Gold accumulation in the metavolcanic-hosted orogenic gold deposit constrained by pyrite paragenesis coupled with in-situ trace elements and sulfur isotope: The Sarekuobu example in the Chinese Altay Orogen

Abstract

Origin and accumulation process of gold in the metavolcanic-hosted orogenic gold deposits is still open to debate. Approximately fifteen gold-bearing deposits are hosted in the metamorphosed volcanics of the Chinese Altay Orogen, NW China. Among them, the representative Sarekuobu gold deposit, occurs as lodes in a Devonian volcanic-sedimentary succession that was deformed into a series of multiple overturned synclines and metamorphosed to (garnet)-biotite-chlorite schists. In previous studies, little attention has been paid to pyrite varieties and their in situ geochemistry to constrain their genetic understanding. Herein, five types of pyrite with distinct textures and mineral assemblages at Sarekuobu are divided into four generations based on the crosscutting and replacing relationship. The earliest pyrite (PyI) occurs as magnetite-pyrite parallel laminated layers comparable to those in VMS deposits, and some of its beddings are crosscut by PyⅡ that is scattered in foliation of the host greenschists and likely formed during syn-deformation. The colloidal PyIIIa and inclusion-rich PyIIIb are present in the same auriferous quartz veins continuously cutting across the PyⅡ-rich schists. It is notable that PyIIIb acts as the most important gold-bearing pyrite containing some micro-mineral inclusions such as visible gold and chalcopyrite. The final inclusion-poor PyⅣ with coarse and euhedral grains replaces or envelops all above three pyrite generation. The highest gold concentrations are presented in PyIIIa and PyIIIb by LA-ICP-MS spot analysis, with the maximum values up to 16 ppm and 426 ppm, respectively. In contrast to the substituted invisible Au nano-particles in lattice structures with time-resolved LA-ICP-MS signal parallel to Fe, the visible Au occurs as sizeable micro-inclusions in PyIIIb revealed by highly variable LA-ICP-MS signal spectrum. Further LA-ICP-MS trace element mapping show Au and other trace elements (e.g., Cu, Pb, Zn, Bi, Sb and Ag) are unevenly distributed in PyIIIb but barren in coexisting PyⅣ, in agreement with the microscopic textures of inclusion-rich PyIIIb and inclusion-poor PyⅣ. Another mapping displays these trace elements are homogeneously distributed along colloidal layers in PyIIIa but depleted in the coexisting PyI. Collectively, PyIIIb is believed to experience relatively longer-time crystallization co-precipitating with native gold and chalcopyrite, whereas PyIIIa is rapidly deposited from the same auriferous ore fluids without sufficient time for crystallization and captures some clay minerals and organic matters. In-situ δ34SV-CDT values of four pyrite generations by fsLA-MC-ICP-MS range from +2.34 to + 10.14‰, and gradually decrease from PyI (mean, +9.59‰), through PyⅡ (+8.67‰), PyIIIa (+6.16‰) and PyIIIb (+4.97‰), to final PyⅣ (+3.66‰). In combination with published sulfur isotope composition of host rocks (maximum, +18.7‰), the formation of the auriferous fluids and deposition of Au in pyrite at Sarekuobu might be attributed to the sulfur pathway of progressively enhanced thermal sulfate reduction (TSR) in the Devonian marine strata. Thus, two process including the Devonian seafloor sedimentation and Triassic orogeny-related metamorphism are critical for formation of the Sarekuobu gold deposit.