Early Cretaceous bimodal magmatism related epithermal mineralization: a case study of the Gaosongshan gold deposit in the northern Lesser Xing’an Range, NE China

Abstract Low-sulfidation (LS) gold mineralization is partially associated with bimodal volcanism in origin. Sources of ore-mineralized fluids and relevant dynamic processes remain poorly understood. To better understand these issues, a study including deposit geology, petrology, zircon U–Pb dating, and element and Sr–Nd–Hf isotope geochemistry of bimodal volcanic rocks at Gaosongshan was conducted. The bimodal volcanic rocks are composed of andesitic rocks (basalt andesite–andesite–trachyandesite–andesitic pyroclastic) of the Meifeng Formation and overlying rhyolite lava of the Fuminhe Formation cut by equivalent porphyry stocks, which contain pyrite-bearing granular-textured quartz cavity with a sugary appearance, with trapped single-phase magmatic fluids. The andesitic suite formed at approximately 106–108 Ma, prior to eruption of the younger rhyolitic magmas (105.6±1.7 Ma; n=13, MSWD=1.2). The andesites show calc-alkaline affinities, which is a subduction-related characteristic, and show moderate La/YbN (7.91–11.36) and weakly negative europium anomalies (0.80–1.02), indicating a source in the asthenospheric mantle with varying contamination by both subduction and continental materials. A-type rhyolitic rocks are geochemically comparable to the andesitic suite, and their higher La/YbN (13.01–30.05) and lower Eu/Eu* (0.66–0.73) indicate evolved products of andesitic magmas. The estimated oxygen fugacity of dry (＜1.2% water) rhyolite lava is FMQ –1.87, whereas rhyolite porphyry is more oxidized (FMQ +0.12) and water rich (2.5–2.8%) with probably being water saturated at 2.0–2.4 km depths. Therefore, it is assumed that the essential constraints of bimodal magmatism on epithermal gold mineralization are sequential processes from ascending and gradual differentiation of crustal-level mantle-derived basaltic magmas to releases of hydrothermal solutions from the silica-rich magma reservoir at shallower levels. Most importantly, the oxygen fugacity of and the fluid fluxes from dry rhyolitic melts are critical restricting parameters for the limited reserves at Gaosongshan; thus, this research contributes objective evidence for investigations of deposit genesis and prospecting targets.