Tectonic-magmatic-metallogenic system, Tongling ore cluster region, Anhui Province, China

Abstract

The Tongling ore district of the Middle-Lower Yangtze metallogenic belt is a famous Cu-Au-Fe-S polymetal region, and its tectonic deformation, magmatic evolution, and metallogenic processes have been studied for decades. In this article, we propose a comprehensive tectonic-magmatic-metallogenic model of the ore-forming mechanism constrained by magmatism and regional deformation. In the Tongling district, the tectonic regime underwent two transitions. (1) In the Middle Triassic, the tectonic regime transitioned from quiescence to intense compression. During contraction, the lithosphere thickened and a series of NE-trending folds developed in the cover sequence; because of the multi-layered structure of this caprock, bedding faults, typically cut by steeply dipping faults, developed widely. (2) From 134 to 150 Ma, the tectonic regime changed from compression to extension. During this transition, mantle–crust interaction was prominent; ore-bearing magma was generated by the mixing of crust-derived and mantle-derived melts triggered by delamination of the thickened lithosphere. Meanwhile, detachment faults developed along the interfaces, for example between the lower and upper crust, serving as emplacement sites for several magma chambers. Ore-bearing magma dikes containing large amounts of volatiles derived from a shallow chamber at about –10 km depth migrated into the cover sequence along the pre-existing steeply dipping faults. Melt injection reworked the structural framework, facilitating further development of steeply dipping faults, as well as the vertical transport of ore-bearing fluids. Hydrothermal fluids derived from the emplaced magmas not only formed a range of deposits, including skarns, porphyries, and cryptobreccias around the intrusions but also widely replaced carbonates along bedding-parallel faults and formed so-called stratabound skarn ore bodies, as well as superimposing synsedimentary orebodies developed in the quiescence stage to form several large polymetallic hydrothermal ore deposits. Various types of ore deposits at different depths are clustered in a single orefield, composing a multi-layered mineralization network. In the network, skarn deposits dominate and are characterized by fluid immiscibility processes and diverse element enrichments. The intense mineralization in the Tongling region was caused by the abundance of metals derived from the mantle, favourable ore-controlling structures, and widespread fluid boiling of magmatic hydrothermal fluids, which facilitated metal deposition during the Mesozoic, as well as the superposition of Mesozoic hydrothermal reworking of earlier Palaeozoic sedimentary ore bodies.