Broken Hills Mine, Hikuai, Thames-Coromandel District, Waikato Region, New Zealandi

Regional Level Types
Broken Hills Mine	Mine
Hikuai	- not defined -
Thames-Coromandel District	District
Waikato Region	Region
New Zealand	Country

Rhyolite-hosted epithermal Au-Ag deposit. Active in the early 1900s. Being reworked in 2014 as a small operation targeting high grade veins about 10cm wide, in narrow adits.

Rhyolite-​hosted epithermal adularia-​sericite Au-​Ag deposit.

Stuart Rabone, a geologist often appears in references for this region. For around thirteen years until 2012, the Rabone family headed by Stuart conducted limited mining within the historic Broken Hills Mine using hand methods, and employing up to seven people. The money beyond this was used to restore the historic workings, and tours were conducted for the public of the underground workings. This unique experience was brought to an end in 2012 by government red tape, when the Department of Labour insisted on safety measures for the mine in line with industrial scale mining. We hope the information loaded at the beginning of this article is correct, but no information was found of activity beyond 2012.

The Broken Hills gold mining area can be accessed from the north via the Puketui Valley Road to the campground, or the south via the Puketui Road. Neither meets. Access to walking tracks from the southern road require fording the third branch of the Tairua River, although is possibly quieter during peak periods. The area contains a number of walking tracks to the remains of the Broken Hills battery, Golden Hills battery, Government battery, Puketui settlement site, water races, old tram lines, and various tunnels which can be walked through. The Collins Tunnel is a 15 minute walk through, and requires a torch.

Gold was discovered in the area in 1893. The London based Broken Hills Gold Mining Company Limited was formed, who constructed a 20 stamp battery, tramway, water races etc. After two years, a local syndicate took over in 1899, named the Tairua Broken Hills Gold Mining Company Limited, and re-opened the mine in 1903. The entire area produced 55,199 ounces of gold from 39 385 tonnes of ore (Moore, 1979), the majority from the Broken Hills Mine ranging from 30,000 to 51,000 ounces depending on the sources.

The country rock at the mine site is flow banded rhyolite overlain with silicified rhyolite pyroclastics, consisting of fine to coarse pyroclastic flow lapilli sheets, airfall ash, and lapilli. The pyroclastics contain abundant poorly welded pumice fragments, and rhyolite fragments, with phenocrysts of quartz, albite, andesine, and rare K feldspar. Massive porcellaineous and opaline siliceous sinter deposits overlie all this in parts of the area, representing former hot springs and geysers. The pyroclastics frequently contain silicified or more commonly carbonaceous, with colloform pyrite and/or marcasite twigs and branch fragments.

Mineralisation is described from a large breccia pipe, Bluchers Reef, and Sievers Leader. Historic mining was confined to narrow fissure veins, richest where the shoots steeply pitched as the reefs ran obliquely through the ore zone. This is a fault bounded breccia zone, with the reefs outside the zone far lower in Au-Ag values. The mine was accessed by four adits.

Argillic alteration as poorly crystallised illite, illite-montmorillonite, amorphous clay, and rare kaolinite occurs near the mineralised veins. Adularia is also found in the wall rocks near the veins. Anatase is common in the wall rocks associated with adularia, and anatase is also found associated with electrum, and silver sulphoselenides in the veins.

Supergene minerals include acanthite, covellite, chalcocite, Au rich electrum, barite, various iron oxyhydroxide minerals, and replacement of pyrite and marcasite with goethite.

The large breccia pipe contains chert and clay, capped on the surface in the past by a rich gossan. Chert is found as banded pods or nodules associated with complex and irregular chert veining, and silification, enclosed by illite. Bluchers Reef is a stock work in pyroclastics, but as it traverses rhyolite forms a distinct one metre zone. Sievers Leader is 2-5 cms wide.

Bluchers Reef contains a 10 cm wide banded chert zone on the Hanging Wall containing electrum and native silver; an auriferous central breccia zone with chert cementing breccia clasts; and a barren massive botryoidal iron oxide zone on the Footwall, associated with kaolinite, within this a 10 cm thick chert vein.

Pyrite is the most common sulphide found as aggregates, irregular discontinuous veins, individual crystals, framboidal, and rare skeletal needles, and pseudomorphed after marcasite. Pyritohedral is the most common habit, and rare small cubes. The pyritohedral pyrite is zoned with a greyish pink core with thick yellow cream rim; or as intricate quartz-pyrite zoning patterns with a thin lemon yellow outer rim; or when associated with electrum and silver sulphoselenides as creamy pink with small rounded blebs of electrum and aguilarite. The zonings may not be apparent in hand specimens.

Marcasite as radiating clusters, framboidal, and xenomorphic grains associated with pyrite, or disseminated in quartz. Some have an iron sulphide core, with crystallised marcasite rims.

Electrum is found in relatively small amounts in the deposit, as straw yellow, whitish yellow, or yellow cream depending on the silver content, as isolated blebs in quartz, carbonaceous material, and clay. In the breccia pipe it is intergrown with silver arsenic sulphoselenides, aguilarite, or pyrite. Native silver is found as blebs in quartz, or thin rinds associated with electrum, pyrite, and aguilarite in the breccia pipe.

Barite is uncommon in the area, largely restricted to the mine, as colourless in the No. 2 level, and black with pyrite dust in the battery level. The iron oxyhydroxides form large masses, frequently associated with kaolinite, with lepidocrocite confined to the rims.

The rare species trechmannite occurs as black disseminations in altered tuff, and occurs with electrum/silver sulphoselenides in clay veins in the breccia pipe. Specialist equipment is needed apparently to distinguish it.

Acanthite rims the silver minerals, and occurs as thin cross-cutting veins in silver arsenic sulphoselenides. Aguilarite is a relatively rare mineral, but fairly common in Au-Ag deposits in the Coromandel region. It is the most abundant silver mineral in the breccia pipe, intergrown with electrum, and pyrite.

The deposit contains a complex association of Au-Ag sulphoselenides and electrum as disseminated in clay veins, associated with trechmannite and minor barite, with some Au-Ag found in simply pyrite veins. A number of rare minerals are found in the deposit reflecting its Se rich status. These are often not apparent in hand specimens, and require specialist equipment to detect. These include uytenbogaardite, fischesserite, petrovskaite, naumannite, an un-named silver chloride, and un-named Ag-Au minerals.

Other Regions, Features and Areas containing this locality

References