Fluid inclusion and stable isotope (C, O and S) constraints on the genesis of the high-grade Langdu Cu skarn deposit in Yunnan, SW China

Abstract

The Langdu Cu skarn deposit is the highest grade Cu deposit (average 6.49% Cu) in the Zhongdian polymetallic district in SW China. Skarns and ore bodies occur primarily in the contact zone of Upper Triassic Qugasi Formation (Fm.) and Late Triassic (ca. 217 Ma) intrusions. Disseminated, massive, stockwork, and vein-like sulfide ores occur in the skarn, quartz-calcite veins, porphyry and marble. Skarn minerals include pyroxene, garnet, actinolite, and epidote. Four types (six subtypes) of fluid inclusions (FIs) are identified in the sulfide-bearing quartz veins. Type-I liquid-rich two phase FIs have varying vapor/liquid ratios, and yielded homogenization temperatures of 110–338 °C and salinities of 1.7–29.5 wt% NaCl eqv. Type-II vapor-rich FIs contain CH4-N2 bubbles and homogenized to vapor at 262–425 °C. Type-III FIs are single phase at room temperature (20 °C). A vapor phase is present at below −120 °C, and the FIs homogenized at −117 to −114 °C. Type-IV daughter mineral-bearing FIs homogenized by halite disappearance at 295–392 °C, and have salinities of 37.8–46.6 wt% NaCl eqv. Such characteristics suggest that FIs were trapped by fluid immiscibility and phase separation. The δ34SCDT values of chalcopyrite and pyrrhotite are of −5.3 to 0.7‰. On the δ34S histogram, all Langdu data show a cluster of 32S-rich (δ34SCDT = −5.3 to −4.6‰) and a cluster of 34S-rich (δ34SCDT = −1.1 to 0.7‰). Type-I calcite (in skarn) and type-II calcite (in quartz-sulfide vein) have δ13CPDB and δ18OSMOW values of −8.4 to −7.8‰, −6.3 to −6.1‰ and 10.1 to 12.3‰ and 12.0 to 13.0‰, respectively. Type-III calcite from calcite-sulfide veins has δ13CPDB = −5.6 to 0.2‰ and δ18OSMOW = 12.5 to 16.3‰. Biotite, garnet and pyroxene, syn-ore quartz, and chlorite yielded δ18OSMOW values of 6.9‰, 4.1–5‰, 13.2–15.3‰, and 6.8‰, respectively. Sulfur, carbon and oxygen isotopic features of these hydrothermal minerals and the FIs microthermometric data suggest that the ore-forming fluids were magmatic sourced and contaminated by meteoric/formation water during ascent and/or fluid/wallrock interactions. Low δ34S (=−5.3 to −4.6‰) in sulfides and the CH4 in the FIs are interpreted to reflect the reaction between the magmatic fluids and the carbonaceous slate in Qugasi Fm. The methane likely expanded the vapor/liquid immiscibility field and reduced the sulfates (SO42−) to sulfides (S2−). And extensive water/rock reaction and fluid mixing may have occurred, leading to the high-grade Cu mineralization at Langdu.