BM37
is a bonanza-grade (>30 ppm Au) ore shoot of the Maria Lode of the
Karangahake adularia-sericite epithermal deposit in the southern Hauraki
Goldfield. 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 wtper
centAu (ave. 52 wt per centAu). Electrum occurs in sulphide-rich
bands in association with pyrite, chalcopyrite, acanthite, galena, sphalerite,
and local covellite. Lower grade samples contain local to rare mckinstryite
[(Ag, Cu) 2S] and polybasite [(Ag, Cu) 16 (As, Sb) 2S11]. 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. Acanthite also contains low
quantities of trace metals, and typically has less than 2.5 per centSe.
The vein shows complex textures. Bonanza grade sulphide-rich ginguro bands form
discrete layers adjacent to the vein margins, and occur as breccia fragments
encased by later massive to comb quartz with local quartz after platy calcite.
Both the breccia fragments and ginguro bands are cross-cut by later quartz
veins. Textures thus indicate that formation of the high-grade portions of BM37
resulted from one or more discrete events that differed markedly from the events
that formed the bulk of the vein.