Rare earth elements and carbon-oxygen isotopes of calcite from the Tongjiachong Cu deposit, South China: Implications for fluid source and mineral precipitation

Abstract Calcite is a major gangue mineral in many types of hydrothermal deposits, and its chemical and carbon-oxygen isotopic signatures can shed light on the metallogenic evolution. The Tongjiachong Cu deposit, located in the northern margin of the Yangtze Block in South China, is hosted by dolostone of the Ediacaran Doushantuo Formation. Empirical geothermometric calculation indicates that the pre-ore chlorite was formed at 265–305°C. Copper mineralization occurs mainly as veinlets in a calcite-chalcopyrite-bornite±ankerite assemblage. Exsolution lamellae of chalcopyrite from bornite indicate that both minerals were formed coevally at 200–250°C. Mineral chemical analysis on the syn-ore hydrothermal calcite from Tongjiachong suggests that all the calcite samples are enriched in rare earth elements and yttrium (REYs) (ΣREY = 53.7–119.9 ppm). They have similar Y/Ho (38.3–42.3) and varying La/Ho (21.8–86.7), and display consistent convex PAAS-normalized REY patterns. The distinct positive Eu anomalies (2.38–3.45) and lack of Yb anomalies (0.92–1.05) suggest initial fluid temperatures of 250 to 420℃ and calcites deposition temperatures of Dolomite samples from the dolostone ore host show a marine carbonate C-O isotopic affinity (13CPDB = -4.0 to -2.7‰; δ18OSMOW = +19.8 to +20.2‰). Comparatively, the hydrothermal calcite contains lower 13CPDB (-7.4 to -6.1‰) and δ18OSMOW (+14.1 to +14.8‰), and display a positive 13CPDB vs. δ18OSMOW correlation. Simulated calculations of water-rock (W/R) interactions and ore geological constraints suggest that the initial fluids were likely originated from organic-rich formation water. We proposed that W/R reactions may have been the main drive for calcite precipitation at Tongjiachong, whilst decreasing temperature and W/R reactions may have led to the chalcopyrite precipitation.