Pyrite textures, trace element and sulfur isotopes of Yanlinsi slate-hosted deposit in the Jiangnan Orogen, South China: Implications for gold mineralization processes

Abstract

Yanlinsi (7.3 t Au @ 6.1 g/t) is a slate-hosted gold deposit in the Jiangnan Orogen. Regional gold mineralization remains enigmatic, especially in terms of the mineralization process, and the ore-fluid nature and origin. In this study, we use petrography, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and LA-multi-collector (MC)-ICP-MS analyses to document texture, trace elements geochemistry and sulfur isotopes of pyrite from the Yanlinsi gold deposit. Five types of pyrite are identified, which signify multistage hydrothermal evolution. The alteration/mineralization can be divided into three stages: (I) disseminated pyrite and arsenopyrite in altered slate; (II) polymetallic sulfide-gold in laminated-quartz veins; (III) quartz-carbonate veining. Three types of pyrite of stage I are identified: euhedral coarse-grained Py-0; porous and sulfide-rich core Py-Ⅰa; and homogenous rim Py-Ⅰb. The Py-Ia has porous and sulfide (galena, sphalerite and chalcopyrite)-gold-rich textures compared to Py-0, Py-Ib have lower As contents (10,862–32,932 ppm) than Py-0 (19,318–64,946 ppm), which may represent a new (poor-As and rich-Au) fluid pulse. The poor-As and rich-Au hydrothermal fluids locally overprinted Py-0, precipitating sulfides (galena, sphalerite and chalcopyrite)-gold-rich Py-Ia. With the evolution of hydrothermal fluids, the concentration of Au and As in the residual fluid decreases, and Au (2.228–23.10 ppm)/As-poor Py-Ib precipitated around Py-Ia. In stage II, Py-Ⅱ and Py-0 formed obvious texture and composition contrast. Specifically, the As contents (687.1–22,639 ppm) decreased significantly but the Au contents (5.180–238.4 ppm) increases in Py-Ⅱ. In addition, Py-Ⅱ is characterized by sulfide (arsenopyrite, galena, sphalerite, chalcopyrite and tetrahedrite)-rich inclusions and complex compositional zoning, which we attribute to the fluid phase separation by pressure fluctuation. Due to the abrupt destabilization of metal complexes, the Au-rich Py-Ⅱ precipitation starts from native gold deposition. The Py-III is significantly different from Py-Ⅱ in texture and composition. Only pyrite is present in stage III, whereas gold or other sulfides are absent, representing the final hydrothermal stage. The δ34S values (−4.32 to −1.07 ‰) of the different pyrite types are relatively similar, indicating that the Yanlinsi ore sulfur source is predominantly from the deep ore causative magmas.