Metallogenesis and hydrothermal evolution of Middle Jurassic porphyry Mo deposits in the southern Zhangguangcai Range, NE China: evidence from fluid inclusions, H–O–S isotopes, and Re–Os geochronology

A major metallogenic belt with substantial resources of gold, lead, zinc, copper, and molybdenum is present in the southern Zhangguangcai Range, NE China. Several large porphyry Mo deposits are located in this belt, as for example at Jidetun, Fu’anpu, and Daheishan. Five molybdenite samples from the Jidetun deposit yielded an Re–Os isochron age of 168.6 ± 2.1 Ma (mean standard weighted deviation = 0.20), and this is consistent with the Re–Os isochron ages of the other Mo deposits in the southern Zhangguangcai Range, giving a Middle Jurassic age for metallogenesis. The Jidetun, Fu’anpu, and Daheishan deposits all tend to have weakly enriched ³⁴S values of 0.80‰–3.20‰ and relatively low Re contents ranging from 3.073 to 43.567 ppm, which indicates the ore-forming materials were derived mainly from granitic magmas that had an origin in the mixture of crust and mantle. Three stages of mineralization can be identified in the deposits at Jidetun, Fu’anpu, and Daheishan. The original ore-forming fluids in stage I were characterized by high-temperature magmatic hydrothermal fluids that were most likely derived by exsolution from the Middle Jurassic ore-bearing magmas. However, two different fluid systems, NaCl–H₂O–CO₂ fluids and NaCl–H₂O fluids, were widespread in stage I of porphyry Mo deposits in the southern Zhangguangcai Range. Taking into account the regional geological characteristics and tectonic setting, we suggest that two different emplacement modes of the ore-bearing magmas explain the different fluid systems in stage I: the first magmas were emplaced along the contact zones between the strata and earlier granitoids, whereas the second magmas were emplaced entirely within the earlier granitoid intrusions. The stage II and III fluids were characterized by relatively lower temperatures and low H–O isotopic values, indicating a gradual evolution from magmatic to meteoric sources.