Glacial dispersion of hydrothermal monazite in the Prominent Hill deposit: An exploration tool

Abstract

Light rare earth elements (LREEs) are commonly enriched within iron oxide–copper–gold (IOCG) deposits within the Gawler Craton, South Australia. The LREEs are host within a number of phases including monazite, which is a resistate phase that can withstand processes of physical transport and weathering without significant chemical alteration. Recognition of this elevated LREE signature within rocks that have been transported away from mineralised zones can therefore be used as a geochemical vector towards potential IOCG mineralisation.In the northern Gawler Craton, South Australia, monazite occurs within basement rocks within and proximal to the Prominent Hill IOCG deposit. These basement rocks have been physically transported and dispersed during glacial activity subsequent to the mineralisation event, and redeposited in the cover sequence as a glacial diamictite. Here we show that the hydrothermal monazite within the mineralised zone has a characteristic geochemical signature, and that this signature has been preserved within monazite grains within the overlying glacial diamictite. The hydrothermal monazite is characteristically enriched in La and Ce, and depleted in Y and Th. A chemical criterion for exploration is derived. Monazite chemistry showing concentrations of La+Ce >63wt.% and Y and Th <1wt.% is considered compelling. Concentrations of 57.5wt.%<La+Ce<63wt.% are considered interesting, and compositions of La+Ce<57.5wt.% are considered background.Using the assumption that all light rare earth elements in the cover sequence are host within monazite, this mineral chemistry signature can be recognised in whole rock geochemistry. Data showing La >75ppm and Ce >155ppm is considered anomalous. Data that also shows (La+Ce):Y ratios between 10:1 and 30:1 and (La+Ce):Th ratios between 16:1 and 32:1 is considered interesting. Ratios of (La+Ce):Y and (La+Ce):Th greater than 30:1 and 32:1 respectively are considered compelling. In the northern Gawler Craton, the scale of the footprint associated with the anomalous whole rock geochemical signature characteristic of Prominent Hill-style IOCG mineralisation is 2–3 times larger than the orebody itself. Dispersion of this signature is related to glacial processes and palaeotopography. This chemical criteria may potentially be used as a geochemical vectoring method towards Prominent Hill-style IOCG mineralisation, and may be applicable to exploration for other IOCG deposits within the Gawler Craton and further afield.