Fluid-rock interaction of the early Cambrian black shale in the South China Block: Implications for low-temperature mineralisation

Abstract

Chalcophile elements hosted by black shales of the Lower Cambrian Niutitang Formation, South China Block, contribute to low-temperature mineralisation in southwest China. To study the mobilisation and migration of ore-forming elements, hydrothermal experiments were performed to simulate leaching of Au, As, Sb and Ag from an Au-rich black shale from the Kaiyang Phosphate Mine (KYPM), China. The experiments were designed to simulate low-temperature mineralisation at 150 °C, with solvent concentrations of ~5%, durations of 15 d and 30 d, and water/rock mass ratios of 13:1 and 26:1. Reagent-grade NaCl, NaHCO3 and Na2S were used to provide the common anions found in hydrothermal systems. The results show that solutions containing Cl− ions in oxidised acidic systems leached Au most effectively. Weakly alkaline systems containing HCO3− ions leached Au less effectively. Solutions containing HS− ions did not leach Au effectively, and Au precipitated within the HS−-bearing system. The results for Ag were similar to those for Au, and Ag-leaching was most effective in the oxidised acidic system containing Cl− ions. As-leaching does not occur in the Cl−-bearing system, but As is leached effectively by HS−-bearing and HCO3−-bearing solutions. Sb was not leached by our experiments. Oxidised fluids produced by deep-sourced magma during the Indosinian and Yanshanian tectonic events infiltrated black shales affected by extensional deformation through faults, joints and fractures. Fluid–rock interaction leached a range of elements in these structural settings. These experiments provide insights into migration of chalcophile elements, particularly Au and Ag, and the source of large-scale low-temperature mineralisation within the South China Block.