Grasmere 1:100 000 Sheet, Home to a Few Surprises Mapping of the Grasmere sheet near Broken Hill was undertaken as part of the Koonenberry Mapping Project, part of the Exploration New South Wales program. This sheet joins Bunda and Kayrunnera as the third, first edition 1:100 000 scale map completed to date. A further two maps, Nuchea and onnaminta, are currently in progress and the remainder of the outcropping block is scheduled to be mapped at 1:100 000 scale. The Grasmere map area is within the southern portion of the outcropping Koonenberry Belt which defines the eastern margin of the Curnamona Craton in western NSW. The belt is separated from the outcropping Broken Hill Block by the northwest striking Bancannia Trough, containing up to 6 km of Devonian and Mesozoic section. The Koonenberry Belt consists of multiply deformed Late Proterozoic and Cambrian sedimentary and volcanic rocks with less deformed, post Delamerian cover sequences that range from Late Cambrian to Cretaceous. The recent mapping has considerably extended the strike length of the horizon prospective for Cu, Zn, Ag and Au bearing massive sulphide mineralisation at Grasmere. The mapping has also located previously undocumented shafts sunk into carbonate altered mafic volcanic rocks, and an undocumented shaft sunk into gold bearing Devonian basal conglomerate. The delineation of the structural framework of the area also offers new exploration possibilities. While most mapping concentrated on the Cambrian and older basement rocks that host the Grasmere mineralisation, examination of Devonian strata revealed details of faulting and the structural setting of the shear controlled lode system up to 8 km in length along the Koonenberry Fault. Within both the Nuntherangie silver field and Wertago copper field, felsic volcanics and intrusions are present and subsurface intrusions are postulated. The presence of felsic intrusions, numerous historic mines and a favourable fault system suggest a prospective area for epithermal mineralisation.
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