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Tetrahedrite-(Ni) (IMA2021-031), ideally Cu6(Cu4Ni2)Sb4S13, is the first natural Ni-member of tetrahedrite group mineral found in Luobusa chromite deposit, Tibet, China. The new species occurs as anhedral grains 2 to 20 μm in size, associated with gersdorffite, vaesite, and chalcostibite, which are disseminated in a matrix of dolomite, magnesite, quartz, Cr-rich mica, and Cr-bearing clinochlore. Tetrahedrite-(Ni) is black in color with a reddish-black streak and metallic luster. It is brittle with uneven fractures and has a calculated density of 5.073 g·cm–3. The mean values of 9 electron micro-probe analyses (wt%) are Cu 39.83, Ni 5.67, Fe 1.45, Sb 21.69, As 5.45, S 25.39, total 99.48, and the empirical formula calculated on the basis of cation = 16 apfu is M(2)Cu6.00M(1)[Cu4.03(Ni1.55Fe0.42)Σ1.97]Σ6.00X(3)(Sb2.85As1.16)Σ4.01S12.67. Tetrahedrite-(Ni) is cubic, with space group I43m, a= 10.3478(4) Å, V= 1108.00(14) Å3, and Z= 2. Its crystal structure has been solved by X-ray single-crystal diffraction on the basis of 188 independent reflections, with a final R1= 0.0327. Tetrahedrite-(Ni) is isostructural with tetrahedrite group minerals. It represents the first natural tetrahedrite-group mineral with a Ni-dominated charge-compensating constituent. Tetrahedrite-(Ni) may be the product of late-serpentinization at moderately high-temperature conditions around 350 °C. In this case, tetrahedrite-(Ni) and its mineral paragenesis record an entire geological process of nickel enrichment, migration, activation, precipitation, and alteration from deep mantle to shallow crust.

Abstract: The Quxu (曲水) complex is a typical intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang (棉将) and Niedang (聂当) villages in Quxu County, including gabbro, mafic micro-enclaves (MME), and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis (47–51 Ma). In this article, the same zircons of the 6 samples were applied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176Hf/177Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to ɛ Hf(t) values of 6.3–14.7. Their Hf depleted-mantle modal ages (T DM) are in the range of 137–555 Ma, and the zircon Hf isotope crustal model ages (T DMC) range from 178 to 718 Ma. The mantle-like high and positive ɛ Hf(t) values in these samples suggest a mantle-dominated input of the juvenile source regions from which the batholith originated. The large variations in εHf(t) values, up to 5-ɛ unit among zircons within a single rock and up to 15-ɛ unit among zircons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.