Unconventional non-magmatic sulfur source for the Mazarrón Zn-Pb-Cu-Ag epithermal deposit (SE Spain)

Abstract

Geological evidence and mineralogical, fluid inclusion temperature and geochemistry data suggest that the Mazarrón Zn-Pb-Cu-Ag-Fe deposit is a volcanic-hosted epithermal deposit. It is hosted by dacites with advanced argillic alteration, argillic alteration, propylitic alteration and silicification halos. K-Ar dating of illite from the argillic alteration gives an age of 9.7 ± 1.2 Ma, which suggests that mineralization took place very close to the volcanism. The fluid inclusion study in the ore veins shows epithermal temperatures (between 190 and 260ºC), but moderate salinities, between 12 and 18 wt. % NaCl eq), compatible with mixing between surface waters and polygenic deep hydrothermal fluids. Two mineralization types are identified: iron sulfide mineral association and polymetallic mineral association (sphalerite-galena-chalcopyrite-fahlore bearing). According to isotopic data, the first mineralization was mainly formed by interaction between basinal fluids that leached Triassic sulfates (+ surface waters), and C-bearing reduced deep waters. This interaction developed a Thermochemical Sulfate Reduction (TSR) supporting extreme and geologically unusual fractionation, with a range up to +30‰ δ34S, and reaching +53‰ δ34S. The second mineralization event is compatible with the irruption of a magmatic fluid discharge. Although this study suggests that Mazarron could be classified as an intermediate sulfidation deposit, the genetic model includes processes such as TSR that are not typical features of volcanic-hosted epithermal deposits.