Genesis of the Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China: Constraints from geochronology, fluid inclusion and geochemistry

The Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China, is located in the southwestern margin of Ailaoshan orogenic belt. Here, we use H-O isotope, quartz fluid inclusion, sulfide Re-Os and zircon UPb isotopic dating of copper ore and element contents of basalts to constrain the age and genesis of the deposit. The basalts in the study area formed in the shallow part of the Emeishan mantle plume by low degree of assimilation-contamination in intraplate environment. Investigations revealed two stages of mineralization in the deposit. The ore mineral assemblage in the early stage was pyrite-chalcopyrite-calcite. The early-stage ore forming fluid was characterized by medium-high temperature (206.6 degrees C-279.8 degrees C) and high salinity (12.28%-22.44%), and it was related to water-rock interaction between the Emeishan sulfur-carbonate-bearing volcanic hydrothermal fluid and gypsum layer in the Carboniferous, and experienced boiling. The ore mineral assemblage in the main metallogenic stage is quartz-chalcopyrite-bornite-calcite. Re-Os dating of chalcopyrite in the main stage in No. 1 ore body yielded an age of 246 +/- 17 Ma, and zircon ages of copper ore in No. 3 and No. 4 ore bodies revealed that they formed at 245.6 +/- 1.0 Ma and 244.9 +/- 2.0 Ma, respectively, suggesting that the age of main metallogenic stage is ca. 245 Ma. The ore-forming fluid in the main metallogenic stage was characterized by high temperature (294 degrees C-425.4 degrees C) and low salinity (6.3%-16.05%). Combined with analysis of H-O isotope of quartz samples, we suggested that the main-stage hydrothermal fluid was mainly derived from intermediate acid magmatic water and experienced degasification. The organic substances may act as the reducing agent during the mineralization process. The main-stage mineralization was related to the continental collision between the Simao-Indosinian massif and the Yangtze massif in the Middle Triassic. Re-Os analysis in No. 3 ore body indicated that the deposit was reformed by later hydrothermal fluid in Yanshanian period.