Finniss Gravity Survey 2020 Final Report to NTGS

Abstract

Core Lithium Ltd is a lithium explorer and budding developer in the Bynoe Pegmatite Field, southeast of Darwin on the Cox Peninsula. There, lithium-rich pegmatites have intruded the Finniss River Group, the youngest part of the Pine Creek Orogen. The source is currently interpreted to be the geochemically-similar Two Sisters Granite that outcrops to the west and possibly underpins the entire area. The original objective of the gravity survey outlined here was to test the concept that pegmatite presence, style and fertility are at least in part related to the geometry of the underlying granite roof in the Bynoe Pegmatite Field. The previous gravity dataset in the Finniss area is a very broad 11 km spacing but was used to imply a regular upwards slope of the upper granite contact from east to west. This is consistent with the metamorphic gradient and the transition to the high-grade Litchfield Complex to the west. However, there is potential for this to be an accreted terrain that underwent metamorphism prior to emplacement of the Two Sisters Granite. If so, then this broad gravity data might reflect lower crustal architecture rather than the shallower granite roof. The slope might be entirely different. The previous gravity data spacing was also not sufficient to decipher the mesoscale geometry of the contact, which may play an important role of localising pegmatites, such as above apophyses on top of the granite. The apparent clustering of pegmatites in several areas (groups; Frater 2005) within the Bynoe Field was thought to be directly related to this geometry (eg, Leviathan, Kings Table, Observation Hill Groups etc). Core requested and was granted collaborative Government support to collect closed-spaced gravity over the project area to assist in exploration and to improve the NT Governments regional gravity dataset in the Pine Creek Orogen. The Finniss Gravity Survey was carried out by Daishsat Pty Ltd between 3 to 18 August 2020 via the ground collection of a 500x500m or 500x1000m grid of gravity stations through the majority of Core's landholding and along the main gazetted roads. The data has been terrain corrected and processed. Various images and processed data can be found in this document and the Data Delivery. The survey was successful in achieving a number of objectives that were laid out prior to commencement. Processed gravity data neatly maps granite geometry at macro and meso scale. The regional gravity field would appear to be related to deep crustal structure and not granite geometry. Granite has been differentiated into three main bodies: Two Sisters Granite in the west; Ringwood shallow pluton in the southeast; and a linear NNE belt in the middle that has a spatial connection with pegmatites. Pegmatites are largely restricted to the gravity highs that are interpreted to be the shoulders of the central granite body. Steep pegmatites reside on the crest, while shallow-dipping bodies are at the flank and to some extent above the larger granite bodies. The shape of the granite apophysis therefore exerts a strong control on the structural regime above it, into which pegmatites are emplaced. The steep-dipping and shallow-dipping styles currently recognised in the Bynoe Field have a spatial relationship with the gravityimplied roof geometry. The heat-flow created by the regional-scale and meso-scale depth-to-granite is also likely to have controlled the depth at which pegmatites are fertile for lithium precipitation and preservation. The survey also highlights a major NNE-trending gravity high and lithium-pegmatite corridor that extends from King Table Group in the north to the Leviathan Group in the south and includes the lithium-rich Observation Hill Group (main prospect are Grants, Carlton, Hang Gong). There is no reason to believe that these pegmatite groups are clusters. It appears more likely that the current distribution of pegmatites in this belt is due to exploration maturity. Large tracts of ground between Grants and Leviathan are covered in a thick mantle of laterite or estuarine cover and have not been effectively explored to date. The Annie River Group appears to relate to a separate gravity high on the eastern side of the central gravity ridge. There is no logical explanation for the larger pegmatite size or lower fertility of these pegmatites at present. It may also reflect exploration maturity. The gravity survey demonstrates unequivocally that the gravity methodology is a very useful tool for pegmatite exploration and granite mapping.