Geological and mineralogical constraints on the genesis of the Bilihe gold deposit in Inner Mongolia, China

Abstract

The Bilihe gold deposit is a rare example that economical gold mineralization occurred in unidirectional solidification textures (UST) quartz at the cupola of middle Permian granodioritic intrusion. However, there is a key dispute about whether gold precipitated from low-diffusion silicate melt or high-activity fluid phase. In this paper, based on in-situ trace elements analyzing, SEM-CL mapping, and infra-red and Raman spectroscopy, the magmatic fluids are supposed to play a critical role in the gold mineralization by metal extraction from below magma at magmatic-hydrothermal transition stage. At Bilihe, the UST quartz represents the most fertile unit. Statistic results show the quartz–K-feldspar (KV) and gray banded quartz veins (GBV) also host about 22–27% of total reserves. A decreasing trend in Au/Ag, and Au/S, but elevated Au/Cu ratios of gold compositions indicate a relatively continuous fluid evolution in the Bilihe system. Petrography and spectroscopy studies show that the Bilihe UST quartz can be divided into two different domains, gold-rich brownish and barren colorless zones. The former can contain more water, occurring as tiny fluid inclusion or as structural OH– in muscovite, corresponding to sector CL zones (UQ1) or euhedral oscillatory bands (UQ2). The latter usually contain melt inclusions and correspond to obscure CL-textures (UQ3). Besides, the LA-ICP-MS trace elements show that there is a significant decreasing trends on fluid-mobile elements (e.g., K, Fe, B, and Li) from UQ1 over UQ2 to UQ3. In particular, the trace elements in UQ3 show greater affinity with magmatic quartz phenocrysts but with higher Ge, B, and Li contents, and higher Ge/Ti and Ge/Al, but lower Li/Ti ratios, indicating a highly evolved and fluid-rich environment. Based on previous model, we consider that the Bilihe UST quartz grew in the variable growth mediums, namely fluctuant alternation between fluid-dominant pocket (for UQ1 + UQ2) and silicate melt phase (for UQ3) near the melt-fluid interface at the apex of magma body. The relatively H2O-poor nature of parent magma is an important factor to cause the specific crystal growth environment. Mineral thermobarometers demonstrate that the parent magma from deep reservoir has experienced a significant ascent or decompression (from ~ 3–5 kbar to ~ 0.5–1.8 kbar). In addition, high fluxing volatiles, such as B, F and P, effectively protract magma crystallization to form highly evolved melt, and finally resulting in water saturation at shallow level. Thus, gold saturation might reach in earlier exsolving low-salinity fluids at low pressure (as low as 0.5 kbar) and high temperature (~660–700 °C) condition. Then voluminous fluids get release accompanying with hydrofracturing, and some gold sequentially precipitate in the KV with fluids cooling (<550 °C) and later in the GBV with further cooling and flashing during transition from lithostatic to hydrostatic loads. With increasing sulfur fugacity by fluid condensation and further cooling in fluid temperature, secondary gold can precipitate with possible Cu-(Fe) or As-Sb sulfides associated with pervasive intermediate argillic alteration, which intensively overprinted on earlier potassic alteration zone.