Metallogenesis and fluid evolution of the Devonian porphyry Cu deposits in the Central Asian Orogenic Belt: the Xiaotuergen example

Abstract

ABSTRACT The Central Asian Orogenic Belt (CAOB) is among the world’s largest Phanerozoic accretionary collage systems and hosts one of the three best-known porphyry metallogenic domain. Relatively few Devonian porphyry Cu (±Au) deposits (such as Oyu Tolgoi, Lailisigao’er, Halasu, Yulekenhalasu, etc) have been found in the CAOB, and the geological characteristics, fluid evolution and metallogenesis of Devonian porphyry Cu deposits are still uncertain. In this work, we undertake a fluid inclusion and isotopic study of the Xiaotuergen Devonian porphyry Cu deposit and present a comprehensive analysis of the ore-forming fluids to better understood Devonian porphyry deposits in the CAOB. The Devonian porphyry copper (±gold) deposits in the CAOB formed during the Early and Late Devonian (366–412 Ma) and have two peaks (375 Ma and 405 Ma). The Xiaotuergen porphyry Cu deposit, located in the Altay orogenic belt, Xinjiang, NW China, is mainly hosted by an Early Devonian granodiorite porphyry. The ore-forming fluids of the main copper mineralization stages (II and III) in Xiaotuergen were characterized by high to intermediate temperatures (190–320°C) and low to intermediate salinities (3.5–15.7 wt.% NaCl eqv.), similar to those of other the Devonian porphyry copper (±gold) deposits in the CAOB (T = 120–500°C, salinities = 0–21.7 wt.% NaCl eqv. and 33.5–51.0 wt.% NaCl eqv.). The temperatures and salinities of the ore-forming fluids changed, and phase separation might have been one of the important factors in the precipitation of chalcopyrite and pyrite. The Devonian porphyry copper (±gold) deposits in the CAOB have similar hydrogen and oxygen isotope characteristics, and the ore-forming fluids were dominated by magmatic fluids, with some minor contributions from meteoric water. The coexistence of CO2-bearing fluid inclusions with aqueous fluid inclusions in stage III suggests that fluid phase separation occurred at pressures of 110 to 320 bar (depths of 0.4–1.3 km). The formation depths of the Xiaotuergen deposit are shallower than those of the Lailisigao’er Cu deposits (palaeodepths of 1.7–2.2 km). The total sulphur value in the Xiaotuergen deposit is +12.2‰, indicating that the sulphur could have originated from crust. The δ34SV-CDT values of the most Devonian porphyry copper (±gold) deposits in the CAOB are concentrated near 0 ± 7‰, indicating a homogeneous sulphur source and derivation of sulphur from deep-seated magmas.