Origin and evolution of a porphyry-breccia system: Evidence from zircon U-Pb, molybdenite Re-Os geochronology, in situ sulfur isotope and trace elements of the Qiyugou deposit, China

Abstract

The Qiyugou Au deposit, located in the Xiong'ershan ore cluster of central China, consists of breccia pipe hosted Au ore bodies (>60 t Au) and newly discovered porphyry ore body (8 t Au). The porphyry Au mineralization occurs in the silicified core of the apical part of porphyritic granite stock which shows close spatial relationship with the auriferous breccia pipes. Here we present an integrated study involving detailed field investigation, textural studies, in situ pyrite trace element and sulfur isotope analysis, and U-Pb and Re-Os geochronology to elucidate the genetic relationship and hydrothermal evolution of the porphyry and breccia pipe mineralization. The SIMS sulfur isotopic analyses for the five pyrite generations show consistent δ34S values between −3.6‰ and 0.6‰, indicating that the two styles of Au mineralization had a mixed source including crustal and mantle-derived components. Zircon LA-ICP-MS U-Pb geochronology of two porphyritic granites yield weighted mean ages of 133.0 ± 0.6 Ma and 130.3 ± 0.9 Ma, and six molybdenite samples display an isochron age of 131.3 ± 1.1 Ma, representing the timing of granitic magmatism and Au mineralization in the 189 porphyry ore body. Available geochronological data allow us to constrain the timing of granitic magmatism and gold mineralization to be 135.6–134.1 Ma in the breccia pipes and 133.0–128.6 Ma in the porphyry ore. Among the five pyrite generations, Py1b and Py2b from the J4 breccia pipe are characterized by higher Co, Ni, and comparable contents of other elements (e.g. Au, Ag, Te, Pb, Zn, Cu and Bi) than those in Py1p, Py2p and Py3p from the porphyry ore. The widely consistent metal concentrations and linear relationship of metals in pyrite between the breccia pipe and porphyry ore are most probably the result of fluid exsolution from the same magmatic-hydrothermal system, whereas the enriched Co and Ni values from the J4 breccia pipe can reflect the composition of pyrite precipitating fluid at the early stage of the prolonged magmatic-hydrothermal system. The Qiyugou porphyry and breccia pipe mineralization show close spatial-temporal association and consistent source materials, and thus we propose that they resulted from multiple pulses of fluid exsolution from a large, deep-seated magma chamber.