Granitoid-hosted gold deposits in the Anjiayingzi District of Inner Mongolia, People's Republic of China

Abstract

The Anjiayingzi district contains several mesothermal, quartz vein-type gold deposits which are hosted by a Mesozoic quartz monzonite (Jiguanzi pluton). A second pluton (Kalaqin granite), which is older than the quartz monzonite, is not mineralized. The granitic plutons are situated in a basement sequence of early Precambrian gneisses. Rhyolite dikes occupy the same fracture systems as the mineralized quartz veins and the dikes crosscut the veins; thus the age of mineralization is constrained between that of the host pluton (124-132 Ma, K-Ar and Rb-Sr biotite ages) and the dikes (122 + or - 1 Ma, Rb-Sr whole-rock isochron).Gold deposits consist of quartz sulfide veins and alteration zones in faults 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 outward from the selvages. Mineralizing fluids removed Na, Ca, Sr, and Ba from wall rock, and added Mn, K, Rb, S, CO 2 and the ore elements Au, Cu, Pb, and Zn. Fluid inclusions in vein and selvage quartz contain low-salinity fluids with about 10 to 50 mole percent CO 2 . Total homogenization temperatures of 300 degrees + or - 30 degrees C (2Sigma ) are consistent with temperature estimates from chlorite geothermometry. Inclusion evidence of CO 2 - H 2 O unmixing fixes the trapping pressure at 0.5 to 1.1 kbars. Fluid unmixing may have been a factor in causing gold precipitation.Most whole-rock delta 18 O values of the Jiguanzi host pluton range from 7.6 to 8.2 per mil (SMOW) and these probably represent magmatic compositions. Lower values (0-5ppm) were obtained 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 hydrothermal fluids were of meteoric origin and that S was leached from the basement gneisses. Neither the Jiguanzi quartz monzonite, which hosts the gold ore, nor the rhyolite dikes, with which gold mineralization is most closely associated in the field, are anomalously enriched in gold. Therefore we rule out the granitic intrusions as a source of ore elements or fluid.