VCMSM Report 3 - An evaluation of high-purity alumina and rare earth elements in select clay occurrences of central Victoria.

Category: Victoria’s Critical Minerals and Strategic Materials	Product Code: MP-R-172463
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Executive Summary:
As part of the Developing Victoria's Critical Minerals Initiative, new pre-competitive geoscience was acquired to characterise the presence and form of a select number of Victoria’s known critical mineral occurrences. Kaolin and other clay deposits are potential sources of critical minerals, including high-purity alumina and rare earth elements.

Samples of select Victorian clay deposits show that clays formed by the weathering of different protoliths have characteristic geochemical and mineralogical signatures.

Clays derived from Palaeozoic granite are most suitable to produce high-purity alumina, due to a high proportion of Al₂O₃ in kaolinite, large quartz phenocrysts amenable to mechanical separation, and low deleterious elements. Sampling of granite derived clays from existing clay operations and surrounds, were from metaluminous (‘I-type’) granitic rocks, which typically do not have strong high field strength element and rare earth element enrichment. A single sample of clay in legacy drill cuttings formed from the in situ weathering of Ordovician sedimentary rocks contains elevated rare earth elements (1,001 ppm total rare earth oxide). X-ray diffraction mineralogy on this sample suggests that the rare earth element mineralisation is in apatite and therefore is not ionically adsorbed, although further work is required to confirm this.

While not replicated in this study, rare earth element enrichment has been identified in clays in Victoria. This includes, in situ kaolin formed during weathering of the Mount Egerton Granodiorite, and in fine-grained fluvial sediment deposited by the palaeo Murray River.

Most in situ clays in Victoria formed during an indiscriminate Palaeocene-Eocene weathering event, which affected all rocks exposed at that time. A revised understanding of Victorian landscape evolution developed as part of this study, including the nature and extent of post-Eocene deep erosion and modification, has identified the geographical extents of in situ Palaeocene-Eocene weathering regolith remnants, helping to define the search-space for in situ clay-hosted critical minerals in Victoria.

The thickest Palaeocene-Eocene weathering profiles typically developed within, beneath, and proximal to pre-Eocene fluvial units, such as the distinctive and extensive White Hills Gravel. The mapped distribution of White Hills Gravel and other cover strata of similar age therefore serve as a proxy for the distribution of remnant Eocene clay rich regolith. Mineral exploration for in situ clay deposits should be focussed on granite and other feldspar rich igneous rocks proximal to the White Hills Gravel and on other landscape remnants Eocene or older in age. Alkaline (‘A-type’) felsic igneous rocks, which can contain elevated high field strength elements and rare earth elements, do occur in Victoria and in situ clays derived from these are a priority for further sampling and analysis.

Bibliographic Reference:
Andrews, T.M. and Cayley, R.A. 2024. An evaluation of high-purity alumina and rare earth elements in select clay occurrences of central Victoria. Victoria’s Critical Minerals and Strategic Materials Report 3. Geological Survey of Victoria, Department of Energy, Environment and Climate Action, Melbourne, 54 pp.

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The downloadable version of this report is supplied as (PDF 27 MB), Attachment A1 data (PDF 92 KB) & Attachment A2 data (XLSX/ODS 120 KB).