The (U-Th)/He Chronology and Geochemistry of Ferruginous Nodules and Pisoliths Formed in the Paleochannel Environments at the Garden Well Gold Deposit, Yilgarn Craton of Western Australia: Implications for Landscape Evolution and Geochemical Exploration -

Mining & Minerals - Evaluations / Mineral Exploration Activities - General

Anand and 8 co-authors reported in mid-2021 that ferruginous nodules and pisoliths that cap deeply weathered profiles and transported cover are characteristic of the Yilgarn Craton, Western Australia. Here we show how ferruginous nodules and pisoliths formed in the paleochannel sediments during Miocene can be used to locate buried Au mineralization. Three types of ferruginous nodules and pisoliths were identified in paleochannel sediments and saprolite, representing different parent materials and environments covering the Garden Well Au deposit:

i) ferruginous nodules formed in saprolite on the flanks of the paleochannel (NSP),

ii) ferruginous pisoliths formed in the Perkolilli Shale in the middle of the paleochannel (PPS) and

iii) ferruginous nodules formed in the Wollubar Sandstone at the bottom of the paleochannel (NWS).

The appearance, mineralogy and geochemistry of ferruginous nodules and pisoliths vary according to their origin. The PPS and NWS are goethite-rich whereas NSP is a mixture of goethite and hematite which make them all suitable for (U–Th)/He dating. The average age of goethite in the NSP is 14.8 Ma, in the NWS is 11.2 Ma and in the PPS is 18.6 and 14 Ma. The goethite ages in ferruginous nodules and pisoliths are thought to be younger than the underlying saprolite (Paleocene-Eocene) and were formed in different environmental conditions than the underlying saprolite.

Anomalous concentrations of Au, As, Cu, Sb, In, Se, Bi, and S in the cores and cortices of the NWS and the PPS reflect the underlying Au mineralization, and thus these nodules and pisoliths are useful sample media for geochemical exploration in this area. These elements originating in mineralized saprolite have migrated both upwards and laterally into the NWS and the PPS, to form spatially large targets for mineral exploration.

The authors concluded that the combination of dating, geochemical and mineralogical data from the paleochannel and its associated ferruginous nodules and pisoliths gives insight into the conditions during landscape evolution at Garden Well. This study also fills in the gaps in the geochemical dispersion model for the Garden Well Au deposit that has been extensively researched. Three types of ferruginous nodules and pisoliths were identified in the paleochannel sediments and on the margins of the paleochannel.

The NWS and NSP are much simpler texturally and larger than the PPS. The relative abundance of Si, Al, Fe, goethite, hematite, quartz, gibbsite, talc, and kaolinite in whole samples varies according to the origin of these ferruginous materials. All ferruginous nodules and pisoliths record goethite precipitation in the mid Miocene and are thought to be young relative to the saprolite beneath.

Geochronology has revealed that ferruginization of the aggraded channels becomes younger with depth. The multi-element anomaly in the NWS and the PPS reflects the underlying mineralization. Gold dispersion in the ferruginous materials is much greater than in the underlying saprolite. Thus, the NWS and PPS provide a broader target to mineralization itself and negate the requirement to drill deep into the regolith.

Explorers can afford to have a wider spaced sampling regime of ferruginous nodules and pisoliths compared to the saprolite. Not all pisoliths show the same geochemical dispersion due to their complex history of formation and consequently, their cores and cortices show different element behavior.
 Relatively fewer mobile elements (Cr, Sb, Sn, and Ti) are mainly hosted in cores, due to physical dispersion, whereas mobile elements (Au, As, Cu, Mn, and Fe) reside in cortical laminae and veins due to chemical dispersion.

An integrated approach, combining different mechanisms of dispersion with the formation of the transported cover, geochronology of weathering, and climatic setting needs to be considered to provide the best prediction of metal transfer.