Genesis of the Naneng gold deposit in southeastern Yunnan: Evidence from in-situ trace elements and isotopes of sulfides

Naneng gold deposit is an important medium-sized Carlin-type gold deposit in southeastern Yunnan. Objective and MethodsTwo stages of pyrite (PyⅠ and PyⅡ) were identified to develop in the Naneng gold deposit via detailed field investigations and petrographic observations, and in-situ trace elements analyses of gold-bearing minerals (pyrite and arsenopyrite) were conducted by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS), combined with in-situ and sulfur isotopes analyses, to constrain the source of ore-forming materials and the genetic mechanism of the deposit. ResultsLA-ICP-MS analyses show that PyⅠ contains a certain amount of Au (mean 6.37×10−6), and is relatively enriched in Co, Ni, Se, W and other elements; The distribution characteristics of trace elements in PyⅡ and PyⅠ are similar, but the content of Au (mean 68.02×10−6) is relatively high, and As, Sb, Cu elements are enriched in PyⅡ; The average Au content of arsenopyrite is 36.02×10−6, and arsenopyrite is mainly enriched in elements such as As, Ni, Sb, Se, Au, whereas the contents of Zn, Ag, Hg, Tl are relatively low. In addition, the in-situ δ34S values of gold-bearing minerals in the Naneng gold deposit are consistent, ranging from 13.7‰ to 16.5‰, indicating that the sulfur in these minerals mainly come from the surrounding rocks. It is preliminarily concluded that PyⅠ was formed in a relatively stable environment by medium to low temperature hydrothermal fluids from the same source rich in trace elements such as Au, As, Sb, and a small amount of Au was precipitated simultaneously with PyⅠ in the form of solid solution (Au+). In the PyⅡ stage, the intense tectonic activities in the study area caused ore-forming fluid to upswell, and after sulfidation reaction with surrounding rock strata, the concentration of H2S in the fluid decreased, causing the Au-HS complexes to become unstable. Consequently, Au was supersaturated and precipitated, and then highly enriched in PyⅡ in the form of nanoscale inclusions (Au0). ConclusionThe research results are of great significance for the prospecting of Carlin-type gold deposits in southeastern Yunnan.