Abstract

The Xiaoqingling Mt. area is one of the most productive gold districts in China. The gold-bearing quartz veins are controlled by a series of east-west shear zones within the Archean Taihua Group, and related to the late Yanshanian (Cretaceous) granite. The major metallic minerals are native gold, electrum, pyrite, galena, chalcopyrite and sphalerite, with gangue minerals dominated by quartz, sericite, ankerite, and calcite. The host rocks are mainly amphibolite, plagioclase gneiss, and migmatites. Principal alteration is sericitization, silicification, pyritization, and carbonization. There are four generations of hydrothermal quartz in altered rocks of gold deposits. The earliest generation of quartz, (Q 1 ), occurs in altered amphibolite gneiss and migmatite, and typically contains fluid inclusions with halite crystals (type A) and two phases, (L+V), of fluid inclusions (type B). The salinities of type A inclusions are 32 wt% to 52 wt%, with dissolution temperatures of halite crystals being from 155 degrees C to 465 degrees C. The second generation of quartz, (Q 2 ), occurs as a fine-grained aggregate associated with sericite, and has small (less than 3 mu m) type B inclusions. The third generation of quartz, (Q 3 ), occurs in mylonitic, intensively altered rocks and specially beresite (a type of rock composed of quartz, sericite, and pyrite) near the veins, and contains large amounts of CO 2 -rich fluid inclusions (type C-fluid inclusions with liquid CO 2 and type D-fluid inclusion with high density CO 2 ). The CO 2 contents of type C range from 72.6 to 87.4 mol% in gas phases, and 35.7 to 68.3 mol% in liquid phases, based on Laser Raman analysis. The homogenization temperatures of fluid inclusion type A and C are from 215 degrees C to 380 degrees C, and from 210 degrees C to 325 degrees C, respectively. The minimum trapping pressure of fluid inclusions is estimated as 80 MPa to 170 MPa for type A, and from 57 MPa to 190 MPa for type C, which approximate to those of fluid inclusions in vein quartz. The delta 18 O of quartz are from 10.4 per mil to 11.0 per mil for Q 3 of altered rocks, and from 10.7 per mil to 12.8 per mil for stage I quartz. The calculated delta 18 O of fluid inclusions varies from -1.3 per mil to 6.0 per mil. The delta D of fluid inclusions ranges from -67 per mil to -59 per mil and from -81 per mil to -48 per mil, respectively. It is concluded that the water in the hydrothermal fluids both reacting with wallrocks and precipitating orebodies could have been mainly of magmatic and/or metamorphic nature. However, local meteoric water might have been important in late mineralization, because of a distinct oxygen-shift toward the meteoric water. The delta 34 S values for pyrite of the ores range from -7.1 per mil to +7.1 per mil, with an average of +2.9 per mil. Those of other sulfides (chalcopyrite, galena, and sphalerite, range from -12.5 per mil to 8.3 per mil. The ratios of lead isotopes for galena of the ores are: 17.06 to 17.31 ( 206 Pb/ 204 Pb), 15.43 to 15.74 ( 207 Pb/ 204 Pb) and 37.58 to 38.45 ( 208 Pb/ 204 Pb). In summary, ore-forming fluids in the Wenyu-Dongchuang gold deposits may have originated from a mantle and/or lower crust source, and ore materials were derived from the Precambrian basement, the Taihua Group.

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