Supercritical fluids as recorded in quartz megacrysts of the Late Jurassic porphyritic granitic dyke in the giant Dongping gold deposit, Northern China

The giant Dongping gold deposit, hosted in Devonian alkaline complex, is the first deposit of this kind discovered in China. In this deposit, a newly found porphyritic granitic dyke that contains abundant quartz megacrysts was studied. The dyke occurs along a NE-SW striking shear zone and has gold mineralization within the shear zone. Orthoclase from porphyritic granitic dyke has good plateau ages of 144.3 ∼ 148.2 Ma and isochronal ages of 145.4 ∼ 146.7 ± 1.53 Ma, similar to zircon U-Pb ages of a previous study for the porphyritic dyke. This indicates that the porphyritic granitic dyke is coeval with the adjacent Late Jurassic Shangshuiquan K-feldspar-rich granitic pluton. Plentiful primary fluid inclusions in those quartz megacrysts have homogenization temperatures (300 °C to 400 °C) similar to those in Au-bearing quartz veins of the Dongping deposit, with lower salinities of 5.0–19.0 wt%NaCl equiv. A small number of melt inclusions which can be classified as melt-fluid inclusions were seen in quartz megacrysts together with fluid inclusions. It is suggested that the initial fluids related to the formation of porphyritic granitic dyke could be H₂O-silicate supercritical fluids. We propose that H₂O-silicate supercritical fluids containing Au were originated from Late Jurassic K-feldspar-rich granitic magmas and might further leach Au from country rocks of alkaline complex. These fluids, which migrated along NE-SW shear zone that controlled the porphyritic granitic dyke, replaced alkaline complex, formed granitic dyke by replacement related to granitization, and, later, Au-bearing quartz veins.