Trace element contents and in situ sulfur isotope analyses of pyrite in the Baiyun gold deposit, NE China: Implication for the genesis of intrusion related gold deposits

摘要

Trace elements distribution in minerals and formation mechanism of visible gold are poorly understood in intrusion-related gold systems. Here, we choose the Baiyun gold deposit, which is hosted within the Paleoproterozoic metamorphic rocks in the Liaodong Peninsula, NE North China Craton (NCC), as a case study, to address these issues. The ore deposit geology characteristics and paragenesis are described in details; In situ geochemical and sulfur isotopic compositions of pyrite and bulk Pb isotopic ratios of pyrite were presented to define the genesis of pyrite that is closely associated with gold mineralization. Three generations of pyrite were recognized based on petrographic studies: metamorphic pyrite (Py0), hydrothermal pyrite (Py1) coexisting with milky quartz, and hydrothermal pyrite (Py2) coexisting with smoky quartz. In situ laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) spot analysis suggests that both Py0 and Py1 contain low contents of invisible gold (mean 0.19 ppm and 0.17 ppm, respectively). However, Py1 is low in most other trace element contents and contains Au-Ag-Cu-Pb-Bi micro-inclusions. In contrast, Py2 contains both invisible gold (mean 0.50 ppm) and Au-Ag-Cu-Pb-Bi micro-inclusions. Furthermore, LA-ICP-MS mapping reveals the distributions of Ag, Cu, Pb, Bi and Te closely mimic that of Au, indicating an episode of ore-forming fluid rich in Au, Ag, Cu, Pb, Bi and Te. In situ sulfur isotope analysis of pyrite shows that Py0 has significantly positive delta S-34 values ( + 11.74-17.33 parts per thousand), suggesting that sulfur was derived from sedimentary sources. Py1 and Py2 have negative delta S-34 values (-8.53-6.19 parts per thousand, 10.44-6.86 parts per thousand, respectively), indicating that sulfur was derived from oxidized magmatic fluids. The magmatic fluids are concluded to be released from the early Cretaceous microdiorite magmas, providing most of the sulfur and ore-forming metals. Proofs include the close spatial association of the ore bodies and the dykes, as well the uniform Pb isotopic ratios of Pyl and the early Cretaceous microdiorite. Lines of geological, mineralogical, geochemical and isotopic evidence consistently suggest that the gold mineralization in the Baiyun deposit is intrusion-related, and was resulted from the similar to 126 Ma magmatic-hydrothermal activities in the region. Visible gold precipitating in pyrite and fractures was from metal-rich magmatic fluids rather than being remobilized from the metamorphic pyrite.