Characteristics of gold bearing quartz veins at the historic Garibaldi goldfield, Central Otago, New Zealand

Stewart, James Alistair

A set of more than 30 mineralised normal faults and associated fractures cuts upper greenschist facies textural zone 4 Otago Schist between the North Rough Ridge and Rough Ridge drainage divides near Garibaldi Diggings, Central Otago. The mineralised structures strike northeast and dip steeply (60•) to the northwest. The zone of mineralisation is intense and occurs as a swarm of veins within a well defined area under 1 km2. Mineralised structures contain 1 cm to 1.8 m thick quartz veins which are traceable as an intricate network of structures for several meters along strike. In the near surface the mineralised structures are hosted within a thick zone of kaolinitized greenschist which developed beneath a regional unconformity surface and an overlying suite of auriferous terrestrial sediments. The thick zone of alteration forms a low northeast trending ridge and is heavily stained by secondary iron and manganese oxides. The source of the iron oxides may be due to the breakdown of iron bearing sulphide minerals within the mineralised structures or the oxidation of associated hydrothermal ankerite. Alternatively the breakdown of chlorite within the westerly dipping greenschist layer could also provide a source for the iron oxides. Quartz veins contain abundant breccias with variably altered clasts of quartzofeldspathic schist. The veins contain sparsely distributed scheelite mineralisation although no sulphide minerals can be observed in hand specimen due to the intense alteration within the host greenschist layer. Hydrothermal alteration of the host greenschist layer adjacent to veins cannot be differentiated from alteration developed beneath the regional unconformity surface due to the pervasive nature of this alteration. The normal faults and associated fractures of the Garibaldi vein swarm developed during middle to late Cretaceous extension whicih accompanied the uplift and exhumation of the Otago Schist belt. The vein swarm at Garibaldi may share affinities with the Manuherikia Fault Zone; a major northeast striking fault zone on the east side of the Manuherikia Valley, Central Otago. Detrital gold grains collected from both the Miocene and Quaternary paleoplacers at Garibaldi Diggings have secondary gold structures preserved on their surface. There is a consistency in morphology and surface textures displayed by detrital gold grains collected from the Garibaldi and Dell Creek placer deposit in the Lower Nevis Valley, Central Otago. The Garibaldi and Dell Creek placer deposits represent two separate parts of the Central Otago alluvial gold system.

79 pages : illustrations, maps, 2 folded maps and 4 sheets in pocket ; 30 cm. + 1 CD-ROM (4 3/4 in.)

2012Stewart

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