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Geology and genesis of the Debao Cu polymetallic skarn deposit, southwestern China

2021; Elsevier BV; Volume: 131; Linguagem: Inglês

10.1016/j.oregeorev.2021.104046

1872-7360

Jiahao Chen, Qingfei Wang, Qihai Shu, Weijun Weng, Xiaojie Xu, Tingyi Wang, Qizuan Zhang,

Geochemistry and Geologic Mapping

The Debao deposit, with stratiform orebodies in Middle-Upper Cambrian carbonate rocks, is a rare Early Paleozoic skarn copper deposit in the South China Block. Garnet U-Pb dating yielded a lower intercept 206Pb/238U date of 438.8 ± 7.3 Ma, which is coeval with the related granite (439 ~ 441 Ma). Skarns contain garnets with andraditic compositions (Ad53~93Gr4~46Py1~8) and clinopyroxene (Hd5~11Di89~94Jo0~1 and Hd52~85Di13~46Jo1~4) with low Mn/Fe ratios (<0.1), showing typical mineral compositions of Cu skarn deposits. The proximal garnet skarn mainly developed at the bottom and both sides of the orebody, while distal pyroxene skarn developed in its interior and upper parts. The garnets in the bottom of the orebodies contain more andraditic compositions than their upper parts. The prograde skarn minerals have a range of δ18Ofluid values from 8.81 to 9.53‰, and the sulfide minerals of the main ore stage have a range of δ34S values from −2.40 to 2.57‰, both suggesting a magmatic origin of the ores. In the Debao deposit, hydrothermal magnetite from different skarn zones contains substantial amounts of Al, Sn, Ga, Mn, Mg, Ti, Zn, Co, V, and Ni. The content of Al and Ti in the magnetite, which was considered to derive from the wall rock due to fluid-rock interaction, was positively correlated with temperature. Magnetite in the retrograde skarn stage from the proximal garnet skarn zone contains more Ti and V than that from the distal pyroxene skarn zone, indicating a gradual decrease in temperature and increase in oxygen fugacity of the ore-forming fluid in this stage. The magnetite has higher Sn content (mainly hundreds to thousands of ppm) than normal skarn deposits, implying that high oxygen fugacity occurs during the retrograde skarn stage. Subsequently, the formation of magnetite led to a decrease in the oxygen fugacity of the ore-forming fluid. The decrease of oxygen fugacity and temperature provided favorable conditions for subsequent precipitation of Cu sulfides. Minor vein-type Cu mineralization related to late magmatic fluid extraction of sulfur from the sedimentary strata was recognized. All geologic and geochemical data suggested that the deep skarn in the region transformed from Early Paleozoic carbonate has copper mineralization potential.