Fluid evolution of a hematite-dominated, magmatic-hydrothermal Cu-Au deposit at Qibaoshan, Shandong Province, China

Abstract

Abstract Qibaoshan is a magmatic-hydrothermal, polymetallic ore deposit located to the northwest of Wulian district, Shandong Province, China. The deposit hosts high grade copper and gold, which are the primary targets of currently active open-pit mining. However, the type, origin and mineralization mechanism(s) at Qibaoshan are still debated. Here, we studied the mineral assemblages, hydrothermal alteration, paragenesis of mineralization, and fluid evolution of the Qibaoshan deposit. Qibaoshan shares many similarities with some iron oxide-copper-gold (IOCG) deposits such as Mantoverde (Chile). The ore bodies of the Qibaoshan deposit are hosted in and structurally controlled by breccias, and are composed almost invariably of chlorite, coarse specular hematite, quartz and carbonates with subordinate pyrite and chalcopyrite. Passing inwards from the deep and peripheral parts towards the central ore zones, the intermediate- to felsic-host rocks mainly follow a sequence of hydrothermal alteration consisting of pervasive potassic and later acid alteration (chloritic and sericitic) events. The acid alteration is associated with the first pulse of copper deposition; which is followed by the deposition of specular hematite + euhedral quartz; then the deposition of (2nd) chalcopyrite + pyrite; and finally, late calcite + siderite precipitation in the vein centerlines. Each successive stage of this vein paragenesis shows clear evidence of precipitation into open space. The fluid evolution at Qibaoshan shows a transition from early, highly saline (47 wt. % NaCleq), high temperature (396°C), and low pressure fluids during the acid alteration; towards a moderate salinity (26 wt. % NaCleq), intermediate temperature (272°C) and pressure fluid during hematite growth; and then towards a low salinity (11 wt. % NaCleq), low temperature (77°C) fluid during the precipitation of euhedral quartz. The initial, high temperature stage showed features indicative of magmatic-hydrothermal origin. This first sequence included a metal-bearing (iron > copper) fluid pulse. Over the course of this sequence, the fluids showed gradual cooling coupled with a gradual decrease in salinity, probably indicating dilution as the early magmatic fluids mixed with externally derived meteoric water. After that, another metal-bearing (copper > iron) fluid pulse resulted in abundant (2nd) copper deposition in the veins. This vein sulfide stage was then followed by a trend towards lower salinity (14 wt. % NaCleq), moderate temperature (225°C) fluids that drove the precipitation of late carbonates. Overall, these results suggest that Qibaoshan is a magmatic-hydrothermal, IOCG-like deposit, with at least two magmatic fluid pulses, each followed by subsequent cooling and dilution by mixing with meteoric water that was tied strongly to the ore deposition.