Generation and alteration conditions, fluid regime features of the ore-magmatic system of the South Saryshagan granite intrusion (Western Balkhash region)

Abstract

Research subject . For the first time, the mineral composition, petro- and geochemical features, generation and metasomatic alteration conditions, fluid regime (based on the behavior of F, Sl, and S in apatites), and ore-generating potential of granitoids from the South Saryshagan intrusion (Western Balkhash) were studied. Materials and methods . X-ray fluorescence and atomic emission methods of rock analysis (drilling well core), as well as microprobe (polished sections) studies of mineral composition. Results . The rock composition was found to correspond to moderately alkaline granites, formed at a generation pressure of about 2 kbar and T = 670°C. Subsequent metasomatic transformations were mediumtemperature (313–350°C) and multistage, i.e., phyllitization followed by chloritization. The ferrous composition of chlorite and the confinement of its development area to fracture zones indicate the local scale of the process. Ore mineralization is mainly represented by chalcopyrite, occasionally containing Au. According to the morphology and composition features, apatites from granites can be divided into two groups, i.e., magmatogenic grains and products of their transformations. In a diagram of F-Cl-S ratios, most points of their composition lie in the fields of rocks productive of Cu-porphyry mineralization. Conclusions . Mineral associations and the analysis of F, Cl, and S behavior in apatites indicate changes in the composition of the fluid phase over time under the conditions of an open system. In apatites, sulfur accumulates to the maximum level (0.012 wt % S) simultaneously with an increase in the amount of Cl and a decrease in the amount of F. After S reaches this level, the Cl content in apatites decreases followed by a notable increase in F; as a result, the late portions of the fluid become depleted of F, but enriched with Cl and S. In general, the content of the latter two components is low, compared with that typical of rich Cu–porphyry systems. Since the amount of mobilized copper depends on both the contents of Cl and S in the fluid and the volume of this fluid (determined by the size of the intrusion itself; in our case, it is a small body), the predicted copper reserves in the upper horizons of the South Saryshagan intrusion are small.