Update on the geophysical expression of the Abra sedimentary replacement Pb-Ag-Cu-Au deposit, Western Australia

Abstract

Summary The Abra sedimentary replacement Pb-Ag-Cu-Au deposit is located in the Paleoproterozoic Edmund Basin, 900km NNE of Perth. Mineralisation at Abra has no surface expression and the deposit was discovered in 1981 by drill testing coincident magnetic and gravity anomaly highs. The deposit is hosted within siliciclastic and carbonate deposits of the Edmund Group and consists of a stratabound apron of lower grade Pb-Ag-Ba mineralisation in a laminated iron oxide and barite altered siltstone unit that overlies a funnel shaped feeder zone of chlorite altered, brecciated and veined carbonaceous siltstone containing high-grade Pb-Ag in the core, transitioning to Pb-Cu and Cu-Au at depth. As at December 2018, the Abra deposit remains unmined and has an estimated resource of 37.4Mt at 7.5% Pb and 18g/t Ag. Mutton and McInerney (1987) and McInerney et al. (1994) described the geophysical expression of Abra, and recent geophysical survey results are presented here. Abra is characterised by discrete anomaly responses in magnetic, gravity and TDEM survey data. A +450nT magnetic anomaly is observed in ground and airborne magnetic data, which is caused by magnetite within the lower part of the stratabound zone. Dense galena, barite and iron oxide mineralisation in the stratabound zone, and galena in the feeder zone, is surrounded by low-density sedimentary host rock, resulting in a +1mGal gravity anomaly. TDEM surveys have resolved massive sulphide mineralisation as EM conductors, and petrophysical testing on core samples show this is mostly caused by galena. Inverted AMT-MT data sections resolved the deposit halo as a conductive anomaly. ZTEM data failed to resolve a distinct anomaly response. DDIP surveying failed to resolve a chargeable anomaly coincident to known high-grade mineralisation, despite significant disseminated sulphide mineralisation occurring within the deposit. An IP chargeability anomaly observed on the southern side of the deposit is thought to be associated with an alteration zone and low-grade disseminated sulphide mineralisation in a fault zone. A 2D seismic reflection survey line resolved the deposit envelope as strong seismic reflectors surrounded by a seismically bland zone, and this is related to the significant density contrast between the high-density stratabound mineralisation in contact with low-density sedimentary host rocks, as the mineralisation and host rock have similar seismic velocities. Passive seismic HVSR surveying resolved the top of Abra as a subtle HVSR response below a flat impedance contrast horizon interpreted as weathered siltstone over diagenetic cemented siltstone.