An in situ sulfur isotopic investigation of the origin of Carlin-type gold deposits in Youjiang Basin, southwest China

Abstract

• In situ sulfur isotope analysis of pyrite and arsenopyrite in Carlin-type deposits. • Study has focused on relationships between different deposits in the Youjiang Basin. • There are two source end-members from NW to SE basin, δ 34 S ~ +0‰ and δ 34 S > +10‰ • Results lead to a differential deep metamorphic–hydrothermal genetic model. Debate over the origins of ore-forming fluids and metal accumulation in Carlin-type gold (Au) deposits has hampered the development of genetic models for such deposits. Youjiang Basin in southwest China contains Carlin-type deposits, including the Nibao, Banqi, and Yata deposits in Guizhou, and the Gaolong and Jinya deposits in Guangxi, but there has been little study of the relationships between these deposits, and the origins of the ore-forming fluids and metals remain enigmatic. We conducted in situ laser ablation multi–collector inductively coupled plasma mass spectrometry (LA–MC–ICP–MS) sulfur (S) isotope analyses of different stages of pyrite and arsenopyrite to improve our understanding of the sources and genesis of the deposits. Average δ 34 S values for ore-stage pyrite in the Nibao, Banqi, Yata, Gaolong, and Jinya deposits are −0.05‰, +4.04‰, +7.02‰, +11.51‰, and −3.80‰, respectively, and those of ore-stage arsenopyrite in the Banqi, Gaolong, and Jinya deposits are +7.33‰, +10.92‰, and −3.84‰, respectively. Average δ 34 S values of sedimentary–diagenetic (i.e., pre-ore) stage pyrite in the Gaolong and Nibao deposits are −6.41‰ and −15.61‰, respectively. Two source end-members contribute δ 34 S values of ~0‰, which are deep metamorphic fluids and volcaniclastic rocks in the Nibao deposit, and δ 34 S values of greater than +10‰ are from mixed basinal fluids or marine sulfates in the Gaolong deposit. The Yata and Banqi deposits are intermediate between the other two deposits. The δ 34 S values of ore-stage pyrite increase from northwestern platforms to southeastern basins, with this trend also occurring in other Carlin-type deposits in Youjiang Basin. The sources of S and Au were deep Proterozoic metamorphic basement rocks, which were mixed with a sedimentary source. We propose a genetic model for these Carlin-type Au deposits based on a deep metamorphic–hydrothermal fluid system, with the initial ore-forming fluid generated by the Indosinian or Yanshanian orogenies. Ascending fluids mixed with deeply circulating meteoric waters and/or basinal fluids and reacted with wall-rocks in the shallow crust to form the Au deposits. This study confirms that in situ S isotope analysis is effective for investigating ore genesis and assisting in exploration for new ore fields.