THE BM37 SHOOT OF THE KARANGAHAKE AU-AG DEPOSIT, NEW ZEALAND: A BONANZA SHOOT IN AN ADULARIA-SERICITE EPITHERMAL OREBODY

Epithermal Au deposits are attractive exploration targets because of their high grades, and some veins have exceptionally high-grade shoots. BM37 is a high-grade ore shoot of the Maria Lode of the Karangahake adularia-sericite deposit in the southern Hauraki Goldfield in the North Island of New Zealand. The Karangahake deposit was the second largest producer of bullion in the Goldfield; it produced over 1 Moz Au from veins that were mined for over 500 m vertically and over 1 km along strike. Ninety percent of the Au was recovered from the Maria lode, and the remainder from the Welcome lode. Both lodes are discrete 2-3 m wide veins that are hosted by andesite flows and breccias. Where the andesite passes upwards to rhyolite, the veins break into a stockwork that is not economic.

Samples from BM37 contain up to weight percent gold. The ore shoot is a vein that pinches and swells, but is typically tens of cm thick, and locally exceeds 0.5 m in true width. Gold occurs as irregular anhedral electrum grains that range up to 300 microns across and contain 38 to 59 wt% Au (ave. 51 wt% Au). Electrum occurs in sulfide-rich bands in association with pyrite, chalcopyrite, acanthite, galena, sphalerite, and local covellite. Pyrite from BM37 is nearly stoichiometric; it lacks the As that commonly occurs at up to percent levels in pyrite from other veins in the goldfield. The acanthite also contains low quantities of trace metals, and typically has less than 2.5% Se. The vein shows complex textures, including brecciation. Bonanza grade electrum in sulfide-rich bands occur as discrete layers adjacent to the vein margins, and as breccia fragments encased by later massive to comb quartz with local quartz pseudomorphed platy calcite. Both the breccia fragments and sulfide-rich bands are cross-cut by later quartz veins. Textures thus indicate that formation of the high-grade shoot of BM37 resulted from one or more discrete events that differed markedly from the events that formed the bulk of the vein.