Control of crustal deformation on orogenic Au mineralization in Himalaya: A case study from Buzhu

Abstract

The Buzhu Au deposit in the Himalaya orogen, experiencing Cenozoic uplift of the Himalaya, provides a window to study the control of crustal deformation on Au mineralization. The Au mineralization is characterized by quartz veins controlled by an extensional fault system, comprising W- to WNW-trending shear zones/faults and superposed NNE- to N-trending normal faults. The vein system experienced three deformation stages, stages I and II occurred in shear zones, and stage III developed in normal faults. The deformation comprises hydraulic brecciation in stage I and crack-sealing processes during stage II shear deformation in the brittle-ductile transition, followed by matrix-supported breccias progressively crosscut by veins in newly-formed normal faults in stage III. Stages I and II contain invisible-Au-dominated sulfides, while native Au and pyrrhotite formed in stage III.

Blocky quartz with oscillatory and sector zoning patterns implies fluid pressure build-up processes in stage I. Pyrite displays coupled Au-As variation, high Au contents (<19 ppm), and restricted δ³⁴S values (−3.6 to −2.4‰), supporting that fluid-rock reaction was responsible for Au precipitation. Elongated quartz of stage II displays contrasting bright and dark cathodoluminescence bands with corresponding high and low Al-Li concentrations, indicating fluctuating fluid pressure. Pyrite shows a negative correlation between Au (<45 ppm) and δ³⁴S values (5.4–1.7‰, from cores to rims), consistent with fluid oxidation associated with a fluid pressure drop. Minor late quartz in open spaces transected earlier quartz, implying the hydrothermal system dropped to near-hydrostatic conditions. The partial replacement textures of pyrite and arsenopyrite from stage III, with varied δ³⁴S values (5.9–9.3‰), and the existence of micro-inclusions and visible Au along the contact, suggest a fluid-mediated dissolution-reprecipitation process. Thus, it is demonstrated that mineralization occurred as the hydrothermal system transitioned from the relatively ductile to brittle domain, with varying Au precipitation mechanisms. This study further summarizes the similarities of Au mineralizing systems in young collisional orogens worldwide, particularly with respect to relationships between structural-hydrothermal system and rapid uplift.