Hydrothermal copper mineralization in the Mesoproterozoic Huashugou banded iron formation, Northwest China: characteristics, timing of formation and genesis

Abstract

Precambrian banded iron formation (BIF)-related copper deposits constitute a rare category in the world with their origins remaining equivocal. Here we investigate the Huashugou Fe-Cu deposit in the North Qilian Orogenic Belt (NQOB), Northwest China, containing approximately 428 Mt iron ore @ 40 wt% Fe and 0.2 Mt Cu (metal) @ 1.82 wt% Cu. The deposit consists of the upper BIF and the lower copper orebodies, which are hosted in the dominantly clastic-carbonate sedimentary Mesoproterozoic Jingtieshan Group. The distribution of the copper orebodies is controlled by NNW-SSE-trending faults. The BIF and phyllite hosted mineralization includes abundant Cu-sulfide minerals distributed along foliation planes, as transgressive veinlets, fissures, and irregular lenses and aggregates, with or without quartz and siderite. Sulfides (chalcopyrite and pyrite) from the copper ores show large variation in δ³⁴S values (−2.7‰ to 32.2‰), which partly overlap with the δ³⁴S values of pyrite and slightly lower than δ³⁴S of barite from the Jingtieshan Group rocks. Most of the sulfides from copper ores have high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios relative to the country rocks although there are minor overlaps. The lead isotope data show linear correlation between ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb. The sulfur and lead isotope compositions suggest that the ore-forming materials were derived from the host strata. Fluid inclusion studies constrain the trapping temperature of copper mineralization as 180–200 °C, and are related to low salinity (<10.2 wt% NaCl equiv) fluids. The δ¹⁸O and δD values of quartz (13.3‰–17.3‰ and −82‰ to −63‰, respectively) and δ¹³C and δ¹⁸O of siderite (−6.7‰ to −4.3‰ and 15.4‰–18.7‰, respectively) intergrown with sulfides, along with the characteristics of the wall rock alteration in the Huashugou area, suggest that strata-sourced ore-forming fluids with a contribution of meteoric water remobilized sulfur and copper from the country rocks. Chalcopyrite from the ores yield Re–Os isotope age of 583.6 ± 7.9 Ma (2σ, MSWD = 1.6), providing tight constraints on the timing of the copper mineralization. We classify the late Neoproterozoic Cu deposit associated with Huashugou BIF as an epigenetic hydrothermal-type deposit.