Source of fluids in the Longquanzhan gold deposits in the Yishui area, Shandong, China

Abstract

The Longquanzhan gold deposit, hosted in Archean gneiss is located along the Tanlu fault zone, on the southeastern margin of the North China craton. The orebodies occur as veins striking 15 degrees-18 degrees and dipping SE at 35 degrees-62 degrees. Wall rock alteration types include silicification, pyritization, and sericitization, and chloritization, calcitization. Metallic minerals in ores are dominated by pyrite. Gold occurs mainly in the form of electrum. All inclusions are two-phase (L+V) NaCl- H2O type. The inclusions generally range in diameter from 2 to 8 mu m with a vapour/liquid ratio of 5-90% and mainly 5-10%. The homogenization temperatures of fluid inclusions in the Longquanzhan gold deposit are between 108 and 300 degrees C. The ice-melting temperatures vary from -2.0 to -8.6 degrees C, at a peak of -2.5 to -7.0 degrees C. The salinities determined from the ice-melting point of the fluid inclusions range from 3.39 to 12.39 wt.% NaCl equiv. According to isotope fractionation equation and mean homogenization temperatures, the delta O-18 values of the mineralizing fluids are calculated, ranging from -0.28 parts per thousand to +4.07 parts per thousand, showing mixing of ore-forming fluids and meteoric waters. The He-3/He-4 ratios of fluid inclusions in pyrite are 0.14-0.24 Ra, suggesting the crustal source of ore-forming fluid. The assemblage of alteration minerals, the characteristics of fluid inclusions and stable isotopes indicate that the Longquanzhan gold deposit belongs to epithermal type deposit.