Carboniferous-Permian tectonic transition envisaged in two magmatic episodes at the Kuruer Cu-Au deposit, Western Tianshan (NW China)

Abstract

The Western Tianshan in NW China is one of the most important gold provinces in the Central Asian Orogenic Belt (CAOB). The recently discovered Kuruer Cu-Au deposit has been interpreted to represent a transition from high-sulfidation epithermal to porphyry mineralization system. In this study, we present new LA-ICP-MS zircon U-Pb ages for the many magmatic rock types at Kuruer, including the Dahalajunshan Formation andesitic tuff (333.2±1.6Ma), diorite porphyry (269.7±2.0Ma), slightly-altered (264.4±2.6Ma) and intensively-altered (270.5±2.5Ma) albite porphyry. These ages reveal two distinct magmatic episodes: The Early Carboniferous Dahalajunshan Formation (wall rocks) andesitic tuff samples contain narrow ranges of SiO2 (60.29–61.28wt.%), TiO2 (0.96–0.98wt.%), Al2O3 (16.55–16.57wt.%) and Fe2O3T (5.36–5.57wt.%). The tuff is characterized by LREE enrichment and HFSE depletion, as well as LREE/HREE enrichment ((La/Yb)N=8.31–8.76) and negative Eu anomalies (δEu=0.64–0.76). Zircon εHf (t) values are 5.4–8.2, and two-stage Hf model ages (TDM2) are 821–1016Ma, indicating partial melting of a moderately depleted mantle wedge with Precambrian continental crustal input. The ore-forming Middle Permian diorite porphyry and (quartz) albite porphyry have variable major oxide compositions (e.g., SiO2=53.09–53.12wt.% for the diorite porphyry, 70.84–78.03wt.% for the albite porphyry, and 74.07–75.03wt.% for the quartz albite porphyry) but similar chondrite-normalized REE and primitive mantle-normalized multi-element patterns. These porphyries display LREE enrichment and HFSE depletion, as well as elevated LREE/HREE enrichment and negative Eu anomalies. The positive zircon εHf(t) values (11.7–15.9 for the diorite porphyry, 8.9–14.9 for the albite porphyry) and young two-stage Hf model ages (TDM2) (282–542Ma for the diorite porphyry, 337–717Ma for the albite porphyry) indicate a major juvenile continental crustal involvement. We propose that the Carboniferous and Middle Permian magmatism was formed in a continental arc and post-collisional settings, respectively, with the latter episode responsible for the Cu-Au mineralization.