In-situ sulfur isotope characteristics of pyrite and chalcopyrite from the Naruo porphyry Cu (Au) deposit in Xizang: Implications for geological significance

The Naruo deposit is the third largest porphyry Cu (Au) deposit within the Duolong ore district in Xizang. Previous studies have extensively investigated the petrogenesis, metallogenetic geological chronology, and metallogenic geological background. However, the ore-forming mechanism, including the source of sulfur, remains unclear. This study focuses on the widespread occurrence of pyrite and chalcopyrite in the Naruo ore deposit. Through methods such as optical microscopy identification and LA-MC-ICP-MS isotopic analysis, mineralogical characteristics and isotopic geochemistry were investigated. The aim is to elucidate the in-situ sulfur isotope characteristics, reveal the ore genesis of the deposit, and provide guidance for mineral exploration. Based on microscopic observations, three types of pyrite are categorized, from earlier to later: Py-Ⅰ→Py-Ⅲ→Py-Ⅱ→Py-Ⅲ. Except for Py-Ⅰ, all others are closely associated with the occurrence of chalcopyrite. Pyrite exhibits δ³⁴S values ranging from －4.05‰ to 3.49‰ (with a mean of －0.2 ‰, n=53), while chalcopyrite demonstrates smaller δ³⁴S values, ranging from －7.24‰ to 0.32‰ (with a mean of －2.44‰, n=24). These test results closely approximate values found in other deposits within the ore cluster. The total sulfur value of the ore-forming fluid (δ³⁴S_Σ) is －3.06‰, indicating that the source of sulfur is primarily associated with magmatic sulfur. The sulfur isotope mineral pairs of pyrite-chalcopyrite indicate ore-forming temperatures ranging from 255℃ to 590℃, with a central ore-forming temperature of 320℃, revealing a mesothermal ore-forming environment in the ore deposit center. The spatial distribution patterns of sulfur isotopes indicate a gradual decrease in δ³⁴S values from the mineralization center to the periphery, which is notably different from some alkaline porphyry-type deposits. This study suggests that such variation is attributed mainly to ore formation in a mesothermal environment and the degassing of remote SO₂, thus serving as crucial exploration indicators for the Naruo deposit. This study has enriched our understanding of the sources of sulfur and ore-forming temperatures, among other ore-forming mechanisms, in the Naruo ore deposit. It lays the foundation for further research in ore-forming theories and mineral exploration.