Mineralization processes involved in the formation of the Jinya Carlin-type Au deposit, northwestern Guangxi, China: Evidence from in situ trace element and S isotope geochemistry of Au-bearing zoned pyrite

Abstract

The Jinya Au deposit is a representative example of the Carlin-type Au mineralization located within the “Golden Triangle” area of the Yunnan, Guizhou, and Guangxi provinces, China. The deposit is located within a fault zone at the contact between a carbonate platform and contains Au mineralization hosted by arsenopyrite and pyrite, with the latter providing evidence of changes in fluid composition during pyrite growth. Back-scattered electron (BSE) images indicate that this Au-bearing zoned pyrite can be divided into dark cores (Py1), lighter colored central sections (Py2), and dark rims (Py3). Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) and secondary ion mass spectrometry (SIMS) analyses provide information on compositional and S isotopic variations within this pyrite. These data indicate that each pyrite zone is compositionally homogeneous, showing Co/Ni ratios of <1. Pyrite has δ34S values ranging from –8.54‰ to –4.05‰, with Py1, Py2, and Py3 having average δ34S values of –6.89‰, –5.60‰, and –5.99‰, respectively. These values are indicative of thermochemical sulfate reduction (TSR) processes. Mineralizing fluids that formed the deposit derived from a sedimentary source mixed with variable amounts of meteoric water (Py1 = 3%; Py2 = 32%; Py3 = 23%). The ore-forming fluids within the sedimentary basin metasomatized the Fe-rich dolomitic rocks of the forming the early pyrite cores (Py1). This fluid flow focused along the main fault zone, which at this time was permeable, allowing with an influx of meteoric water. This mixing process lowered the temperature of the system, causing the precipitation of large volumes of Au and As. This caused the formation of the light-colored and As- and Au-enriched central parts of the pyrite within the deposit (i.e., Py2). A subsequent decrease in the amount of meteoric water in the system led the formation of the darker Py3 rims that contain low concentrations of As and Au.