Granite intrusion and copper-molybdenum mineralisation in the Greenland group, Seventeen Mile Bluff, Westland.

Banks, Michael Jeffrey.

The Greenland Group sediments in the mapped area, adjacent to Seventeen Mile Bluff, consist of interbedded sandstone-siltstone units with a sandstone to siltstone ratio invariably greater than one.
The sediments exhibit great maturity with an abundance of detrital quartz and only minor feldspar. The original provenance for mineral grains was probably a Pre-Cambrian granitic terrain although their subsequent history has been polycyclic. Chemically the rocks exhibit K20/Na20 ratios greater than one and Si02 ranges from 65 to 75%.
The Greenland Group in the Seventeen Mile Bluff area exhibits mineral assemblages characteristic of the biotite zone of the greenschist facies. The siltstones have a well developed slaty cleavage and the sandstones have an imperfect schistocity. This foliation is attributed to axial plane sericite crystallization and the sequence is deformed into a series of open folds striking west-north-west.
Greenland Group sediments are intruded by a small stock of muscovite granite with porphyritic margins and associated aplite dykes of probably Cretaceous age. The stock was emplaced at a relatively high level (2-3Km.) in the crust at a temperature less than 650°C.
Cu-Mo mineralisation associated with the intrusive occurs as disseminations and veins in the country rocks and the stock itself. Minor lead, bismuth, tellurium and nickel mineralisation is also present.
Hydrothermal alteration of the stock, country rocks near contacts and adjacent to veins is locally intense. Alteration assemblages developed include: quartz-albite-microcline (potassic), quartz-albite-sericite (phyllic) and carbonate-epidote-chlorite (propylitic).
The chemistry and mineralogy of the intrusive rocks exhibit close affinities to other copper-molybdenmn mineralised granites in Westland and Nelson province. They exhibit high Na20/K20 ratios and frequently have Sl02 greater than 75%. This is chemically more akin to granites of the Cretaceous Separation Point belt than the strongly potassic Karamea and Paparoa belts of Carboniferous to Permian Age.
Alkali basalt and dolerite dykes intrude Greenland Group rocks at the Seventeen Mile Bluff headland. They are probably of mid to late Cretaceous age as correlative flows are found in the Paparoa beds (Mata-Roukumara series).

77 leaves : illus., map (fold. col. in pocket) ; 30 cm.

1978Banks

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