Withdrawal of ELA 31504 after evaluation of its phosphate potential

Abstract

ELA 31504 is on Aboriginal Land near Kunayangku and was applied for to evaluate an NTGS report of phosphate in waterbore RN011690 (Khan et al 2007, MODAT Kunayangku). An AAPA register search, a check of previous basement IOCG drilling by Giants Reef, and in-house waterbore study were undertaken. The study of previous mineral exploration drilling and waterbores reached the following conclusions: there is between 3 m and 24 m of Cenozoic cover above the Cambrian Wiso Basin stratigraphy, some of this cover is an aquifer; many of the bores did not get deep enough to test the most prospective interval; they were abandoned prematurely in running sand at 40 m-50 m, such sand would present a potential problem in mining through it. Furthermore, it would make the even-deeper prospective stratigraphy prohibitively deep for open pit phosphate mining; the basal Cambrian stratigraphy varies considerably over quite short distances, probably reflecting different basement topography and geology and variable facies-specific Cambrian onlap. The top of the preserved Cambrian is erosional, probably with the loss of tens of metres of section in places; the interval most prospective for phosphate was interpreted to be a Cambrian carbonate with chert, but this unit is prohibitively deep in the west and there was little prospect of it shallowing sufficiently to the east. It was also apparently absent in one waterbore which went right through the Cambrian stratigraphy into granite basement and it may also be absent in mineral exploration holes in the southeast of the ELA; in all waterbores checked, the standing water level (as shallow as 3 m on the ELA) was above the prospective interval and flow rates are moderately high. This would mitigate against open pit mining. In several cases, the most prospective interval corresponds to the main aquifer; in the specific case of the NTGS waterbore study which reported that RN011609 intersected 3 m at 2.43% P2O5 from 81 m depth, the standing water level is 16.32 m and the phosphatic interval is the main aquifer in the bore. Even allowing for some dilution of the chip samples used by NTGS due to drilling contamination from uphole, the grade and depth reported by Khan et al 2007 are no longer considered prospective by Verdant Minerals. The phosphate prospectivity of ELA 31504 has been downgraded and the ELA is being withdrawn. As an aside, the majority of groundwater in this area that was tested is not fit for human consumption due to high fluorine. pH readings indicate that the shallower groundwater is generally slightly to moderately acidic. That from the deeper Cambrian carbonate aquifers is generally closer to neutral.