A review of the geological, geophysical and geochemical data of the Kakadu Conservation Zone as a basis for assessing the resource potential.

The Bureau of Mineral Resources, Geology and Geophysics (BMR) has developed a model for the distribution of gold, platinum, palladium and uranium at Coronation Hill. There is strong evidence that deposits of this type may occur at other places within the Conservation Zone.
This report presents the results of systematic field investigations carried out by the BMR for the Resource Assessment Commission inquiry into the Resources of the Kakadu Conservation Zone. The brief was to "conduct and interpret geological, geochemical, and geophysical surveys as required to provide the best possible basis, within the limited time available, for estimating the resource potential elsewhere in the Conservation Zone" (ie. excluding the defined resource at Coronation Hill).
During June/July 1990, regional stream sediment geochemical surveys were undertaken and limited soil sampling was done. Detailed geochemical rock chip sampling was also carried out at the old mines, prospects and occurrences and a review was made of company work undertaken at these localities. From these surveys a relative evaluation of the likelihood for mineralisation (i.e.. prospectivity) of each anomaly and/or prospect is made. An quantitative estimate of the undiscovered resource of the region is the topic of a separate BMR report.
The Kakadu Conservation Zone was the site of intense regional uranium exploration from about 1954 to 1964 and again from 1968 to 1976. There are numerous signs of mining activity including seven open pits and 5000m of underground workings. At most of the prospects there is a network of access roads, benches and costeans.
The Conservation Zone has never been systematically or rigorously tested for minerals other than uranium. Except for the recent work at Coronation Hill, exploration for gold consisted only of analysing those drill cuttings in the uranium drilling programs that had visible gold.
The Conservation Zone contains many areas with geological fault structures, rock types and rock alteration favourable to the occurrence of further gold+palladium+platinum (± uranium) mineralization of the Coronation Hill type. Other localities contain high levels of rare earth elements, representing the potential for styles of mineralisation other than precious metals.
Economically significant gold, palladium and platinum assay results were obtained in stream sediment and soil surveys. High gold values from 14 stream sediment sites are comparable to assay results adjacent to some demonstrated economic resources of gold elsewhere in Australia. Some of these zones occur in areas not previously known to have mineralisation.
The majority of the prospects and mines in the Conservation Zone were located as a result of geophysical surveys using either radiometric or self potential surveys. A review of the geophysical data shows that there are many untested airborne radiometric anomalies and self potential anomalies which are potentially related to mineralisation.
Systematic variations in mineralisation styles and structural settings, rock alteration characteristics and rock types indicate that all the mineralisation styles are probably related to one major mineralisation system. An empirically based new ore genesis model is presented which integrates these variations and which is used as a predictive tool in assessing the potential mineralisation within the Conservation Zone.
The model relates gold and palladium/platinum to feldspar-bearing rocks; uranium is usually only found where carbon-rich shales or chloritic rocks are present. Where there is quartz veining present gold is associated with minor amounts of arsenic, copper and/or uranium. These deposit types are surrounded by altered rocks that are depleted in sodium, calcium, silica and thorium. These same elements have been moved to higher levels, and therefore rocks enriched in these elements are considered as possible indicators of gold, platinum, palladium and uranium mineralisation at depth.
Zones of quartz veining at these higher levels are potential sites for gold and minor uranium mineralisation. At even higher levels, zones of thorium enrichment are related to high concentrations of rare earth elements, which offer secondary but significant economic targets. Therefore the possibility exists for a tiered arrangement of differing types of mineral deposits. Gold may also be associated with iron stones similar to the Tennant Creek deposits, offering yet another deposit style for consideration in any resource assessment analysis.
The mineralisation is located in positions where openings in the rocks have been created by differential movement along fault zones. Two types of openings control most of the Conservation Zone mineralisation. Near-horizontal tabular ore zones are related to openings created along unconformities (major breaks in the geological rock record). Steep cigar-shaped ore zones are formed in openings created at bends in near-vertical fault lines. Past exploration (including drilling) is evaluated to determine how well these geometries have been tested.
Coronation Hill is an intensively faulted zone in which many rock types are juxtaposed. Mineralisation consists of gold, palladium and platinum, with both uranium-poor and
uranium-rich zones. Most of the uranium-rich zones are outside of the proposed open pit. The deposit is open at depth and the structural setting and mix of rock types suggest that extensions of mineralisation will occur at depth.
Repetitions of the Coronation Hill style of mineralisation are likely at Coronation Hill West, where the geology is a direct mirror-image of the Coronation Hill deposit.
Gold mineralisation at El Sherana is high grade but discontinuous. It occurs in spatial association with, but often slightly removed from, uranium mineralisation. Both occur in shallow northwest-pitching cigar-shaped bodies which do not appear to persist at depth owing to this geometry. The recent five hole drilling program by the Coronation Hill Joint Venture Partners failed to test the distribution of mineralisation, and the area still has a very high potential. El Sherana is a small part of a highly prospective area that extends for over three kilometres from Stag Creek through to High Road and beyond.
The Palette area is structurally and stratigraphically complex, and is so far largely untested for gold, palladium and platinum. Four drill holes by the Coronation Hill Joint Venture Partners tested a Coronation Hill-like structural target but obtained no significant results. More work is needed to properly test the potential of the many possibly mineralised structural sites of this area, which extends along strike for at least two kilometres from Cliff Face to Skull 2.
Other old prospects which warrant further exploration for gold, platinum and palladium include the Airstrip region, the Monolith-Koolpin Creek area, Scinto 5 North, the Saddle Ridge area and the Clear Springs Fault Group between Scinto 5 and Scinto Camp. Areas indicated as moderately to highly prospective by this study which have not previously been intensively prospected include Saddle Ridge Northeast, Pul Pul Hill, Gimbat Ridge, and along the Fisher Fault.
The history of uranium development in this region is of small, pod-like high grade deposits. This study suggests that such pods are controlled by the intersection of near-vertical trending carbonaceous or chlorite-rich basement rocks and subvertical faults. Additional uranium deposits are likely to be found.
The unusual style of platinum and palladium mineralisation is dependent upon several distinctive geological parameters. As these are rarely coincident elsewhere in Australia there is a low probability of locating other deposits of this style in other regions. This is endorsed by the failure of intensive exploration in the last five years to locate a new economic resource of these metals in Australia.
Of the 50 sites described in this report several are rated as highly prospective. A comparison with exploration case histories in other regions of Australia suggests that the presence of significant economic resources additional to Coronation Hill is very likely.
Any future exploration would be assisted by the extensive existing network of about 300 km of roads, including access to the top of the Scinto Plateau, and numerous trenches, benches and adits. Most of the roads are in reasonable condition and would require minimal bulldozing to repair them.

Tags: Conservation zone, EARTH SCIENCES, Kakadu, Published_External, mineralisation
Organization: Geoscience Australia Data