In–Situ LA-ICP-MS Trace Elements Analysis of Pyrite and the Physicochemical Conditions of Telluride Formation at the Baiyun Gold Deposit, North East China: Implications for Gold Distribution and Deposition

Abstract

The Baiyun gold deposit is located in the northeastern North China Craton (NCC) where major ore types include Si-K altered rock and auriferous quartz veins. Sulfide minerals are dominated by pyrite, with minor amounts of chalcopyrite, sphalerite and galena. Combined petrological observations, backscattered electron image (BSE) and laser ablation analysis (LA-ICP-MS) have been conducted on pyrite to reveal its textural and compositional evolution. Three generations of pyrite can be identified—Py1, Py2 and Py3 from early to late. The coarse-grained, porous and euhedral to subhedral Py1 (mostly 200–500 μm) from the K-feldspar altered zone is the earliest. Compositionally, they are enriched in As (up to 11541 ppm) but depleted in Au (generally less than 10 ppm). The signal intensity of Au is higher than background values by two orders of magnitude and shows smooth spectra, indicating that invisible gold exists as homogeneously or nanoscale-inclusions in Py1. Anhedral to subhedral Py2 grains (generally ranging 500–1500 μm) coexist with other sulfides such as chalcopyrite, sphalerite and galena in the early silicification stage (gray quartz). They have many visible gold grains and contain little amounts of invisible Au. Notably, visible gold has an affinity with micro-fractures formed due to late deformation, implying that native gold may have resulted from mobilization of preexisting invisible gold in the structure of Py2 grains. Subsequently Py3 occurs as very fine-grained disseminations of euhedral crystals (0.05–1 mm) in late silicification stage (milky quartz) and coexists with tellurides (e.g. petzite, calaverite and hessite). They contain the highest level of invisible gold with positive correlations between Au-Ag-Te. In the depth profiles of Py3, the smooth Au spectra mirror those of Te with high intensities, revealing that gold occurred as homogeneously/nanoscale-inclusions and submicroscopic Au-bearing telluride inclusions in pyrite grains. The high Te and low As in Py3, combined with high Au content, imply that invisible gold can be efficiently scavenged by Te. Abundant tellurides (petzite, calaverite and hessite) have been recognized in auriferous quartz veins. Lack of symbiosis sulfides with the tellurium assemblages indicates crystallization under low fS2 and/or high fTe2 conditions and coincides with the result of thermodynamic calculations. High and markedly variable Co (from 0.24 to 2763 ppm, average 151.9 ppm) and Ni (from 1.16 to 4102 ppm, average 333.1 ppm) values suggest that ore-forming fluid may originate from a magmatically-derived hydrothermal system. Combined with previous geochronological data, the textural and compositional evolution of pyrite indicates that the Baiyun gold deposit has experienced a prolonged history of mineralization. In the late Triassic (220,230 Ma), the magmatic hydrothermal fluids, which had affinity with the post-collisional extensional tectonics on the NCC northern margin, caused initial gold enrichment. Then, as a result of deformation or the addition of new hydrothermal fluids, visible gold-rich Py2 was formed. The upwelling of mantle–derived magma brought in a lot of Te-rich ore-forming hydrothermal fluids during the peak of the destruction of the NCC (~120 Ma). Amount of visible/invisible gold and Au-Ag-Te mineral assemblages precipitated from these mineralized fluids when the physical and chemical conditions changed.