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

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 subductionrelated 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.