GRANITOID-HOSTED GOLD DEPOSITS IN THE ANJIAYINGZI DISTRICT OF INNER-MONGOLIA, PEOPLES-REPUBLIC-OF-CHINA

The Anjiayingzi district contains several mesothermal, quartz vein-typ e gold deposits which are hosted by a Mesozoic quartz monzonite (Jigua nzi pluton). A second pluton (Kalaqin granite), which is older than th e quartz monzonite, is not mineralized. The granitic plutons are situa ted in a basement sequence of early Precambrian gneisses. Rhyolite dik es occupy the same fracture systems as the mineralized quartz veins an d the dikes crosscut the veins; thus the ape of mineralization is cons trained between that of the host pluton (124-132 Ma, K-Ar and Rb-Sr bi otite ages) and the dikes (122 +/- 1 Ma, Rb-Sr whole-rock isochron). G old deposits consist of quartz sulfide veins and alteration zones in f aults and fractures within the Jiguanzi quartz monzonite. Hydrothermal alteration formed quartz-sericite-pyrite assemblages as vein selvages and propylitic assemblages which extend up to several decimeters outw ard from the selvages. Mineralizing fluids removed Na, Ca, Sr, and Ba from wall rock, and added Mn, It, Rb, S, CO2, and the ore elements Au, Cu, Pb, and Zn. Fluid inclusions in vein and selvage quartz contain l ow-salinity fluids with about 10 to 50 mole percent CO2. Total homogen ization temperatures of 300 degrees +/- 30 degrees C (2 sigma) are con sistent with temperature estimates from chlorite geothermometry. Inclu sion evidence of CO2-H2O unmixing fixes the trapping pressure at 0.5 t o 1.1 kbars. Fluid unmixing may have been a factor in causing gold pre cipitation. Most whole-rock delta(18)O values of the Jiguanzi host plu ton range from 7.6 to 8.2 per mil (SMOW) and these probably represent magmatic compositions. Lower values (0-5 parts per thousand) were obta ined from two samples of the pluton and from the rhyolite dikes. These low delta(18)O values reflect postmagmatic. alteration. Limited S-O-H isotope data from pyrite and vein quartz indicate that the hydrotherm al fluids were of meteoric origin and that S was leached from the base ment gneisses. Neither the Jiguanzi quartz monzonite, which hosts the gold ore, nor the rhyolite dikes, with which gold mineralization is mo st closely associated in the field, are anomalously enriched in gold. Therefore we rule out the granitic intrusions as a source of ore eleme nts or fluid.