Ore-forming fluid source of the orogenic gold deposit: Implications from a combined pyrite texture and geochemistry study

Interpretation of bulk sulfur isotope data of the orogenic gold deposit is frequently hampered by complex zoning in pyrite, which calls for in-situ determination of sulfur isotope composition of sulfide minerals. The Qiuling gold deposit, located in Qinling orogen, is representative of orogenic type and selected here to further constrain the source of sulfur (and Au) and test the current popular ore genesis models for deposits of this type. Detailed backscattered electron imaging (BSE) study is applied to characterize intragrain texture, and subsequently in-situ sulfur isotope and trace element determination using secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are used to reveal sulfur and metal source and evolution of the ore-forming fluid.

Two generations of pyrite, i.e., pre-ore stage (Py1) and ore stage pyrite (Py2), were investigated in this study. In most cases, Py1 occurs as pyrite framboids, while Py2 as euhedral grains, in which Py2a as anhedral core, Py2b and Py2c as euhedral inner and outer rim, respectively. The delta S-34 values of both Py1 and Py2 have unusual wide ranges, i.e., a delta S-34 variation from - 31.1 to + 106.7 parts per thousand for Py1 (average + 0.8 parts per thousand) and from -2.7 to +24.1 parts per thousand for Py2 (average + 13.2). The distribution pattern of 834 s values in Py1 is accommodated with Rayleigh fractionation in sulfate-restricted pore water. Modeling results show that neither temperature nor oxygen fugacity change could result the variations in delta S-34 values of Py2, and contribution of both preexisting pyrite and a pulse of ore-forming fluid with delta S-34 about + 14.2 parts per thousand may be responsible for the observed variation in Py2. Such fluid may have been sourced from metamorphism of old strata such as the Neoproterozoic basement. Furthermore, Py2a shows an intimate relationship with Py1 in its delta S-34 values, indicating Py1 may have acted as part of sulfur source in the formation of Py2a.

Py1 is rich in a suit of element such as Bi, Co, Mn, Ni, Tl and has the lowest As and Au. Py2a is similar with Py1 but with elevated As and Au. Py2b contains more abundant As and Au compared to Py2a. Py2c is rich in As and especially Au, but depleted in other elements compared to Py2a. The distribution pattern, coupled with sulfur isotope composition, indicates that Py2a was likely formed with a contribution of trace elements and sulfur from Py1 at the start of hydrothermal process. Py2b was formed with elevated proportion of contribution from the ore-forming hydrothermal fluid, while Py2c was formed solely by the ore-forming fluid without a contribution from previous pyrite. Consequently, the ore-forming fluid in the Qiuling gold deposit was rich in As, Au, Cu and Sb while depleted in other metals such as Bi, Co, Mn, Ni and Tl, indicating that the pre-ore framboid pyrite may have not contributed any Au and As during mineralization.

This study highlights the importance of detailed texture characterization coupled with relatively large data set of in-situ sulfur isotope and trace elements for orogenic gold deposit research, and further proves that metamorphic fluids may have significant contribution for Au mineralization in orogenic gold deposits.