Final report EL 3607 Williery South, NT

Abstract

EL 3607 is believed to cover an extension of the McArthur Basin of Carpentarian age. Black shales of the Barney Creek Formation of the McArthur Group, the host to the major HYC Zn, Pb, Ag deposit has been intersected (in EL 3607 ) in a trough bounded by syndepositional faults similar to and parallel to the Emu Fault system bounding the HYC deposit. The McArthur Group sediments outcrop poorly and have been subjected to deep oxidation leaching and silicification. The McArthur Group sediments are covered by Cainozoic Sand and black soil, Cretaceous sediments, Cambrian basalts and Roper Group and Nathan Group sediments ranging from a five metres up to several hundred metres plus in thickness. The Barney Creek Formation intersected was essentially barren. The McArthur Group carbonates intersected in the host block immediately east of the black shale sub basin contained epigenetic pyrite mineralisation but no significant base metals. EL 3607 was originally granted to Clifford Minerals Limited on 7th October 1982. EL 2835 located immediately to the north was originally pegged because it was thought that rocks of Carpentarian age may be present and could therefore be prospective for large tonnage base metal deposits. The exploration programme was supervised by Don F. Ward and Associates Pty. Limited. A preliminary field visit suggested that the area may be a hitherto unrecognised extension of the McArthur Basin and may therefore be prospective for large tonnage Zn, Pb, Ag (Cu) deposits of the Mt. Isa-McArthur style. Other potential extensions of the McArthur Group were subsequently applied for and EL 3607 and 1079 sq km was granted on 7th October 1982. The two Exploration Licences were then examine as one geological unit. An option was granted to Esso Australia from Sept.,l982 to May 1983. During this period the two licences were mapped at Air Photo Scale (1:84,000 ) assisted by a hired helicopter. This mapping confirmed the presence of McArthur Group Sediments ( Umbolooga Sub Group ). Structures similar to the Emu Fault were recognised while formation thicknesses similar to those in the Batten Trough in the McArthur Basin indicated the possibility of locating a base metal deposit similar to the HYC deposit at McArthur River ( Published reserves 232 million tonnes 9.2% Zn, 4.1 % Pb, 4 1 g/t/ Ag ). A higher metamorphic grade than at McArthur River, but lower than at Mt. Isa suggested that mineralisation in the Helen Springs area would be coarser grained than that at McArthur River, and hence would have lesser metallurgical problems. The exploration programme consisted of detailed geological mapping on airphotos flown for the purpose and also on pegged grids. The mapping was augmented by airborne and ground magnetic surveys and extensive RMIP Surveys. The RMIP was found to be particularly useful in areas of extensive aeolian sand and thick Cretaceous cover where the pattern of resistive and conductive formations was successfully followed. Extensive gravity both semi regional and detailed was employed in an attempt to define structure and perhaps locate local accumulations Extensive rotary percussion drilling was conducted for stratigraphic and geochemical information. In some areas of silicified highly resistive rocks, electrode holes were drilled to the water table to ensure good electrical contact. Diamond core drilling was employed when bad ground conditions and or strong water flows prevented further rotary percussion drilling. Continuous core sampling is frequently essential in the McArthur Group to reliably identify formations. Sirotem was employed in an attempt to define depth to conductive horizons below thick younger cover ( EL 2835 only ). Lead Isotope analyses were conducted on lead minerals by CSIRO (Sirotope) in an attempt to indicate proximity to major base metal deposits assuming the mineralisation to be similar in style and isotope composition to the mineralisation at Mt. Isa and McArthur River. The target host formation, the Barney Creek Formation was inter sected in three holes in the Hunter Creek area EL 2835. Weak McArthur style zinc lead mineralisation was also intersected in all three. However, lead isotope ratios indicated it was possibly distal to a major ore body at best. Attention was then transferred to the parallel Willieray Fault which appeared to be a stronger structure but where outcrop was extremely poor. RMIP was employed to aid sub surface mapping. Barney Creek FormatIon was intersected in the first hole targeted to intersect this formation. It was intersected below 20 to 40 metres of Cretaceous sediments in the Willieray South area within EL 3607 Willieray 3 DD ). Unfortunately, the formation here was barren. Attention was then focussed on the Willieray North area where possible Barney Creek Formation/Teena Dolomite was intersected along strike from Willieray 300 in the same graben below 300 to 400 metres of younger cover. The intervening distance of 13 kilometres is covered by an unknown thickness of younger sediments. A new mineralised shaly horizon associated with a thick tuff horizon was discovered in the Willieray North area in the basal Tooganinie Formation and Tatoola Sandstone. This horizon has not been tested in EL 3607. The oldest rocks outcropping in this area have been assigned to the Tompkinson Creek Beds of possible Lower Proterozoic Age ( Helen Springs Explanatory Notes. ) It was reported that some major unconformities may be present. It was during the present survey that correlation was made with the McArthur Basin sediments of Carpentarian age. The correlation of the oldest rocks mapped in the area is tenuous at best however, the correlation of the overlying suite with the Umbolooga Subgroup of the McArthur Group is quite clear with all formations being present (except perhaps the topmost Reward Dolomite) and most of the Members both formal and informal as well. The measured thicknesses of the formations in outcrop and in drill holes are similar to those in the Batten Trough of the McArthur Basin, indicating the presence here of sedimentation in a separate and parallel graben to the Batten Trough. This sedimentation laps onto the Tennant Creek Block to the south and probably marks the true southern boundary of the McArthur Basin. A notable variation between sedimentation in the Helen Springs area and the Batten Trough occurs in the Tatoola Sandstone and Tooganinie Formation where a major tuffaceous event similar to that in the Barney Creek Formation in the Batten Trough is present. This tuffaceous horizon is possibly thicker and more continuous than

the discontinuous Barney Creek Formation where significant thicknesses are confined to sub basins (500 metres plus) with thicknesses of the order of a few tens of metres in intervening areas. The normal tuffaceous event in the Barney Creek Formation (which extends down into the Teena Dolomite) is also present in the Helen Springs area. This lower tuffaceous horizon also culminates in a thinly bedded shaly zone strongly anomalous in base metals. It has not been intersected below the oxidised zone but appears to have been originally bituminous and pyritic. This lower tuff probably exists in EL 3607 but has not been intersected in any drill holes.