Summary:
The Mt. Holland project is focused on the exploitation of the resource in the Earl Grey pegmatite group. The Earl Grey pegmatite group consists of a main tabular pegmatite body flanked by numerous narrower hanging wall and footwall apophyses. The pegmatite has a strike length of at least 1 km, and a dip extent of over 2 kilometers and a thickness of up to 100 meters.
Deposit Types and Mineralization
The Earl Grey pegmatite group consists of a simple albite-quartz-microcline-spodumene petalite dominated assemblage with minor biotite, muscovite, and tourmaline (Covalent, 2020). The lithium aluminosilicates spodumene and petalite are by far the most abundant lithium-bearing minerals in the Earl Grey pegmatite; however, a wide array of trace lithium phases has also been documented in distinct domains. These are mostly late-stage alteration related phases, and except for cookeite, are a rare occurrence. Textures range from extremely coarse pegmatite through to finer grained seriate granitic to aplitic and late-stage replacement textures. The Earl Grey pegmatite group does not display the strong concentric mineralogical zonation commonly associated with complex rare element pegmatites.
Bedrock Geology
The Earl Grey pegmatite is hosted within the north-south trending amphibolite facies volcano sedimentary stratigraphy of the mideastern ultramafic belt. The stratigraphic succession broadly progresses up-dip towards the west, although potential repetition along major north-south trending shears makes the original sequence difficult to ascertain.
The base of the sequence is dominated by high-Mg basalt with intercalated horizons of andesite, mafic sediments, Banded Iron Formation (BIF), komatiitic basalt and tholeiitic sills. A package of komatiites with intercalated BIF sits atop the high-Mg basalt, with this contact appearing at least partly structural. At the far west of the deposit, pelitic and carbonaceous schists of the upper sedimentary succession occur in faulted contact with the komatiites,
Two major Proterozoic dolerite dikes intersect the greenstone sequence in the vicinity of Earl Grey, including the 400 m wide Binneringie dike, which marks the southern extent of the deposit.
Surface geology
The residual weathering zone around the Earl Grey pegmatite extends 30 to 40m below surface, with few instances of outcrop or subcrop in the area. Shallow depressions of limited extent contain minor alluvial and colluvial sediments; however, no significant channels have been identified in the immediate area. The area is predominantly covered by a veneer of laterite, up to 5 m in thickness, which is underlain by a 10 to 15 m deep alluvial zone of pallid grey to mottled clay material. The regolith becomes increasingly iron-rich toward the base of the weathering profile, with ferric induration common.
Earl Grey pegmatite
The Earl Grey pegmatite group consists of a main tabular pegmatite body flanked by numerous narrower hanging wall and footwall apophyses. The pegmatite has a strike length of at least 1 km, and a dip extent of over 2 km and a thickness of up to 100 m. The pegmatites become progressively narrower and more branched to the south and the east of the main pegmatite until even the main body divides into several narrower dikes. Narrow blocks of enclosed wall rock rafts are present within some areas of the pegmatites.
The pegmatites intrude with an approximate strike of 210° to 220° and dip of 5° to 15° to the northwest. At their western margin, the pegmatites appear to be affected by gentle folding. The dip of the pegmatites is variable, with the pegmatite steepening from sub-horizontal in the south to 10° to 15° to the northwest north of the Earl Grey gold pit. Several footwall pegmatite branches dip to the southwest at around 20°, potentially intruding the same set of structures as the Bounty pegmatites.
The pegmatite group is truncated to the south by the east-west trending Binneringie dolerite dike. Similarly, a 20 m thick dolerite dike crosscuts the pegmatite south of the Earl Grey gold pit. The full down-dip depth extent of the pegmatites is not currently understood, with deep drillholes suggesting the main pegmatite pinches out and another pegmatite of similar thickness develops in the hanging wall. The eastern extents of the pegmatites have not been well defined at this stage, with the pegmatites narrowing to sub-meter thickness at around 1.5 km east of the Mt. Holland Shear (Covalent, 2020).