Grasmere Copper and Gold Mineralisation High-grade secondary copper minerals were mined at Grasmere during the late 1800s to early 1900s. Primary Cu, Zn and precious metal mineralisation occurs along a line of quartz and magnetite lode with some 4 km of outcropping strike length. Drilling results indicate a continuous, steeply dipping, primary massive sulphide lode. The lode dominantly consists of pyrite with lesser chalcopyrite and sphalerite, occurring down dip from siliceous and ferruginous gossans with secondary copper minerals. Better drill results include: 6.10 m @ 2.73% Cu; 0.64% Zn; 6.0g/t Ag and 0.21 g/t Au and 8.50m @ 2.95% Cu; 0.71% Zn; 6.8 g/t Ag and 0.02 g/t Au. Typical results with estimated true width sulphide intersections are: 1.6 m @ 3.54% Cu and 3.3g/t Au (61.4-63 m), and 3.6 m @ 3.78% Cu, 1.12% Zn and 0.16g/t Au. Although limited exploration has been conducted along the 4 km strike length of exposed Grasmere lode, most drilling undertaken by previous explorers has concentrated on a 400 m longitudinal section of the lode. The recent mapping project has identified a prospective horizon extending beyond the exposed lode to the northwest and southeast, greatly enhancing the prospectivity of the area. At map scale outcrops of the gossan appear to transgress the mapped local stratigraphy of interlayered tuff, quartz magnetite rock and magnetite, epidote and pyrite altered basalt of the Grasmere beds. In drill core the lodes are finely banded, suggestive of syngenetic origin. The lodes may therefore be simply transposed and upgraded by later en echelon fault movement. Fault movement is evident at the sharp, sheared sulphide lode margins and at outcrop and map scales. Further Exploration Potential Other notable prospective areas are a group of pits and shafts sunk into copper and gold-bearing fault breccias at Rawlins Tank and pits and alluvial workings where Prospectors Creek crosses the Koonenberry Fault. Other, possibly prospective areas include a three metre deep shaft with a drive of indeterminable length developed within the basal conglomerate member of the Pevril Peak outcrops of the Devonian Snake Cave Sandstone. A grab sample of mullock returned anomalous values of 0.044 ppm Au and 13.0 ppm U. Dilational zones adjacent to the Koonenberry Fault are also a valid target for lode gold deposits and possible Cobar-style structural traps. A grab sample taken from an unprospected gossanous quartz vein returned an assay of 0.33 g/t Au, 1.5 ppm Te and 118 ppm Co. Also previously undocumented are numerous shafts developed within quartz and epidote altered basalt of the Paddys Creek beds. Within the Mount Daubeny Basin (see page 21), Late Silurian-Early Devonian red-bed dominated sediments and contemporaneous volcanism occur. The development of volcanism in a rapidly subsiding basin occurs in response to high heat flows in stretched and attenuated basement rocks and provides the ideal mechanism for the development of intrusive related mineralisation. Within or immediately adjacent to the Mount Daubeny Formation on the adjoining Kayrunnera 1:100 000 sheet lie the Nuntherangie Silver field (Pb, Ag), Wertago Copper field (Cu +/- Au) and Great Wertago Mines (Cu). Epithermal gold and porphyry copper systems are possible in such an environment. This article is based on a paper presented by Peter Buckley at the 2000 BHEI Conference, with an abstract published in AGSO Record 2000/10. For further information contact Peter Buckley on (08) 8080 0632, fax (08) 8087 8005 or e-mail: [email protected]
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