Evaluation of the EL 555 Ammaroo Turquoise Deposit, Tosca (Tasca) Hope.

Abstract

Turquoise production has commenced from the Ammaroo deposit. Stone of good second and third quality has been won in some abundance; but top quality material is only found in trace amounts. The turquoise layers occur within claystones that overlie a conglomerate bed that was thought to be a false basement. The occurrence of first quality tone below the conglomerate was therefore thought a possibility. A study was made of the previous drilling programme ( Morrison 1968), recently exposed cuts and pits were mapped and sampled. Five percussion holes were drilled into the basement, one directly; samples at two foot intervals were logged and some selected for copper and phosphorous assay. Within the main pit area a conglomerate forms the basement to any turquoise bearing claystones. Immediately underlying the conglomerate and in places the claystone occurs a quartzitic sandstone that forms part of the Proterozoic metamorphic basement. The quartzitic - sandstone in places grades into, or is interbedded with, a purplish phyllite that is a metamorphosed sandy shale to siltstone. The Proterozoic has been intruded by granite and granite porphyry that were intersected in the previous drilling programme. The main turquoise horizon is close to the base of the claystones particularly where ferroan and phosphatic beds are abundant. The basal four feet of claystone contain abundant beds of kaolinite and khaki - rusty brown - tobacco coloured clay rich rocks thought to be tuffs. The coloured beds contain up to 4000 ppm copper, while all are alumina rich. Phosphate beds and lenses are commonly interbedded with rocks described above. The phosphate occurs as an aluminium variety, a mineral that can convert to turquoise by the addition of copper. Light coloured claystones and solid basement rocks contain from 35 -- 250 ppm copper. Values to 740 ppm occur in weathered basement rocks where leached copper from the basal claystone beds is thought contaminatory.