In situ trace element and sulfur isotope of pyrite constrain ore genesis in the Shapoling molybdenum deposit, East Qinling Orogen, China

Abstract

The Early Cretaceous (ca. 128 Ma) Shapoling vein-type molybdenum (Mo) mineralization, associated with the Huashan intrusion is a newly discovered deposit in the Xiong'ershan district, East Qinling, China where pyrite is a ubiquitous mineral phase. Here we present results from an integrated study on the texture, in situ trace element chemistry from laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and sulfur isotopic composition using secondary-ion mass spectrometry (SIMS), with a view to understand the geochemical variation, genesis of Mo mineralization and ore-forming process. The distinct textural patterns as revealed by petrographic observations suggest three types of pyrites: Py1 is composed of coarse-grained euhedral pyrite in the wall rock and early barren quartz vein at Stage I, Py2 is represented by subhedral-anhedral grains coexisting with molybdenite in quartz veins at Stage II, and Py3 forms irregular interstitial grains in the quartz + K-feldspar + molybdenite dominated veins at Stage III. The Mo enrichment mainly occurred in Py3 (with Co/Ni ratios >1 and d34S values mainly <0‰). Our study shows that Mo, Au, Pb, Zn, Ag and Cu are selectively partitioned into pyrite as mineral micro/nano-inclusions, and that the siderophile and chalcophile elements (such as Co, Ni, As, Se and Te) occur substituted within different textural types of pyrite. The d34S values of the pyrites in Py1 and Py2 range between -2.33‰ and -0.37‰ indicating a dominantly magmatic origin. The d34S values of Py3 increase from -2.55‰ to +1.48‰. In situ d34S analysis of pyrite from Py1, Py2 to Py3 shows a progressive and gradual change in redox conditions. The negative values indicate oxidized sulfur in the early generation of pyrite (Py1 that coexists with magnetite). Conversely, positive values represent reduced sulfur found in the later generations of pyrite (Py3), which contain higher concentrations of Cu, Pb, Zn and Ag. The Mo-bearing pyrite identified in our study could be used as a possible pathfinder for the exploration of high-grade Mo mineralization.