The Characteristics Of The Ore-forming Granitoids And Ore Genesis Of The Dewulu Copper Deposit, Western Qinling

Dewulu copper deposits is in the north of the Yangtze plate, south of north China plate, pine Song pan-Ganzi Basin triangle zone in geotectonic position. is a typical skarn deposit in the western section of West Qinling. Based on detailed study of geological and geochemical characters of the Dewulu deposit, According to petrogenesis and tectonic environment of ore-forming granitoids, this study focus on the relationship between magmatism and mineralization1. The skarn and disseminated Cu orebodies mainly occur as stratoids or lenses at the contact between the Triassic Dewulu intrusive complex and Permian marine clastic and carbonates. Alteration minerals in the mining area include prograde skarns(garnet, diopside, wollastonite), plagioclase, hornblende, actinolite, and tremolite, epidote, chlorite, calcite, quartz, and sericite. The main ore types include the early disseminated skarn-type replacement orebodies and late-stage quartz-sulfide veins. Chalcopyrite is the major ore mineral in the skarn, along with pyrite, bornite, and sphalerite.2. The Dewulu intrusive complex consists of quartz diorite, quartz diorite porphyry, and dioritic mafic microgranular enclaves(MME). The MMEs are spheroidal in shape, and have igneous mineral assemblages, acicular apatites, and complex oscillatory zoned plagioclase, quartz megacrysts surrounded by mafic minerals. The MMEs are metaluminous, and calc-alkaline to high-K calc-alkaline in composition, and yield relatively higher Ni, Cr, and MgO contents, and Mg# values. They display sub-parallelld trace element spider diagrams, and rare earth element(REE) patterns, characterized by enrichment of large-ion lithophile elements and light REEs, depletion of high-field-strength elements and heavy REEs, and negative Eu anomaly.3. The zircon LA-ICP-MS U–Pb weighted mean age obtained from the dioritic MME is 247.0±2.2 Ma, consistent with ages of the host quartz diorite(245.8±1.7 Ma) and quartz diorite porphyry(247.6±1.3 Ma). This early Indosinian magmatism is identical to the 40Ar/39 Ar plateau ages of ore-related sericite(249.1±1.6 Ma, and 249.0±1.5 Ma) within analytical uncertainties. Trace element compositions of zircons from the dioritic MME yield a high average oxygen fugacity of FMQ + 3.3. The zircons have initial negative εHf(t) values in a range of-8.0 to-3.3, corresponding to two-stage model ages ranging from 1.48 to 1.78 Ga.4. The integrated petrology, geochronology and whole geochemistry suggest that the granitoids associated with Cu skarn mineralization at Dewulu were products of arc magmatism formed by magma mixing in an active continental margin setting. They were derived from partial melting of enriched sub-continental lithospheric mantle that had been previously modified by slab-derived melt during the continuous northward subduction of the Paleotethys oceanic slab.