Reconnaissance exploration for diamonds and unconformity- style mineralisation, proposed Kakadu III Extension, Northern Territory. June, 1987.

Abstract

The report records the results of follow- up exploration on various projects outside the main prospects within the South Alligator Valley such as Coronation Hill. Heli-borne radiometrics was used as a tool to explore for unconformity - related deposits which might have some uranium component and would hopefully contain precious metal mineralisation as at Coronation Hill. Forty hours of helicopter flying at 40 to 60 knots was completed. Navigation was manual to a 1:100000 topo base. Escarpments in the valley of Koolpin Creeek, around Sleisbeck in the Katherine Valley, and in the Gertrude Springs and Motor Car Creek Synclines to the west of the South Alligator Valley were covered. Twelve radiometric anomalies, A to L, were selected for follow- up stream sediment sampling. Multiple sources are indicated at Anomalies A, B and E. 5kg of -2mm sample and about 100gm of -80# sample were collected from creeks draining the radiometric anomalies. the results are thought to be low because of extensive dilution from sand. The radiometric and stream sediment results defined several anomalous areas, A major zone extends from (whiteout) Arnhem Land westwards towards Sleisbeck and then south along the escarpment towards Anomaly E. This zone is considered highly prospective. Anomalous results from the Koolpin Valley wereconfined to two cyanide- Au results. Area A had returned a highly anomalous cyaide- Au result of 30ppb in previous reconnaissance (CR5421). The sample site is east of the radiometric anomaly. The drainage pattern in the area is complex. Dense stream sediment sampling was conducted to locate the source. A follow up fire assay value of 0.28ppm Au was found in the main drainage again, though the cyanide- Au results were all low. Detailed follow- up was recommended. Follow- up of anomalous drainage sampling in the Mundogie Ranges (BHP CR5349; CR 1987-0349) was undertaken. Ridge and spur traverses with 100m sampling. 100gm of -80# soil sample was collected at each site. All traverses but one were over Mundogie Sandstone; the other covered BIF of Koolpin Formation. There was no observed hydrothermal alteration and only four spotty values above the 0.1ppm anomaly threshold. It is suggested that the conglomerate horizons of the Mundogie Sandstone are the source of the stream sediment anomalies. Diamonds and kimberlitic chromites have been previously located in the Katherine River in the stretch between Katherine and the southern boundary of Kakadu Stage III. Initial work suggested the source of these was within Kakadu Stage III. The first stage of follow up is described in BHP CR5421 (CR1987- 0329). This resulted in the discovery of a single diamond in Gimbat Creek, though its charactreistics suggested it may have come from a secondary source. In addition a discrete drainage in the south- west of the Stow 1:100,000 sheet area contained numerous Priority 1 kimberlitic chromites. In the work reported on here, a one tonne bulk sample was collected from that creek. This produced over 250,000 kimberlitic chromites, but no diamonds. Dolerites in the drainage basin were not the source. It is believed that the sample was very close to or maybe even on top of a non- diamondiferous kimberlite. The source of the diamonds in the Katherine River and in Gimbat Creek remains unknown. If more time were available, +20 ton samples should be collected from the Katherine River and treated in a jig plant. The large quantity of sand in sediments makes it difficult to get samples, and good sampling locations are hard to find in the wet season.