The Paroo Station Mine supergene lead deposits, Western Australia: geological and geochemical constraints

Abstract

The Paroo Station Mine lead project is situated near Wiluna in the central part of Western Australia. It is located in an outlier of the Paleoproterozoic Earaheedy Group overlying the southeast Yerrida Basin. The total Measured and Indicated Mineral Resource is currently (as at 31st of December 2014) estimated at 31.8 Mt of ore at 4.4% Pb. The deposits are hosted by deeply weathered carbonate and clastic sediments of the Paleoproterozoic Yelma Formation. The lead-only mineralisation contains only traces of sulfides and is composed of few supergene minerals, predominantly cerussite and anglesite with minor pyromorphite and plumbogummite. The Paroo Station Mine demonstrates a high degree of supergene Pb mobility which has resulted in almost complete destruction of primary Pb distribution pattern and development of characteristic for the oxidation zones coupled depletion zone and subhorizontal enrichment blankets below. The deposits are interpreted to have formed from Mississippi Valley-type (MVT), stratabound relatively low-grade sulfide PbZn mineralisation located within dolomites of the Sweetwaters Well Member of the Yelma Formation. The primary mineralisation based on modelling the Pb distribution was controlled by a network of northeast, northwest and meridional oriented fractures and small faults. Lead isotope study yielded from 1.80 to1.88 Ga ages of the mineralisation based on the Stacey and Kramers (1975) two-stage model; the mineralising event is related to the Capricorn Orogeny. This data is well in line with previous UPb dating of monazites interpreted to be related to the mineralisation (1.82 Ga; Muhling et al., 2012). High μ (238U/204Pb) values (10.26–10.58) are indicative of the Archean basement contribution and/or more likely with sediments originally derived from that basement. The Paroo Station Mine mineralisation is thought to be similar in style and formed synchronously with the MVT prospects in the Teague area in the Earaheedy Basin approximately 120 km to the northeast. The mineralisation at the Paroo Station Mine was significantly upgraded through a multi-stage supergene remobilisation and reprecipitation of lead in the course of long weathering history of the region. Lead was almost completely depleted from the upper zone and accumulated below as subhorizontal enrichment blankets at three levels (from top to bottom): (1) the base of the collapse quartz-clay breccia unit marking the dolomite weathering front; (2) the contact of siltstone and sandstone packages coincident with the feldspar/kaolinite weathering front and; (3) the mid- to lower parts of the sandstone unit close to the base of weathering. The lowest enrichment zone extends as a continuous blanket at a significant, up to 500 m distance (at a 1% Pb cut-off) beyond the upper mineralisation projection. This enrichment is interpreted to be formed by lateral Pb transport in waters close to the groundwater table through the most permeable layers of weathered sandstones. This active lateral lead dispersion is of high exploration significance for the region providing a much larger exploration target and vector to the primary mineralisation.