Recognition of Late Ordovician Yudai porphyry Cu (Au, Mo) mineralization in the Kalatag district, Eastern Tianshan terrane, NW China: Constraints from geology, geochronology, and petrology

Although the Eastern Tianshan terrane in Xinjiang, NW China, occupies a critical position in the Central Asian Orogenic Belt (CAOB), the early Paleozoic tectonic evolution and mineralization history of this terrane remain ambiguous.The Yudai deposit is a newly discovered porphyry Cu (Mo, Au) deposit in the Kalatag district of the Dananhu island arc, a key area containing early Paleozoic rocks in the Eastern Tianshan. This paper reports the geologic characteristics, molybdenite Re-Os ages, zircon LA-ICP-MS U-Pb ages, and geochemical and Lu-Hf isotopic findings regarding the ore-hosting porphyries in the Yudai deposit. The Yudai ore-hosting porphyries include an early diorite porphyry and a late quartz diorite porphyry. Alteration and mineralization have formed distinct zones around the quartz diorite porphyry, with disseminated, vein, or veinlet Cu (Mo, Au) mineralization types mainly related to potassic alteration (e.g., biotite, magnetite, and quartz), typical of a porphyry Cu deposit. Two molybdenite samples have an Re-Os weighted mean age of 449.5 +/- 4.2 Ma and exhibit high Re contents (2624 to 4704 ppm), indicating a mantle component. The observed mineralization was probably related to the quartz diorite porphyry, which yielded a mean zircon Pb-206/U-238 age of 452.7 +/- 2.8 Ma. The rock samples from the two porphyries mainly plot in the diorite field, show metaluminous to weak peraluminous features, and belong to tholeiitic to calc-alkaline series, with K2O/Na2O ratios of 0.16-0.34 and Mg-# values of 44.2-50.3. The samples also exhibit steep chondrite-normalized rare earth element and relatively flat heavy rare earth element patterns (indicating residual amphibole), with positive Eu anomalies (Eu/Eu* = 1.09-1.45). The samples are enriched in large-ion lithophile elements relative to high-field-strength elements, typical of arc magma. In addition, the rocks show adakitic features (Sr/Y = 47.8-73.5) and present high Ba/Th and low Th/Nb ratios (indicating slab fluids). The zircons from the quartz diorite porphyry mainly yield high epsilon Hf(t) values (13.5-14.7) and very young two-stage Hf model ages (T-DM2 = 494-556 Ma), consistent with a depleted mantle origin. Combined with the arc tectonic setting, we propose that the ore metals and ore-hosting porphyries at Yudai were predominately derived from the partial melting of the amphibole-bearing and fluid-rich Kangguer Ocean slab that was subducted northward beneath the Dananhu island arc in the Late Ordovician. Therefore, the evolutionary history of the Dananhu island arc can be traced back to the Late Ordovician; the Yudai deposit is the first age-constrained early Paleozoic porphyry Cu deposit in the Eastern Tianshan terrane; and the early Paleozoic was an important epoch of mineralization on this terrane. (C) 2017 Elsevier B.V. All rights reserved.