Formation of Niujuan Ag-Au Deposit, North China Craton: Constraints from Pyrite Textures and In-Situ Trace Element and H-O-S Isotope Geochemistry

The North China Craton (NCC) hosts numerous world-class Au deposits and these Au deposits can be classified into the Au-only and Ag-Au polymetallics, respectively. The former is mostly located in the eastern NCC, such as in the giant Jiaodong Province, and the latter is mostly distributed along the northern and southern margins of the NCC. Compared with Au-only deposits, the ore genesis of the Ag-Au deposits remains controversial. This paper focuses on the Niujuan Ag-Au deposit in the Fengning ore cluster of the northern margin of the NCC. Detailed deposit geology investigation, texture analysis, and analyses of the in situ trace element and sulfur isotope compositions of pyrite, coupled with H-O isotope compositions of quartz from different stages, were conducted to elucidate the ore-forming processes and metal sources. The results showed that the formation of the Niujuan deposit can be divided into four stages, including a pre-ore siliceous breccia stage (stage 1), syn-ore quartz-pyrite stage (stage 2), syn-ore polymetallic sulfide stage (stage 3), and post-ore fluorite-calcite stage (stage 4). Among these, stage 3 represents the major Ag-Au mineralization stage. Pyrite is well developed within stage 2 and stage 3, representing the intensive sulfidation of the wall rock. Microscopic analytical techniques including gamma-enhanced reflected light and scanning electron microscopy backscattered electron (BSE) reveal that pyrite samples from stage 2 and stage 3 have distinct textures. Pyrite (Py1) from stage 2 is homogeneous but with numerous pores. In contrast, pyrite (Py2) from stage 3 has overgrowth textures, and be divided into three sub-stages from core to rim (Py2a, Py2b, and Py2c) with different BSE brightness levels. LA-ICP-MS trace elements analyses results show that these different stages of pyrite show different composition such as Au, As, Ag, Co, and Ni. Py1 has low Au and Ag concentrations ranging from < 0.1 ppm to 0.02 ppm and < 0.1 ppm to 21.8 ppm, respectively. Py2a has low Au and Ag concentrations ranging from < 0.1 ppm to 0.4 ppm and 0.4 ppm to 118.4 ppm, respectively. Py2b is characterized by high As and low Au contents, with average values of 6670.8 ppm for As and 1.4 ppm for Au. Py2c shows relatively low Co and Ni concentrations ranging from 0.02 ppm to 255.2 ppm and < 0.1 ppm to 9.9 ppm, respectively. The sulfur isotope composition of Py1 and Py2 is relatively consistent, ranging from 3.8‰ to 6.7‰. The H and O isotope compositions of quartz from stage 1, stage 2, and stage 3 have insignificant variations, ranging from −119.5‰ to −101.3‰ for δD and −6.8‰ to −3.7‰ for δ18Ofluid, respectively. The results show that sulfur and, possibly, Au and Ag were mainly derived from magmatic hydrothermal fluids, and a significant amount of meteoric water was involved. Combined with the published mineralizing ages (~140 Ma), this paper suggests that the Niujuan Ag-Au deposit formed during the Early Cretaceous under an extensional setting in response to the eastward retreating subduction of the Paleo-Pacific oceanic plate. Evidence from deposit geology and geochemistry reveals that the mixture of magmatic and meteoric water, together with intensive sulfidation, is the key factor controlling Au and Ag deposition.