Geochronology, geochemistry and tectonic significance of the ore-associated granites at the Kaladawan Fe–Mo ore field (Altyn), NW China

Abstract

The Kaladawan Fe–Mo ore field (Altyn, Xinjiang) in Northwest (NW) China contains six deposits, with a total reserve of 60Mt Fe and 10Kt Mo metal. The orebodies are hosted in lower Paleozoic andesite, dacite, phyllite and marble with well-developed skarn alteration. The Kaladawan granites are newly U–Pb dated to be Early Ordovician (476.1±3.3Ma), largely coeval with the Fe–Mo mineralization (molybdenite Re–Os: 480.3±3.2Ma). The granites contain high SiO2, K2O and Al2O3, low TiO2, MgO and CaO, with high K2O/Na2O ratios (1.26–1.58) and A/CNK values (1.00–1.08), showing peraluminous high-K calc-alkaline affinity. The rocks are characterized by large ion lithophile element (LILE) and light rare earth element (LREE) enrichments and depletions of Sr, Ba, Nb, Ta, Ti and P, and with negative Eu anomalies. The rocks have initial 87Sr/86Sr ratios of 0.7066 to 0.7112 and εNd(t) values of −1.4 to −1.1, with TDM2(Nd) ages of 1.32–1.30Ga. Zircon εHf(t) values range from 2.9 to 6.4, with TDM2(Hf) ages of 1.26–1.04Ga. The new geochemical and isotopic data suggest that the Kaladawan granites are highly fractionated I-type, and likely formed by fractional crystallization of a magma that was derived from partial melting of a mixture of crustal and mantle materials. Deposits in the Kaladawan Fe–Mo field are skarn-type and may have occurred in an active continental margin, via the contact metamorphism and metasomatic reaction between granite-derived fluids and the wall rocks.