Geology, Geochemistry and Genesis of the Longhua Low-Temperature Hydrothermal Ni-Co Arsenide Deposit in Sedimentary Rocks, Guangxi, South China

Abstract The Longhua Ni–Co deposit, hosted in Cambrian carbonaceous sandstone and siltstone in South China, consists of disseminated niccolite mineralization and high-grade Ni–Co arsenides–quartz–carbonate vein mineralization. The disseminated mineralization is characterized by niccolite hosted in the center of quartz fragments and in argillaceous concretions in Cambrian banded pisolitic carbonaceous silty mudstone, and by parallel niccolite grains in Cambrian pisolitic pelite. These characteristics indicate that the disseminated niccolite originated through redeposition. The Ni–Co vein mineralization forms high-grade ores and is composed mainly of Ni–Co-arsenides. The homogenization temperatures of fluid inclusions in the mineralized vein quartz range from 118 to 219 °C, with corresponding salinities of 1.2 to 8.8 wt.% NaCl equiv. The ore-forming fluids yield δDV-SMOW (‰) and δ18OH2O (‰) values of −54 to −66 and −3.0 to −7.7, respectively, indicating that the source was circulating meteoric water. The Longhua Ni–Co vein mineralization gives a niccolite Re–Os isochron age of 462.6 ± 8.5 Ma (MSWD = 1.03). The geological and geochemical characteristics of the Longhua Ni–Co deposit indicate that it formed as follows. (1) Precambrian quartz–niccolite mineralization was eroded and transported to the Longhua sediment trap, forming low-grade disseminated niccolite mineralization or Ni–Co rich source beds during the Cambrian. (2) High-grade Ni–Co arsenide vein mineralization was formed by further enrichment of Ni–Co from the Ni-rich source beds by circulated meteoric water during the Ordovician (Caledonian orogeny). The formation of the Longhua low-temperature high-grade Ni–Co vein ore indicates that Ni can be transported in low-temperature fluids that are rich in arsenic and HCO3− or CO32−. The Longhua Ni–Co deposit might represent a new type of sedimentary hosted low-temperature hydrothermal Ni deposit.