A synvolcanic origin for magnetite-rich orebodies hosted by BIF in the Weld Range District, Western Australia

摘要

Iron ore deposits hosted by Archean banded iron-formation (BIF) in the Weld Range greenstone belt are representative of most of the documented iron ore deposits in the Yilgam Craton. They include near-surface, supergene goethite-hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores. The Cenozoic goethite-hematite-rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting BIF in the near-surface supergene environment; however, the deeper and likely older magnetite- and specular hematite-rich orebodies have a more contentious origin. This study is the first to present a fluid-alteration model for hypogene iron mineralization in the Yilgam Craton that uses fluid inclusion and mineral chemistry data to constrain the physical-chemical characteristics and source of hypogene fluids responsible for mineralization. High-grade ( 57 wt% Fe), magnetite-rich iron ore at the Beebyn deposit defines a discontinuous series of 80 m-thick by 1 km-long lenses that extend 3 km along strike in the BIF. These magnetite-rich lenses are surrounded by a broad carbonate alteration halo in the BIF and intense ferroan chlorite and talc alteration in nearby basalt, dolerite, and gabbro country rocks. Magnetite-rich lenses at Beebyn are the product of the replacement of primary quartz bands in the BIF by Stages 1 and 2 hypogene carbonate minerals, followed by their replacement by magnetite and minor ferroan dolomite. Fluid inclusion studies demonstrate that Stage 1 fluids were high-temperature ( 440 degrees C) and CO2-rich. Paired O and C stable isotope data for Stage 1 ferroan dolomite suggest that these fluids had a magmatic source, while Stage 1 magnetite chemistry (e.g. enrichments in Mg, Mn, Ca, and Na) indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite. The presence of monophase carbonic fluid inclusions in Stage 1 ferroan dolomite suggest that phase separation of a bicarbonate-rich aqueous fluid took place in deeper parts of the hydrothermal system, which led to the separation of the resultant volatile-rich and brine phases during transport of the Stage 1 fluid to shallower crustal levels. Cooling of the hydrothermal system during the Stage 2 fluid event involved (i) an early brine ( 275-327 degrees C; 36-40 equiv. wt% NaCl) with Cl/Br and C and O isotopes values that overlap the ranges for magmatic fluids, with minor involvement of Archean seawater, and extensive chemical exchange with country rocks; followed by (ii) pulses of moderate- and lower-temperature Stage 2 brines ( 125-260 C; 2-24 equiv. wt% CaCl2) with Cl/Br, O and C isotope, Na/Br, and Ca/Ca + Na signatures that suggest cooling of magmatic-derived fluids that mixed with Archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways. The last stage of formation of magnetite-rich ore at the Beebyn deposit involved the flow of Stage 3 fluids through the existing fault network that controlled earlier fluids. Stage 3 fluids are lower-temperature ( 98-175 degrees C), low to high-salinity brines with Cl/Br values that overlap reported ranges for 3.2 Ga vent fluids and seawater. Thus, they are likely the product of heated Archean seawater that was chemically-modified through interaction with mafic country rocks.