Petrology And Metallogenesis Of The Ongshan-Hongniu Copper Deposit, In Northwestern Yunnan

The Hongshan quartz monzonite porphyry is one of the Yanshanian intrusions in the souther segment of the Yudun Arc. Detailed zircon U-Pb data of four samples yielded ages of 78.8-80.7Ma, indicating the Hongshan intrusion was emplaced during the Late Cretaceous. The Hongshan intrusion is shoshonitic and high-K cala-alkaline, with A/CNK = 0.64-1.14. Samples show an obvious fractionation between light and heavy rare-earth elements(average [La/Yb]N = 38.85), with negative Eu anomalies(Eu/Eu* = 0.60-0.87) and enrichment in large-ion lithophile elements(Rb, Th, U and K) and depletion in high field-strength elements(Nb, Ta and P). Rocks have high Sr and low Y contents which are of characteristics of adakitic rocks, and were derived from thick lower crust was the magmatic source. The negative and variable εHf(t) values demonstrate that the Hongshan intrusion was derived from ancient crust, without mantle-derived components and was significantly different from the Triassic intrusions in the souther segment of the Yudun Arc. The Sr-Nd values of quartz monzonite porphyry shown a wide range of(87Sr/86Sr)t(0.70165 ~ 0.70731) and a small range of εNd(t)(-6.6 ~-7.1), which is interpreted as assimilation-fractional crystallization during ascent and emplacement.The genetic mineralogy characteristics of garnet and pyroxene from the Hongshan-Hongniu copper deposit are analyzed in the paper. The garnet belongs to the grossular-andradite solid soltion and the andradite is main component and the pyroxene is mainly consisted of sahlite. The result shows the early ore-forming stage has high temperture with high oxygen fagacity and the ore-forming fluid is acid. The pyrite from the Hongshan-Hongniu deposit is enriched in S and depleted in Fe, with Co/Ni>1, which shows the same characteristics with magmatic hydrothermal pyrite. Here, garnet yielded δ18O values varying from 6‰ to 8.5‰,18 D from-93.1‰ to-149‰, and calcite from massive sulfide yielded δ13CPDB(-2.7‰ to-0.7‰) and δ18 OV-NOW(11.4‰ to 16‰), suggesting that the magmatic hydrothermal fluid was dominant and meteoric water was negligible during mineralization in the Hongshan-Hongniu depsoit. Sulfur isotope compositions of sulfides and quartz monzonite porphyry spanned a small range of δ34S from 2.7‰ to 6.2‰, from 4.7‰ to 7.8‰, respectively, which hardly show a contribution from the mantle. Lead isotopes exhibited the features of the lower crust and a minor contribution from the upper crust and orogenic belt.The contribution presents primarily the Cu isotope compositions of the quartz monzonite porphyry and chalcopyrite from the Hongshan-Hongniu skarn deposit by MC-ICP-MS. The δ65Cu values( δ65Cu = [{(65Cu/63Cu) sample /(65Cu/63Cu)NIST 976 }- 1] ×1000) of seven whole rocks, and twenty-two chalcopyrite samples from skarn ore-bodies are relatively narrow, ranging from-0.15‰ to 0.38‰ and from-0.02‰ to 0.77‰, respectively. The overlap of δ65Cu values indicates a genetic relationship between the quartz monzonite porphyry and skarn ore-bodies. We try to evaluate the spatial and temporal variation in Cu isotope in skarn ore-bodies in the Hongshan-Hongniu deposit and compare with some well-documented porphyry deposits in the world. There is a horizontal zonation in the quartz monzonite porphyry that the inner enriched in heavy Cu isotope and the skarn margin depleted in light Cu isotope. Although the data of skarn deposits is relatively limited, together with published studies, it is possible that there are possibly no spatial differences or zoned characteristics in skarn deposits, unlike porphyry deposits at least in the Hongshan-Hongniu skarn deposit. Chalcopyrite formed during the late mineralization tends to enrich in heavy Cu isotope in the Hongshan-Hongniu deposit, analogous to the porphyry deposits. The δ65Cu values of from the quartz monzonite porphyry show the characteristics of typical hypogene mineralizations suggesting that exploring vertical mineralization is reasonable.