McConnochie Creek, Reefton, Buller District, West Coast Region, New Zealandi

Regional Level Types
McConnochie Creek	Creek
Reefton	Town
Buller District	District
West Coast Region	Region
New Zealand	Country

Small granite body with an incipient greisen mineralogy.

A greissenised granite intrusion into the Greenland Group rocks, showing quartz veins with sulphide mineralisation as disseminations, veinlets, and infilling of micro-fractures. The indication is the location would not produce impressive specimens, although if the quartz was sliced and polished, it may reveal the sulphides at their best. Specimens may exist in local collections, but are not generally available.

The intrusion was originally a granodiorite, subjected to postmagmatic greissenisation during the Cretaceous period. It was discovered during prospecting in 1978. It is 11.5 kilometres south-east of Reefton, bordering the McConnochie Creek, the intrusion covering an area 1300 metres long by 300 metres wide. The northern end is overlain by the Quartose Coal Measures bed. The intrusion is emplaced along the Tobin Fault Zone, and is elliptical in shape, orientated north-east to south-west, dipping 70 degrees south-east.

During alteration, muscovite, silification, and albitisation occurred, with red-brown biotite along the margins where alteration was limited. Quartz is most abundant, with also plagioclase, microcline, muscovite, biotite, pink garnet (possibly spessartine), and minor illite-kaolinite, topaz, small tourmaline needles, adularia, Fe-oxides, and sulphides.

The plagioclase is sodic, showing as albite to oligoclase, some replaced with quartz, moderate sericite alteration, and minor clay alteration. Microcline and/or orthoclase forms large plates, where not obliterated by quartz-muscovite replacement. Albite forms a 'chessboard' structure, replacing K-feldspar. Muscovite forms large books, or fine flakes in quartz.

At the south-west end of the granite body, the rock is shattered, fractured and filled with quartz veins, containing sulphide mineralisation. There are at least three sub-parallel sheeted quartz vein systems, across an area 400 metres x 25 metres, dipping 75-80 degrees south-east. Individual veins range from a few mms to 2 metres wide, with sulphides in the quartz forming disseminations, veinlets, and fracture fillings, forming a net like structure.

Ore minerals in order of abundance are pyrite, chalcopyrite, marcasite, molybdenite, sphalerite, native bismuth, bornite, argentite, and rare cassiterite. Pyrite and marcasite form intergrowths, marcasite marginal to the pyrite. In wall rock the only sulphides are pyrite and marcasite, associated with carbonate veinlets, or as disseminations. Chalcopyrite forms massive blebs or veinlets, intergrown with molybdenite and sphalerite, that cross-cuts pyrite, and marcasite. There are locally numerous native bismuth inclusions in the chalcopyrite. Molybdenite is found as isolated grains, and tiny rod-like intrusions in quartz grains. Bornite, argentite, supergene covellite and chalcocite may marginally replace chalcopyrite.

Other Regions, Features and Areas containing this locality

References