Discovery and significance of Cu–Zn intermetallic compounds in the Jingxi gold deposit, western Tianshan Mountains, NW China

Abstract

A group of Cu-Zn intermetallic compounds has been discovered in breccias at the Jingxi gold deposit, NW China. The host rocks, silicified tuff and andesite, are cut by pyrite-bearing quartz veins and are intensely silicified which leads to a high SiO 2 content (over 85%). The main component of breccia is siliceous rock, with the cement mainly siliceous and tuffaceous. Under the reflected light microscope, the Cu-Zn intermetallic compounds have quadrilateral shapes which range in size from 6 to 28 μm. Scratches and pitting can be seen on the surfaces of them. The reflection colour of these intermetallic compounds is golden yellow, similar to gold but slightly more white, with a reflectivity that is lower than that of Au and is more close to that of pyrite. They are weakly bireflectant, ranging from bright yellow to golden yellow. Under polarized reflected light, they show a weak anisotropism and no internal reflections. The EMPA (WDS) results of the intermetallic compounds reveal that the ranges of concentration (wt%) are Cu: 81.35~85.36 (on average 83.75), Zn: 6.08~10.10 (on average 7.39), Sn: 4.13~6.75 (on average 5.23) and Ni: 0.88~1.38 (on average 1.05), with minor S: b.d.~5.05. Compared with other Zn-Cu intermetallic compounds previously described, these Cu-Zn intermetallic compounds can be classified as α portion phase which has an over 80% Cu content. In the Jingxi gold deposit, the Cu-Zn intermetallic compounds occur as xenomorphic grains and are paragenetically closely linked with the formation of planar silicification. The characteristics of ore type, ore mineral assemblages and ore textural relations demonstrate that these Cu-Zn intermetallic compounds were formed during the early stage of mineralization (I stage) under conditions of both low fO 2 and fS 2 . Generally speaking, the native elements and their intermetallic compounds formed at the high melting temperatures under strongly reducing conditions, with O and S absent. Because of the sudden drop of the confining pressure, we believe that fast upwelling and cooling of magmatic hydrothermal fluid make reactions between S and Cu/Zn difficult when S is either absent or present. As a result, Cu-Zn intermetallic minerals will deposit.

Indosinian tectonics and mineral systems in China: an introduction

“…Liu et al . () report an excellent case study of the Jingxi gold deposit in Xianjiang, which originally formed in the Early Carboniferous and was then reworked in the Early Triassic. They discovered a group of Cu–Zn intermetallic compounds, containing minor Sn and trace Ni, and concluded that these Cu–Zn intermetallic compounds were formed under conditions of low f O 2 and f S 2 values.…”

“…Formations and preservations of these deposits are openly debated, due to the fact that investigation of the geological characteristics, mineral compositions, and mineralization styles is very poorly known . Liu et al (2014) report an excellent case study of the Jingxi gold deposit in Xianjiang, which originally formed in the Early Carboniferous and was then reworked in the Early Triassic. They discovered a group of Cu-Zn intermetallic compounds, containing minor Sn and trace Ni, and concluded that these Cu-Zn intermetallic compounds were formed under conditions of low fO 2 and fS 2 values.…”