Summary:
The Merlin kimberlite field is situated on the eastern side of the North Australian Craton, ~100km south-west of the coast of the Gulf of Carpentaria. The Merlin kimberlites are Devonian in age (382 million to 352 million years ago) and intrude mid-Proterozoic shales and dolomites of the McArthur Group and the unconformably overlying Cambrian Bukalara Sandstone. Thin deposits of Cretaceous sediments and laterite overlie the pipes.
The Merlin pipes are small, with the diameter of the upper levels varying between 50m–125m. At surface the shapes of the pipes are circular to elliptical and maintain their regular shape and near vertical sides within the Bukalara sandstone. In the softer sediments beneath the Bukalara/ Proterozoic unconformity, some pipes increase in diameter. In the case of Palomides and Sacramore they coalesce into a larger single vent named PalSac.
The kimberlites are strongly weathered to 60m - 80m depth from surface and appear to have sunk back down into the craters, that have been in-filled with Cretaceous sediments that are up to 40m thick. The sagged nature of the infill sediments, the upturned edges with associated slickensides, the presence of a basal non-kimberlitic conglomerate and the thickened iron pisolite profiles, all suggest that the Cretaceous aged sediments have subsided into the pipe structures possibly due to solution weathering of the kimberlite.
The pipes are located within four main clusters over an area of 10km by 5km and are located close to the projected trace of the north-west trending Merlin Fault, which appears to have had a strong influence on the intrusion of the kimberlite volcanoes. The most northerly Emu cluster, consisting of two pipes (on the exploration licence), lies outside the current mineral lease and from the limited sampling data available are currently not known to be economic.
The remaining 11 pipes (contained in three clusters) are all significantly diamondiferous and are situated within the mining lease. They are named Bedevere, Kaye, Ector, Gareth, Ywain, Gawain, Tristram, PalSac, Excalibur, Launfal, and Perceval.
Five broad categories of kimberlite facies have been recognised: these being epiclastic kimberlite, tuffisitic kimberlite, tuffisitic kimberlite breccia, pelletal tuffisitic kimberlite and micaceous tuffisitic kimberlite. The presence of epiclastic kimberlite in the Emu 1 pipe and in Gawain, indicate the pipes are preserved at the upper diatreme level. The Crater facies evident at Merlin would have been present subsequent to emplacement buthave since been eroded and only the diatreme facies are currently evident.