Nokomai geology : an investigation of the sources and sinks of alluvial gold in the Nokomai catchment, Northern Southland

Kerr, Luke Charles.

Trollheim type alluvial deposition of locally derived gravels, reworked from preQuaternary placers in the Nokomai catchment, produced economic lags of gold at unconformities in the Quaternary sequence. Heavy mineral proportions, occurrence, and composition are indicative of a Central Otago provenance. Individual heavy, metamorphic minerals such as magnetite, clinozoiste, and garnet, are formed during greenschist facies metamorphism and therefore cannot by furnished by the basement rocks exposed in the Nokomai catchment. Nokomai basement rocks are part of the Caples Terrane which has been metamorphosed to sub-greenschist facies and is strained to textural zone IIA-IIB. The Nokomai catchment lies astride the Nevis-Cardrona Fault System (NCFS) in which most movement is accon@lated by a reverse, right-lateral, oblique fault (Garvie Fault). c) Associated with the fault are folds oblique and parallel to the fault system (Steeple Antiform and Slate Range Antiform -respectively). These Pliocene-Quaternary folds are superimposed upon two generations of Mesozoic folding, the Taieri-Wakatipu Synform and isoclinal folding associated with terrane accretion. The Nokomai Fault Zone is currently active on the Garvie and Moa Faults and, in contrast to the northward NCFS, is up to the east on eastwardly dipping structures. Au-bearing Quartz veins have been exposed and Commissioners Reef, the largest of these veins, was historically worked for small production. Commissioners Reef is the product of at least two kinematic styles, strike-slip and normal dip-slip. Arsenopyrite analyses suggested a foramtion temperature of 270±100°C and fluid inclusion studies were inconclusive.

xviii, 230 leaves : ill. (some col.), maps (some folded) ; 30 cm.

1998Kerr

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