Abstract

The Daxueshan deposit is the first magmatic Ni-Cu sulfide deposit that has been discovered in the eastern part of the Tethyan orogenic belt, which stretches from southwest China to Turkey. Although the size of the deposit is small, containing ~0.52 million tonnes of sulfide ore with grades of 0.67 wt % Ni and 0.46 wt % Cu, it provides a unique opportunity to learn more about nickel metallogeny in arcs. The host intrusion of this deposit is composed of gabbro, harzburgite, and lherzolite. Sulfide mineralization occurs as disseminated and massive sulfides (pyrrhotite, pentlandite, and chalcopyrite) in the basal zone of the ultramafic rocks. Sensitive high-resolution ion microprobe U-Pb dating of zircon crystals from the gabbro yields a crystallization age of 300.5 ± 1.6 Ma. The εHf(t) values of these zircon crystals are from −2 to −11. The forsterite contents of olivine from sulfide-bearing (>0.3 wt % S) and sulfide-barren (<0.3 wt % S) ultramafic rocks are from 80 to 83 mol % and from 76 to 80 mol %, respectively. Coexisting pyroxenes are bronzite and augite. The Al/Ti ratios of augite from the Daxueshan intrusion and global arc cumulates are similar. Coeval arc basalts in the area are characterized by light rare earth element (REE) enrichments relative to heavy REEs, pronounced negative Nb-Ta anomalies, elevated initial 87Sr/86Sr ratios from 0.7065 to 0.7071, and slightly negative εNd(t) values from −0.8 to −0.3. The Daxueshan mafic-ultramafic rocks have higher initial 87Sr/86Sr ratios from 0.7116 to 0.7139, lower εNd(t) values from −5.7 and −7.1, and higher degrees of light REE enrichments. These differences can be explained by higher degrees of crustal contamination (up to 20% more) for the mafic-ultramafic intrusive rocks than the coeval basalts. The δ34S and γOs values of sulfide separates from the deposit are from −2.6 to 1.2‰ and from 28 to 482, respectively. The former are similar to the typical mantle value (0 ± 2‰), whereas the latter are significantly different from the primitive mantle value, indicating contamination with organic matter-bearing (and hence Os-rich) sedimentary rocks. Olivine chemistry and Sr-Nd-Hf-Os isotope data indicate that fractional crystallization and crustal contamination played a role in triggering sulfide saturation in the Daxueshan magma, although their relative significance is unclear. Like most arc-type magmatic sulfide deposits worldwide, the platinum group element (PGE) tenors of the Daxueshan deposit are extremely low, indicating a severe PGE depletion of the parental magma due to previous sulfide segregation at depth, including the lower part of the arc crust, to form sulfide-bearing, Cu-PGE–rich cumulates. This finding supports the notion that the formation of sulfide-bearing cumulates in the lower part of the arc crust may be a critical step in continent building or the genesis of porphyry ore deposits because new magma or volatiles may cannibalize sulfides from the previous cumulates in the pathway.

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