Geological and ore-forming fluid characteristics of vein type copper deposits in northern Ningwu area

Abstract：There exist many vein type copper deposits of hydrothermal origin in northern Ningwu area. The authors focused on ore-forming fluid inclusions of Guli and Nanmentou copper deposits in the study area. The results show that the homogenization temperatures of fluid inclusions in quartz of the quartz-metal-sulfide stage range from 202.9 to 299.4℃ for Guli, and from 166.7 to 355.4℃ for Nanmentou; the salinities range from 4.5 to 11.1 wt% NaCl eqv. for Guli, and from 3.5 to 12.0 wt% NaCl eqv. for Nanmentou; the densities range from 0.75 to 1.01 g/cm3 for Guli, and from 0.80 to 0.89 g/cm3 for Nanmentou; the ore-forming pressures and depths range from 18.2 to 30.4 MPa and from 0.69 to 1.15 km for Guli, and from 12.8 to 35.1 MPa and from 0.48 to 1.32 km for Nanmentou respectively. These analytical data combined with data available suggest that the ore-forming hydrothermal fluids of Guli and Nanmentou copper deposits have similar characteristics of middle-low salinity, low density and low depth，but the former is of middle temperature, whereas the latter is of middle-high to middle-low temperature. Raman spectra analysis of fluid inclusions show that there are obvious differences in the gas phase composition. The gas phase composition is mainly H2O+CO2±N2 in the fluid inclusions of Nanmentou, but only H2O has been observed in the fluid inclusions of Guli. Combined with geological characteristics, such as the differences of mineral assemblages, the wall-rock mineralization alteration and ore-forming rock mass, the authors tentatively conclude that there might have existed two main copper metallogenic periods in northern Ningwu area: one was related to early magmatic segregation of diorite porphyrite in Dawangshan cycle ( with the Guli copper deposit as the representative ) and the other was correlated with granite in late Gushan cycle (represented by Nanmentou).