Abstract
The Fancha gold deposit is a representative of the auriferous quartz vein type gold deposits in the Xiaoqingling gold district where the ore-forming fluid is one of the key controlling factors in the deposits formation, and also the main focus of ore deposit research. Here, the geological characteristics and fluid inclusions of the Fancha deposit are presented in detail, with a description of the potential origin and evolution of the fluid and the genesis of the deposit. Field investigation and petrographic observation show that ore-bodies in the Fancha deposit are preferentially hosted in the amphibolites of the Taihua Group and are controlled by brittle fracture. Hydrothermal alteration in the Fancha deposit includes silicification, potash feldspathization, sericitization, pyritization and carbonatization. The mineral assemblage, cross-cutting and replacement relationships indicate that the ore-forming process can be divided into quartz-K-feldspar (I), quartz-coarse pyrite (Ⅱ), quartz-fine pyrite (III), quartz-polymetallic sulfide (IV) and carbonate stages (V). Gold is mainly precipitated in stages III and IV. Three types of fluid inclusions have been identified in the Fancha deposit, based on their characteristics at room temperature: three-phase CO2-rich, three-phase CO2-bearing and two-phase aqueous inclusions. The fluid inclusions demonstrate the modes of homogenization temperatures of 363.3℃, 325.5℃, 273℃, 258.62℃ and salinities of 14.53, 9.59, 11.61, 8.03 wt% NaCl.equiv., for stages II, III, IV, and V respectively. The ore-forming fluid in the Fancha deposit belonged to the CO2-H2O-NaCl-(±CH4) fluid system with mid-low temperature and mid-low salinity. The δ18OH2O of the metallogenic fluids was between 1.36 and 6.28, and the δDH2O was between -87 and -53.1. Hydrogen and oxygen isotope data indicate that the ore-forming fluids were dominated by metamorphic fluids in the early stage, and that the metamorphic fluid gradually mixed with circulating meteoric water during the later stages. Petrographic observation and microthermometry suggest that fluid mixing and immiscibility obviously occurred during the main metallogenic stage. Fluid mixing and fluid immiscibility probably led to gold deposition in the Fancha deposit.
Highlights
The Xiaoqinling gold deposit area is an important rock gold producing areas in China (CHEN et al, 1998; FAN et al, 2000a; MAO et al, 2002; WANG et al, 2010)
The Fancha gold deposit is a representative of the auriferous quartz vein type gold deposits in the Xiaoqingling gold district where the ore-forming fluid is one of the key controlling factors in the deposits formation, and the main focus of ore deposit research
The geological characteristics and fluid inclusions of the Fancha deposit are presented in detail, with a description of the potential origin and evolution of the fluid and the genesis of the deposit
Summary
The Xiaoqinling gold deposit area is an important rock gold producing areas in China (CHEN et al, 1998; FAN et al, 2000a; MAO et al, 2002; WANG et al, 2010). Previous studies have investigated the ore-forming fluids of the gold deposits in the area (CHEN et al, 2007; FAN et al, 2003; LI et al, 2012b; MAO et al, 2002; ). The main theories on the origins of the ore-forming fluids include: (1) metamorphic fluids (WANG and ZHOU, 1996), (2) magmatic fluids (NIE et al, 2001), (3) deep mantle fluids (LU et al, 2003), and (4) mixed fluids (CHEN et al, 2007). In terms of the relationship between these two, there is debate: LI et al (1996) considered that the basic dykes provide a material source for gold mineralization, while CHEN et al (1998), LI et al (2012a), LI et al (2012b) challenged this theory. We explored the metallogenic mecha nism by tracking the source of the ore-forming fluids and analyzing the fluid evolution
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