a dark brown to tan color. The dark brown sphalerite, interpreted to have formed from dominantly metal-rich magmatic hydrothermal fluids in an open system, has elevated concentrations (average) of Ag (2298 ppm), As (2836 ppm), Cu (638 ppm), Fe (3.84 wt.%), Ge (63 ppm), and Mn (119 ppm). The tan sphalerite, interpreted to have formed in a sealed system, has elevated concentrations (average) of Cd (6176 ppm) and In (2.53 ppm), these being attributed to enrichment in the ore fluid due to fractionation/evolution of the fluid. Sphalerite from the most Ag-rich stage has no color banding but has elevated concentrations (average) of Ag (2244 ppm), As (3.17 wt.%), Cd (5731 ppm), Cu (3357 ppm), Fe (6.81 wt.%), In (2258 ppm), and Sn (1849 ppm) relative to that in other stages. Enrichments of Ag, As, Cu, and Sn in sphalerite from all stages likely occur principally as micro-inclusions of sulfosalts and sulfides based on sporadic spikes in LA-ICP-MS count-rate data, whereas Fe, Cd, and In are considered to be structurally bound in the crystal structure of the sphalerite.
Pyrite, another abundant mineral in the CZ, is compositionally zoned with cores overgrown by at least four concentric growth bands. Analyses of three of these showed relative enrichments in Ag, As, Au, Cu, Co, Ni, Sb, Pb, and Bi that can be grouped as follows: (1) As-Cu; (2) Ag-Sb-Pb; and (3) Au-Co-Ni(-Bi). Euhedral growth zones in pyrite with relatively low concentrations of trace elements represent periods of quiescence with slow crystal growth in a closed system, whereas the rounded growth zones with elevated levels of minor and trace elements reflect periods of boiling or fluid mixing with rapid crystal growth in an open system.
Six sulfosalt assemblages were documented: (1) pavonite + galena, (2) proustite + galena, (3) boulangerite + jordanite + galena, (4) Ag-bearing tetrahedrite + miargyrite + pyrargyrite + sphalerite ± kesterite ± Ag-Sb-Pb-S phase, (5) pirquitasite + stannite, and (6) canfieldite (Te-bearing) + galena. The assemblages and their complex intergrowths are interpreted to have formed by unmixing from precursor minerals during progressive cooling, thus reflecting secondary rather than primary processes. Application of the tetrahedrite geothermometer using the composition of Ag-bearing tetrahedrite in assemblage (4) suggests a range in the temperature of formation from 245 °C ± 15 to 270 °C ± 10. It is postulated that high-temperature galena (e.g., >300 °C) was an important precursor mineral to some of the observed Ag-bearing mineral assemblages and that similar galena at depth may have released metals during cooling, contributing to the metal budget higher in the system.