Tracing water-rock interaction in carbonate replacement deposits: A SIMS pyrite S-Pb isotope perspective from the Chinese Xinqiao system

Abstract

Deciphering the sources of metals is a prerequisite to establishing genetic models of ore formation, and is of importance for developing exploration models. For most magmatic-hydrothermal deposits, non-magmatic sourced components (e.g., fluids, sulfur and metals) are suggested to have minor or negligible contributions. On the other hand, there is substantial evidence to indicate that external ore-forming components could be crucial for the mineralization of some magmatic-hydrothermal deposits. Here, we conduct a detailed pyrite sulfur and lead isotope study on the Xinqiao stratabound deposit, South China, with the aim of constraining the contributions of ore-forming components from sedimentary rocks and existing mineralization through water-rock interaction. Traditionally, the source of mineralization at Xinqiao is debated. For example, a Cretaceous magmatic-hydrothermal origin was proposed as the sole source of metals. In sharp contrast, a Carboniferous sedimentary sulfide layer as proto-ore, which was enriched later by the Cretaceous magmatic-hydrothermal fluids, is required in a remobilization model. Crystallized pyrite grains from the stratabound orebody (py1) have a δ34S value of 4.09 ± 1.42 ‰ (2 SD), which is similar to that of pyrite grains from the garnet bearing skarn ore (py3). Colloform pyrite ore, which is composed of fine-grained (∼500 nm) cubic pyrite grains (py2a), has a δ34S value of 0.20 ± 1.14 ‰. The sulfur isotope composition of pyrite grains from the sandstone hosted quartz-pyrite veins is bimodal with δ34S values of 3.28 ± 6.46 ‰ (py4a) and −11.87 ± 5.43 ‰ (py4b). The Pb isotope compositions (206/204Pb ≈ 18.5, 207/204Pb ≈ 15.5) of py1, py4a and py4b are broadly the same as that of K-feldspar from the Cretaceous Jitou diorite, while py2a are more radiogenic (206/204Pb ≈ 21.5, 207/204Pb ≈ 18). Our pyrite S and Pb isotope data show no support for the presence of a Carboniferous sedimentary sulfide layer as proto-ore, but instead suggest that leaching sulfur and metals from Permian shales and an unexposed mineralization through water-rock interaction is a vital mechanism for the mineralization at Xinqiao. In line with previous studies, contributions from the late Permian shales are complementary for the stratabound orebody, and recycling existing Mo-Au rich mineralization is key for the formation of sandstone hosted gold rich pyrite-quartz veins. The existing mineralization is not exposed yet, and should be considered in future mineral exploration. Limited sulfur isotope fractionation between py1, py2 and py3 (0–4 ‰) further indicates a relatively reducing condition during their formation, while significant lower sulfur isotope composition (δ34S = −12 ‰) of py4 could imply an oxidizing condition during its formation. Our study highlights that detailed in-situ sulfur and lead isotope analyses under robust geological and petrographic frameworks can tightly constrain the source of ore-forming components, yield insights into oxygen fugacity during ore formation, and offer clues for future mineral exploration.