Geologic and geochemical insights into the formation of the Taiyangshan porphyry copper-molybdenum deposit, Western Qinling Orogenic Belt, China

摘要

Taiyangshan is a poorly studied copper-molybdenum deposit located in the Triassic Western Qinling collisional belt of northwest China. The intrusions exposed in the vicinity of the Taiyangshan deposit record episodic magmatism over 20-30 million years. Pre-mineralization quartz diorite porphyries, which host some of the deposit, were emplaced at 226.6 +/- 6.2 Ma. Syn-collisional monzonite and quartz monzonite porphyries, which also host mineralization, were emplaced at 218.0 +/- 6.1 Ma and 215.0 +/- 5.8 Ma, respectively. Mineralization occurred during the transition from a syn-collisional to a post-collisional setting at ca. 208 Ma. A barren post-mineralization granite porphyry marked the end of post-collisional magmatism at 200.7 +/- 5.1 Ma. The ore bearing monzonite and quartz monzonite porphyries have a epsilon(Hf)(t) range from - 2.0 to + 12.5, which is much more variable than that of the slightly older quartz diorite porphyries, with T-DM2 of 1.15-1.23 Ga corresponding to the positive epsilon(Hf)(t) values and T-DM1 of 0.62-0.90 Ga corresponding to the negative epsilon(Hf)(t) values. Molybdenite in the Taiyangshan deposit with 27.70 to 38.43 ppm Re suggests metal sourced from a mantle-crust mixture or from mafic and ultramafic rocks in the lower crust. The delta S-34 values obtained for pyrite, chalcopyrite, and molybdenite from the deposit range from + 13 parts per thousand to + 4.0 parts per thousand, + 0.2 parts per thousand to + 1.1 parts per thousand, and + 5.3 parts per thousand to + 5.9 parts per thousand, respectively, suggesting a magmatic source for the sulfur. Calculated delta O-18(fluid) values for magmatic K-feldspar from porphyries (+ 13.3 parts per thousand), hydrothermal K-feldspar from stockwork veins related to potassic alteration (+ 11.6 parts per thousand), and hydro-thermal sericite from quartz-pyrite veins (+ 8.6 to + 10.6 parts per thousand) indicate the Taiyangshan deposit formed dominantly from magmatic water. Hydrogen isotope values for hydrothermal sericite ranging from -85 to -50 parts per thousand may indicate that magma degassing progressively depleted residual liquid in deuterium during the life of the magmatic-hydrothermal system. Alternatively, delta D variability may have been caused by a minor amount of mixing with meteoric waters. We propose that the ore-related magma was derived from partial melting of the ancient Mesoproterozoic to Neoproterozoic middle to lower continental crust. This crust was likely metasomatized during earlier subduction, and the crustal magmas may have been contaminated with litho spheric mantle derived magma triggered by MASH (e.g., melting, assimilation, storage, and homogenization) processes during collisional orogeny. In addition, a significant proportion of the metals and sulfur supplied from mafic magma were simultaneously incorporated into the resultant hybrid magmas. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.