Antrim Project Combined Annual Report for ELs 22642-22644 and 22749-22751 for the period up to 31 December 2002

Abstract

The Antrim Nickel (Ni)-Copper (Cu)-Platinum Group Element (PGE) Project is based on striking analogies between the geological and geophysical settings of the giant Noril'sk Ni-Cu-PGE sulphide deposits in Russia and those of the Antrim Plateau Volcanics (APV) of northwestern Australia. Unpublished, high-quality geochemical data recently acquired by Glass (personal communication, 2000) and AusQuest demonstrate that Antrim basalts are depleted in Ni, Cu, and PGE as well as being strongly contaminated by crustal material. Basalts associated with the Noril'sk Ni-Cu-PGE sulphide deposits in Russia also have these characteristics that are intimately related to the magmatic sulphide forming processes. The >200 km long Milliwindi Dyke, exposed in the Kimberly Basin, is an Antrim intrusive equivalent and is not depleted in Ni, Cu or PGE. This provides strong evidence that Ni, Cu and PGE, in the form of magmatic sulphides, have separated from Antrim magma within the feeder plumbing system in the sedimentary successions of the Birrindudu, Victoria River and Centralian Basins that lie below the APV but above the Kimberly Basin rocks. These successions contain evaporites and pyrite as possible sulphur sources and other reactive lithologies that may trigger formation of magmatic sulphides. Flat-lying sub-volcanic intrusions beneath the basalts centred around feeder conduits are potential trap sites for the concentration of Ni, Cu, and PGE sulphides. A review of geophysical data has identified the magnetic Rib, a distinctive regional magnetic and gravity feature that is interpreted as the main axis of extrusive and intrusive activity for APV magmas. The Rib appears to be controlled by a regional-scale mantle-tapping fault system that was active in pre-, syn- and post-Antrim times. Review of open file data shows that there has been no systematic Ni exploration over the APV and surrounding sedimentary rock successions. Detailed study and geophysical modelling of aeromagnetic, ground magnetic and ground gravity anomalies associated with the Rib and related structures have outlined possible, buried sills. Four of these anomalies have been drilled, but no sills were intersected. Drill holes ANTD001 and 002 which are 18.4 km apart and located to the south of the Neave Fault, intersected several thick and very thick basalt flows that can be correlated between holes on the basis of field characteristics and geochemistry. The thickest flow is ~250 m thick. The composition of the basal flow intersected in ANTD003 and ANTRC004, north of the Neave Fault, are significantly different from the basal flow to the south of the fault and these flows can not be correlated. The extreme thickness of the flows in ANTD001 and 002 are likely to be a result of significant ponding of lava. This suggests that the Neave Fault was active prior to Antrim extrusion and that a half-graben had formed to the south of the fault. The Neave Fault has also been active since Antrim emplacement. Thus it appears that the Neave Fault has a long history of activity and was likely to have been active or at least have been a zone of significant weakness during APV extrusion making it a potential feeder conduit for Antrim magma. The magnetic Rib remains untested by drilling. Based on data from ANTD001 and 002 the source of the Rib anomaly is deeper than previously modelled. This anomaly remains a potential Antrim-related sill or complex of sills within the Precambrian sedimentary succession underlying the APV. The magnetic and gravity anomalies north of the Neave Fault drilled by ANTD003 and ANTRC004, originally interpreted as potential mafic sills within the Precambrian sedimentary succession, are explained by the drilling as thin erosional remnants of the APV. Potential kimberlitic pipes were also recognised from aeromagnetic data as magnetic holes within APV basalts. Ground magnetic and gravity surveys show that some of these 'holes' have steep sides and are 100-200 m in diameter, ideal targets for possible post-Antrim kimberlitic diatremes. Surface sampling failed to locate diamondiferous indicator minerals although many of the holes are covered by transported regolith. None of the holes were drilled and their source remains unknown. Drill testing the best looking of these holes is strongly recommended.