Alluvial gold liberation and compositions downstream of Bonanza mine, east Otago

McCann, Robert Michael.

The Bonanza mine, East Otago is a disused gold mine that operated during the later part of the 19th century. It is a hard rock quartz mine situated about lOkm southeast of Dunback. In the steep schist basement in the area, there are deeply incised valleys. These valleys have concentrated gravels eroded from the catchment area, with Bonanza mine being at the head. The largest of these valleys, which is about 3km long by 1 OOm wide contains up to 4m of alluvial gravels. The sediments consist of the Otago Schist basement (Permian) and Taratu Formation (Cretaceous) conglomerates derived from remnants of the regional unconformity preserved on ridge crests. The aims of this study are 1. To find whether grains of alluvial gold sampled from a 12km length of river change in composition downstream; 2. To determine if all alluvial gold in the river is derived from Bonanza mine or if there is another source such as Taratu Formation and 3. What is the rate ofliberation of free gold from the Bonanza mine. The Bonanza quartz vein is known to be about 500 metres long, strikes northwest and dips 40° to the southwest. The vein has visible gold, with some arsenopyrite and pyrite mineralisation. Gold is primarily liberated from the vein by erosion. Palm-sized vein fragments found up to 12km downstream of the mine contain visible gold. Further downstream fragments containing free gold become smaller indicating a progressive liberation of Bonanza gold within the Back Creek catchment. There are no other mineralised veins which have been mapped in the area. The other source of gold in the catchment area is from the eroded Taratu formation. This population has a different silver content than that of the Bonanza gold. The malleable gold grains have been extensively deformed during transport showing distinctly differing morphologies between the sources. The differing methods of source identification are supported by each other; in this study differing silver content is supported by that of specific morphologies.

49 leaves : col. ill., maps ; 30 cm.

2008McCann

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