Hydrothermal Alteration, Mass Transfer and Genesis of the Ashawayi Gold Deposit, Southwestern Tianshan Orogen, China

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ABSTRACT The Ashawayi orogenic deposit is the third largest gold deposit in the southwestern Tianshan Orogen, Xinjiang, China. Based on an extensive field geological survey, fresh wall rocks, altered wall rocks and mineralised rocks were systematically collected to study the alteration zoning patterns. Major and trace element compositions of these rocks were analysed to calculate the elemental mass transfer during mineralisation processes. The degree of primary alteration, such as pyritisation, silicification and sericitisation, escalates progressively from peripheral fresh wall rocks towards the central mineralised zone. Notably, the pyritisation can serve as the most direct prospecting signal. During the mineralisation, K 2 O, MnO and SiO 2 moved substantially into the central mineralised zone, whereas FeO, MgO and Na 2 O migrated out of the central zone. In addition, Pb and Rb were significantly enriched in the central mineralised zone by external input. In contrast, the moderate or significant depletion in Nb, Ni, Ga and Cu indicated a mass loss of these elements from the altered or mineralised rocks. However, indistinguishable patterns of the other trace elements and rare earth elements (REEs) between fresh, altered and mineralised rock samples indicated that the ore‐forming materials were probably sourced from the ore‐host strata. Reduced gold (Au 0 ) in the fresh wall rocks was reactivated by K‐rich ore‐forming fluids, which transported the gold in the form of Au(HS) 2− . Subsequently, the destabilisation of the Au(HS) 2− complex as a result of intense sulfidation and sericitisation during structural deformation triggered the precipitation of Au from the fluids, leading to the formation of gold orebodies. The deposit is located at the summit of a regional thrust‐nappe structure and is controlled by ductile‐brittle deformation structures. The mineralisation was associated with the tectonic transition from compression to strike‐slip extension.

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