Geology, Ar-Ar Age and Mineral Assemblage of Eocene Skarn Cu-Au±Mo Deposits in the Southeastern Gangdese Arc, Southern Tibet: Implications for Deep Exploration

Abstract

. Medium- and large-scaled skarn Cu-Au±Mo deposits, e.g. Kelu, Liebu, Chongmuda and Chenba among others, are distributed in Shannan area of the Gangdese Cu-Au metallogenic belt. Intrusions-related skarn copper mineralization belongs to high K and calc-alkaline rock series, located in late collision volcano-magmatic arc and formed between 20 to 30 Ma. Copper mineralization occurs at exocontact zone of the lower Cretaceous Bima Group carbonate and other calcareous-bearing sedimentary rocks with intrusions. At present, three main mineralization types are identified, including skarn type, hydrothermal vein type and porphyry type. Mineralizing associations are Cu-Mo, Cu-Au and Cu. In ore districts, those mineralization types form an entire porphyry-skarn Cu-Au±Mo ore-forming system. Alterations of the exocontact are mainly skarnization and hornfelsization, while the alterations of the endocontact are mainly sericitization, silicification, and chloritization of intrusion. In the study area, the endoskarn is not well developed. Copper mineralization occurs mainly in the exocontact in the form of stratoid, lenticular and pockety ore body. Veined mineralization can be seen in marblized and hornfelsed siltstone, being away from the contact zone. In the endocontact, the mineralization is mainly veinlet-like and disseminated. In Shannan area, skarnization can be divided into early skarnization stage and late hydrous silicate stage. The early skarnization stage is featured by mainly andradite and grossular skarn, containing minor diopside, hedenbergite, magnetite and some copper minerals; and the late hydrous silicate stage is of replacement of garnet skarn by chlorite, epidote, quartz and calcite together with sulfides precipitation. The latter is the main stage of copper mineralization. Bornite is the dominant ore mineral associated with minor chalcopyrite and pyrite; and gold as well as silver are distributed in bornite and wittichenite. Results of microthermometry study of fluid inclusions in quartz of late hydrous silicate stage from different deposits show intermediate temperature and low to intermediate-salinity features for all samples. The dominant inclusion type is composed of two phases, being about 4 to 15 % vapor and 85 to 96 % liquid at room temperature. Homogenization temperatures range from 232 to 335dC. Salinities have been recorded between 4.2 and 15.5 wt% NaCl equivalent. Boiling fluid inclusions are not identified and it indicates that metal deposition mainly resulted from water-rock reactions. The results of sulfur isotope analysis indicate that the sulfur isotope values (δ34S 1.29–1.68 %o) of the samples collected from skarns are similar with that from the endocontact (δ34S 1–1.75 %o). Both of them have very close sulfur isotope values (near δ34S 0 %o), which indicate the sulfur of both the skarn type and the porphyry type mineralization was from deep sources. Ages determined on biotite from ore-bearing intermediate porphyries by Ar-Ar methods range from 23.77±0.29 to 29.88±0.56 Ma, showing that skarn copper mineralization in the study area evidently is older than the porphyry Cu(-Mo) mineralization in Gangdese, and likely representing another metallogenic event. The Cu-Au skarn deposits in the Kelu-Liebu-Chongmuda belt are interpreted as the shallow level, skarn-related deposits in a porphyry-skarn mineralization. Appearance of porphyry copper mineralization in some skarn deposits implies that skarn copper mineralization of the study area resemble to those in northern sub-metallogenic belt, having uniform porphyry-skarn ore-forming system. Therefore, it is presumed there should be potential to find deep level porphyry-type Cu-Au mineralization targets.