Geochemistry, U−Pb Chronology and Geological Significance of Garnet in the Skarn−Porphyry Molybdenum Copper Deposit in Tongchanggou, Northwest Yunnan Province

The Tongchanggou skarn−porphyry Mo−Cu deposit, located in the northwest Yunnan Province, is one of the newly−discovered large molybdenum−copper deposits in the western margin of the Yangtze Block. Skarn is widely exposed in the Tongchanggou deposit and is the most important ore−bearing body in the deposit. In this paper, both EPMA and LA−ICP−MS technology have been used to analyze the major and trace element compositions and U−Pb isotopes of garnet grains from the Tongchanggou skarn. Based on microscopic observation, the garnet in the Tongchanggou deposit can be divided into two generations: the early, light yellow to yellowish white, zoning−free to weakly zoning garnet (Grt I) and the late, brown to reddish brown, strongly zoning garnet (Grt II). The results of EPMA show that the two-generation garnet samples have SiO₂ contents of 34.41%～38.45%, CaO contents of 32.7%～35.25%, Al₂O₃ contents of 6.64%～12.57% and FeO contents of 12.03%～22.63%, indicating both the two generations belong to the andradite−grossularite series (And_34-64Gro_34-58). Both Grt I and Grt II have similar “hump−type” rare earth element (REE) pattern with enrichments in middle REE and depletions in light and heavy REE. Relative to Grt I, Grt II have lower U concentrations and more obvious δEu anomalies, indicating it formed under higher oxygen fugacity (f_O2) and more variable pH conditions. Meanwhile, stronger oscillatory zoning within Grt II than Grt I, together with more variable major elements, indicate more intensive water/rock interaction. The two-generation garnet of the Tongchanggou deposit record its an evolving hydrothermal system, accompanying with increasing oxygen fugacity and water/rock interaction degrees. In addition, LA−ICP−MS U−Pb dating results of Grt I yield an age of (85.4±5.6) Ma (MSWD= 0.91), which directly constrains the Tongchanggou mineralization age. Within uncertainties, this age is coeval with emplacement of porphyry intrusions and timing of Mo mineralization in the Tongchanggou area, indicating that the Tongchanggou skarn mineralization consists of an important part of the porphyry−hydrothermal mineralization system. Combined with regional tectonic evolution, it's proposed that the late Cretaceous Tongchanggou and regional coeval Cu−Mo−W deposits are products of post−collisional porphyry−hydrothermal system.