Quartz textures and compositions of the Axi epithermal gold deposit, Xinjiang, NW China: Implications for coprecipitation mechanism of silica and metals

Abstract

The textural characteristics of quartz are crucial for understanding the evolution of ore-forming fluids and the mechanisms of mineral precipitation, as they can record hydrothermal physicochemical conditions. The Axi deposit is one of the largest low-sulfidation epithermal gold deposits in the Western Tianshan, Xinjiang, where four ore-forming stages with various quartz textures developed, but the coprecipitation mechanism between quartz and metals remains enigmatic. In this contribution, scanning electron microscope cathodoluminescence (SEM-CL) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) were conducted to study the formation process of these textures and their composition variations. Four generations of quartz were identified: (1) spheroidal, colloform SiO₂ (Qz_1s), and crustiform quartz (Qz_1c) exhibiting oscillatory CL intensity accompanied by bladed ankerite and electrum; (2) cryptocrystalline/mosaic quartz (Qz₂) with dark CL; (3) microcrystalline quartz (Qz₃) coprecipitated with abundant ankerites and sulfides displaying relatively bright CL; (4) euhedral comb/zonal quartz (Qz₄). Integrated studies suggests that the development of these textures is a record of fluctuations in the fluid boiling degree. The coprecipitation of metals and colloidal silica within stages I and III indicates that metal nanoparticles could be mechanically transported with the protection of colloidal silica in hydrothermal solution. Boiling-induced turbulence could enhance the collision and aggregation of colloidal particles, which might be the principal mechanism leading to mineral precipitation. Simultaneously, quartz composition reveals that ore precipitation is always accompanied by a considerable reduction in Al content, which could be ascribed to the fluid degassing-induced escape of acid volatiles (e.g., CO₂, H₂S) and the precipitation of carbonate minerals, thereby leading to an increase in fluid pH. The Al content in the four generations of quartz exhibited a pattern of initial increase followed by a subsequent decrease, represents a multi-stage injection pattern of hydrothermal fluids. In consequence, the variation of Al content quartz might be the fingerprint parameter contributing to the ore precipitation process in the epithermal deposit.