Genesis of volcanic-hosted Cu vein deposits in the foreland fold-thrust belt, southeastern Tibetan Plateau: Insights from ore geology, C-O-S-Pb-Cu isotopes, and fluid inclusions at Wenyu

Abstract

Copper deposits hosted by volcanic rocks can have different genetic types. Many copper vein deposits at the margin of the Cenozoic Lanping-Simao foreland fold-thrust belt in southwest China are controlled by thrust faults and hosted by Triassic volcanic rocks formed in association with the evolution of the Paleo-Tethys Ocean. The genesis of these copper deposits is poorly understood. Wenyu is a typical Cu deposit hosted in the Middle−Late Triassic basaltic-andesitic rocks and experienced three major hydrothermal stages characterized by mineral assemblages of calcite, anhydrite, and barite (Stage I); early quartz, chlorite, and epidote (Stage II); and Cu sulfides, galena, pyrite, hematite, and late quartz (Stage III). Consistency of the boiling temperature from fluid inclusions in the calcite and the temperature inferred from the C-O isotopes of the calcite, and the oxygen isotopes of the anhydrite and barite indicate that the stage I fluids probably have a δ13C value of ∼−4.5‰ and δ18O values in the range of ∼−3‰ to +4‰. Oxygen isotopes of the early quartz (δ18O = 15.1‰ to 17.4‰), D-O isotopes of chlorite and epidote, and the fluid inclusion data indicate that the stage II fluids have δ18O values of ∼2.4‰ to 5.0‰ and δ18D values of −68‰ to −53‰. Thus, the stage I and II fluids were inferred to be deep-seated brines with temperatures predominantly between 180 °C and 240 °C and salinities of 16−18 wt% NaCl eq. More significant meteoric water was involved during stage III based on the oxygen isotopes of the late quartz (δ18O = 6.2‰ to 9.5‰). The large variation in δ34S values (−21.3‰ to 1.6‰) in the sulfides, the sample- and grain-scale sulfur isotope fractionation, combined with the ore textural evidence that the sulfates and chlorite were replaced by Cu sulfides accompanied by hematite, indicate that the reduced sulfur responsible for sulfide precipitation was generated by inorganic reduction of the previously formed anhydrite and barite (δ34S = 6.3‰ to 8.6‰) or sulfates in the brines by oxidation of chlorite to hematite. Lead isotopes of galena (206Pb/204Pb = 18.67−18.87; 207Pb/204Pb = 15.68−15.72) and copper isotopes of Cu sulfides (δ65Cu = −0.34‰ to 0.51‰) indicate that the ore-forming metals (e.g., Cu and Pb) were derived from the ore-hosting Triassic volcanic rocks. The Wenyu copper deposit cannot be classified as a porphyry, epithermal, or volcanogenic massive sulfide deposit. It is believed that the deep-seated brines, such as the saline formation water in the Permian marine strata, migrated upward along the thrust faults into the Triassic basaltic-andesitic volcanic rocks, and leached metals from the volcanic rocks and precipitated Cu sulfides via inorganic sulfate reduction at Wenyu during the Cenozoic. Such mineralization processes may explain the genesis of the volcanic-hosted copper vein deposits in the Lanping-Simao foreland fold-thrust belt and other similar belts in the world.