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 degrees C and salinities of 1.729.5 wt% NaCl eqv. Type-II vapor-rich FIs contain CH4-N-2 bubbles and homogenized to vapor at 262-425 degrees C. Type-III FIs are single phase at room temperature (20 degrees C). A vapor phase is present at below -120 degrees C, and the FIs homogenized at -117 to -114 degrees C. Type-IV daughter mineral-bearing FIs homogenized by halite disappearance at 295392 degrees C, and have salinities of 37.846.6 wt% NaCl eqv. Such characteristics suggest that FIs were trapped by fluid immiscibility and phase separation. The delta(34)(SCDT) values of chalcopyrite and pyrrhotite are of -5.3 to 0.7 parts per thousand. On the delta S-34 histogram, all Langdu data show a cluster of S-32-rich (delta(34)(SCDT) = -5.3 to -4.6 parts per thousand) and a cluster of S-34-rich (delta(34)(SCDT) = -1.1 to 0.7 parts per thousand). Type-I calcite (in skarn) and type-II calcite (in quartz-sulfide vein) have delta(13)(CPDB) and delta(18)(OSMOW) values of -8.4 to -7.8 parts per thousand, -6.3 to -6.1 parts per thousand and 10.1 to 12.3 parts per thousand and 12.0 to 13.0 parts per thousand, respectively. Type-III calcite from calcite-sulfide veins has delta(13)(CPDB) = -5.6 to 0.2 parts per thousand and delta(18)(OSMOW) = 12.5 to 16.3 parts per thousand. Biotite, garnet and pyroxene, syn-ore quartz, and chlorite yielded delta(18)(OSMOW) values of 6.9 parts per thousand, 4.15 parts per thousand, 13.215.3 parts per thousand, and 6.8 parts per thousand, 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 d34S (=-5.3 to -4.6 parts per thousand) 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.