Sampling of high-nugget conglomerates from the Western Australian Pilbara: bulk sampling at the Beatons Creek gold project, Nullagine

Abstract

Many styles of gold mineralisation pose challenges during sampling because of the presence of coarse gold and high natural heterogeneity (“nugget effect”). The gold-bearing conglomerates of the Western Australian Pilbara provide some challenges. Novo Resources Corporation has addressed many of these over the last five years. Its Beatons Creek open pit operation is the first Pilbara conglomerate to go into production (January 2021) based on a total oxide Mineral Resource of 316,000 oz Au (5.2 Mt at 1.9 g/t Au at a 0.5 g/t Au cut-off). Mineralisation occurs within the Beatons Creek conglomerate member of the Hardey Sandstone formation, which constitutes part of the Fortescue Group. Gold is present within the matrix of multiple, narrow stacked and un-classified ferruginous-conglomeritic reef horizons, which are interbedded with un-mineralised conglomerate, sandstones and grits with minor intercalations of shale, mudstone, siltstone and tuffs. The gold occurs as free particles up to 5 mm across within the ferruginous matrix of the conglomerates. It is closely associated with detrital pyrite and authigenic nodules. Previous owners and Novo have employed several sampling techniques across the project including diamond and RC drilling, trench channel sampling and bulk sampling. Assay methods included fire assay, LeachWELL and more recently PhotonAssay. As part of its 2018 evaluation programme, fifty-eight c. 1-4 t bulk samples were collected from accessible oxide mineralisation and processed via a pilot plant. This paper presents some of the issues and solutions applied by Novo, which have wider implications and impact on the sampling of other heterogeneous orebodies.