Geology and geochemistry of the Gonghudong skarn copper deposit in Darhan Muminggan Joint Banner, Inner Mongolia and its significance

The Gonghudong is a typical skarn copper deposit in central Inner Mongolia. It is located in Mesoproterozoic Bayan Obo rift zone along the middle section of the northern margin of North China plate, and is hosted in the skarn belt formed by the contact of granite porphyry and crystalline limestone of Hujiertu Group. The Cu orebodies are lentoid and stratiform-like in shape. The metallic minerals mainly are chalcopyrite, bornite, sphalerite, chalcocite, pyrite and pyrrhotine, and the nonmetallic minerals mainly are garnet, diopside, calcite and fluorite. The LA-ICP-MS zircon U-Pb dating of granite porphyry yielded an age of (299.6±1.7) Ma, and thus the Gonghudong deposit was the product of Late Paleozoic tectonic magmatic activity. The granite porphyry has high Si, low Al, depleted Ba, Sr, P, Ti, with "V" type REE pattern, and the values of 10000Ga/Al and FeO^T/MgO range from 2.32 to 3.49 and 9.86 to 12.27, respectively. These data indicate that the granite porphyry belongs to A1 subtype of A-type granites and was formed in a post-orogenic extension environment.δ¹³C_V-PDB andδ¹⁸O_V-SMOW of the hydrothermal calcite are from-10.6‰to-8.6‰and from 4.6‰to 24.67‰, respectively, suggesting that the CO₂ was supplied by the interaction of granite porphyry and limestone. Theδ³⁴S values of the sulfide range from 1.2‰to 10‰, indicating that the sulfur was derived from the magma sulfur and marine sulfate. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb of the ores are 17.847 to l8.173, 15.586 to 15.873 and 37.997 to 38.905, respectively. The Pb-isotopic compositions suggest that the ore-forming material was probably derived mainly from the granite porphyry.