Summary:
The Lithium deposits at the Finniss Lithium Project are hosted within rare element pegmatites of the broader Bynoe pegmatite field. The Bynoe Pegmatite Field is 15 km south of Darwin and extends for up to 70 km in length and 15 km in width. Over 100 pegmatites are known, with individual pegmatites varying in size from a few metres wide and tens of metres long up to tens of metres wide and hundreds of metres long. The pegmatites are predominantly hosted within the early Proterozoic metasedimentary lithologies of the Burrell Creek Formation and are usually conformable to the regional schistosity. The Bynoe pegmatites are classified as LCT (Lithium-Cesium-Tantalum) type and are believed to have been derived from the ~ 1845 Ma S-Type Two Sisters Granite that outcrops to the west. The Two Sisters Granite is considered to dip to the east under the Burrell Creek Formation.
Deposit Geology
The Grants pegmatite body is broadly lozenge-shaped, tapering from 20-40m wide in the middle to < 5m at the northern and southern ends, plunging to the south at approximately 60 degrees. Parts of the body swell downplunge while other parts contract. Overall, the pegmatite maintains a consistent shape and internal structure. There is little evidence for brittle structures or fault offsets.
Mineralisation at BP33 is hosted within a large, dominant, sub-vertical pegmatite body and a smaller sill-like body on the northwestern side. The BP33 pegmatite is interpreted to be approximately 350m in strike length and up to approximately 40m in true width. There is a very strong steep southerly plunge component with a depth extent currently more than 800m. The orebody remains open at depth. In the north, the body strikes towards 045° and dips steeply to the east. Approximately halfway along the body to the south, the strike changes to due south, and the body dips steeply to the west. The pegmatite body also thins in a southerly direction, and the average grade of the mineralisation also decreases to the south.
The Carlton deposit is an NNE-striking, steeply east-dipping and south-plunging pegmatite body with a strike length of 280m and true width of up to 25m (average 15m). It has currently been modelled down-plunge to a depth in excess of 800m. A mineralised shallow dipping pegmatite sill exists at the southern end of the main pegmatite body.
Fresh pegmatite at Grants, BP33 and Carlton is composed of coarse grained spodumene in addition to quartz, albite, microcline, and muscovite (in decreasing order of abundance). Spodumene, a lithium-bearing pyroxene (LiAl(SiO3)2), is the predominant lithium-bearing phase. There is no consistent zonation to the pegmatites other than a thin (1-2m) quartz-mica-albite wall facies at the contact with the host rock.
Geology and Geometry Context
To put the Finniss project into context, an understanding of the geology, the geometry of the deposit is critical.
Mineralisation at BP33 is hosted within a dominant, large, sub-vertical pegmatite body and a smaller sill-like body on the northwestern side. BP33 pegmatite is interpreted to be approximately 350m in strike length and up to approximately 40m in true width. There is a very strong steep southerly plunge component with a depth extent currently more than 800m. In the north, the body strikes towards 045° and dips steeply to the east.
Of note is the key variation of a steeply dipping body vs shallow dipping thinner bodies. This is extremely important as shallow dipping orebodies are proven significantly harder to mine, with higher costs, ore loss, and dilution increases, all of which impact the operational performance of the project. This contrasts to the Finniss ore bodies, which are predominantly sub-vertical, large, thick and homogeneous, allowing for much easier controls on mining and designs.