Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 27-
A number of word-class gold deposits in the Kassan area in the west Kyrgyzstan are associated with the Andagul granitic intrusion. These gold deposits share similarities with the class of “intrusion-related gold deposits”, implying that the gold mineralization may be genetically related to the intrusion. In order to evaluate this potential relationship, it is important to study the petrogenesis of the intrusion. This study examines the sources and physiochemical conditions of crystallization of the Andagul intrusion and their implications for gold mineralization, through an integrated study involving zircon geochronology, mineral chemistry, whole-rock chemistry and isotopic geochemistry. The Andagul intrusion consists mainly of granodiorite, which is characterized by metaluminous and medium-K calc-alkaline compositions, very high Ba (>2000 ppm) and Sr (>1500 ppm) abundances, high LREE, LILE, low HFSE , right-inclined REE pattern with a flat HREE pattern, and insignificant Eu anomalies ((Eu/Eu*)N = 0.80 - 0.85). These geochemical characteristics, together with relatively low positive εNd(t) values (+2.1 to +4.2), moderately low (87Sr/86Sr)i ratios (0.70497 to 0.70666), relatively low initial Pb isotopic ratios ( (206Pb/204Pb)t = 17.642 to 18.398, (207Pb/204Pb)t = 15.599 to 15.618, and (208Pb/204Pb)t = 38.034 to 38.297), juvenile εHf(t) (+2.29 to +4.78) and relatively young model ages (T2DM(Hf)=1027 to 1180 Ma, T2DM(Nd)= 729 - 898 Ma) indicate that the magma was sourced from partial melting of juvenile basaltic lower crust with amphibole as dominant residual. The high Zr saturation temperatures (807oC to 825 oC), relatively high Mg# values (43-52) and presence of mafic enclaves, and development of acicular apatite and oscillatory plagioclases and titanite, suggest that the magmatism was related to injection of hot mantle material into the lower crust, which took place in a slab-break off setting at ca. 300 Ma. The high water contents (4.0 to 5.2 wt.% H2O) and moderately oxidized to weakly reduced state (ΔFMQ = -0.29 ~ +2.82) deduced from amphibole composition and zircon Ce anomaly are favorable for enrichment of gold in magmatic fluids.
