A new model for porphyry W mineralization in a world-class tungsten metallogenic belt
Abstract The Dongyuan deposit, in the recently identified world-class porphyry–skarn W metallogenic belt, east Jiangnan Orogen, China, is characterized by a set of nearly parallel and horizontal multi-cycle orebodies within cupolas of porphyritic granodiorite. Almost every orebody from the top to the bottom shows similar vertical changes in vein morphology and mineral assemblage from ≤1 cm quartz–feldspar–scheelite–calcite veins to 1–2 cm quartz–feldspar–muscovite–scheelite veins to 2–5 cm quartz–feldspar–muscovite–scheelite veins to ≥5 cm quartz veins. From the bottom to the top of each orebody in every cycle, the mole number of Al IV in K-feldspar decreases, the concentrations of Mn 2+ in calcite increases, and the mole number of OH − in muscovite decreases. These changes reflect repetitive fluctuations in the concentrations of major elements that were involved in the precipitation of minerals as well as the physicochemical conditions of fluids during vein formation in the multi-cycle orebodies. The concentrations of Mo are obviously lower in pyrite from the middle orebodies than in the top and bottom orebodies. Our data indicate that the ore-forming fluids were sourced from multi-batch fluids that were exsolved from magma. The ΣLREE/ΣHREE ratios in the scheelite decrease from the upper to lower orebodies. Similar trends are found within a single orebody from the top to the bottom of veins. These trends were due to feldspar fractionation in the magmas and ore-forming fluids, respectively, because the crystallization of feldspar tends to capture LREEs. Therefore, the following model is proposed for the formation of the Dongyuan porphyry W deposit: the magma crystallized first at the roof of the intrusion, causing the underlying magmas to become oversaturated in H 2 O; the exsolved H 2 O then accumulated at the boundary between the crystallized upper part and the underlying magmas; as fluid pressures increased, cracks formed in the overlying crystallized granodiorite, and as a consequence the first cycle of vein networks was formed. This process was repeated as the interface between magma and crystallized rock moved downwards, thus forming the multi-cycle W orebodies that are embedded in the pluton.