Geology and mineralisation of the former Broken Hills gold mine, Hikuai, Coromandel, New Zealand

Abstract

In the vicinity of the Broken Hills mine, flow-banded rhyolites are overlain by silicified rhyolite pyroclastics composed of fine to coarse pyroclastic-flow lapilli sheets and airfall ash and lapilli. Within the Broken Hills mine, epithermal subvolcanic gold-silver-selenium-arsenic mineralisation occurs in both fissure veins and in several breccia pipes. The grain size of the mineralisation is typically 50/Lm or less. Some of the gold and silver is inferred to occur either as a solid solution with pyrite, or as submicroscopic inclusions of electrum within pyrite. Microprobe examination of the electrum shows that it is strongly zoned, with silver ranging from 25-64 weight percent, with the highest silver content observed on the outer margins of the e1ectrum grains. Four previously undescribed gold-silver arsenic sulpho-selenides, and trechmannite (AgAsS,j, previously known only from Binnetal, Switzerland are described. Members of the acanthite-aguilarite-naumannite series are also present in association with the above minerals in the large breccia pipe and in the fissure veins in association with electrum. A depth of approximately 200 m is inferred for deposition within the veins, and for the breccia pipe, 60 m. The deposition of gold in the fissure veins and the breccia pipe involves the deposition of electrum in response to changes in the oxygen fugacity, and boiling of the ascending hydrothermal solutions.