Trace element mobility in a deeply weathered regolith profile in the Albany-Fraser Orogen/Yilgarn Craton Margin: the Hercules and Atlantis gold prospects

[摘要] This study focuses on understanding the mobility of specific trace elements from the basement rock suites, through the regolith profile to its surface expression, within the framework of a deeply weathered profile over the Albany-Fraser Orogen/Yilgarn Craton margin, and in particular over the Hercules and Atlantis Gold Prospects (60 km NE of the Tropicana-Havana gold system).Two hundred and six samples were selected for bulk geochemistry, spanning the entire regolith stratigraphy of all regolith units at the Hercules and Atlantis prospects. Specifically, sampling of the entire thickness of regolith was carried out at two metre intervals above mineralised bedrock at drill hole NLD210, and above weakly mineralized bedrock for drill hole NLD071, to provide a“barren” comparison.There is a wide spectrum of basement rocks at the Neale tenement, with igneous rock suites ranging from sub-alkaline and alkaline gabbro, intermingled with granites, expressed in a mixed geochemical signature. There is common metamorphism to greenschist facies, with some localized indications of amphibolite facies. As a whole, these rock suites have a typical Archean geochemical signature of a tonalite-trondhjemite-granodiorite suite, similar to the Yilgarn Craton, resulting from slab melting in a subduction arc setting. No sedimentary packages have been identified within the basement rock suite. Due to the complexity of the basement rock suites, element ratios and metal accumulations in any stratigraphic unit of the regolith should be interpreted with caution for purposes of lithological classification of intrusive mafic-felsic-sedimentary rock units, when targeting specific rock type associations.The Neale tenement has a variable thickness of transported cover between 2 and 25 m. The in situ regolith is divided into three units: (1) top saprolitic silcrete unit (4-20 m thickness); (2) sandy kaolinitic upper saprolite (3-25 m thickness); and (3) lower saprolite, mainly expressed as ferruginized parental rock. The regolith profile typically has a total thickness of approximately 75 m.Results from geochemical sampling of the soil, within the uppermost first 20 cm, do not reflect any significant geochemical proxy that can address basement composition in the area. However, sampling transported cover can lead to the identification of displaced metal anomalies from the basement, which can be tracked using an understanding of the associated sedimentary system. Anomalies located within the transported cover could be traced to possible near source areas at a scale of 100’s metres. Drill core and RC samples from the lower saprolite, preserve the main geochemical features of the basement, and therefore be used for exploration targeting strategies.The silcrete unit (4-20 m thickness) is widely distributed in the Neale tenement, capping the kaolinitic upper saprolite. This unit varies laterally in cementation, and may present more permeable areas. These permeable areas coincide in some cases studied with a more kaolinitic upper saprolite, which has resulted from more intense vertical weathering, and consequently vertical geochemical dispersion of the basement signature to the surface. Surface geochemical sampling above these permeable areas could deliver more reliable geochemical basement signaturesThe weathering overprint in the environment of the Neale area developed in oxidizing acidic conditions within a low saline groundwater environment displaying low vertical hydromorphic dispersion of REE, HFSE and transition metals within the upper saprolite and through the transported cover, with no detectable expression in the aeolian soil cover.The most reliable soil fraction for metal and trace element content is 53-2 and2 micrometres fraction. Metallic elements are more concentrated in the2 micrometre fraction whereas the trace elements associated with the heavy mineral fraction are concentrated in the 53-2 micrometre soil fraction.

[发布机构] CSIRO

[效力级别] Geomorphology and Regolith and Landscape Evolution [学科分类] 地球科学(综合)