Other Metallic Ores
Silver is alloyed with the gold of all fields, but is most common in Hauraki where the silver : gold ratio ranges from an average of 1 : 2 for the whole field, up to 8 : 1 or so in the south near Waihi. Most of the £4,500,000 worth of silver that has been exported has been obtained from gold mines. Silver mines as such (e.g., at Puhipuhi and Great Barrier in Northland, and Collingwood in Nelson) have not been economic. Small quantities of platinum and osmiridium have been recovered from the gold at Orepuki in Southland. Only small amounts of lead and zinc minerals have been found. Trial shipments have been sent overseas from Thames gold mines, and from prospecting in Toi Mine near Te Aroha. The minerals have been found in the western half of the South Island, but are nowhere known in economic quantities.
Like the gold from Hauraki, many New Zealand metallic ores are related to past volcanic activity. The andesitic volcanic rocks of Mt. Egmont, and of its eroded ancestors in the Pouakai Range to the north-west, are exposed along 70 miles of coastline around Cape Egmont. The grinding and sorting action of the sea has released and concentrated titanomagnetite, a resistant iron mineral, which occurs in small amounts in the parent andesite. The beaches and the coastal sand dunes from Kaipara Harbour to Wanganui are known to contain well over 500 million tons of magnetically separable titanomagnetite concentrate, analysing 56 per cent soluble iron and 0·36 per cent vandium oxide. In addition to the iron, 7 per cent titanium oxide is present, and this has so far thwarted attempts to produce iron and steel economically; but research continues. West Coast South Island blacksand deposits, formed by the erosion of granite, are rich in ilmenite with 40–45 per cent titanium oxide, and include a deposit with 5 million tons of concentrate near Westport. Limonite, a brown iron oxide which was indirectly formed by the weathering of basalt lava flows, has been quarried at Kamo, Okaihau, and Puhipuhi in Northland. It is used in improving cement, in gas purification, as a pigment, and, at one time, as a fertiliser, mostly because of its cobalt impurity. The limonite at Onekaka, in Nelson, of less certain origin, was worked to supply a small blast furnace. Some 37,560 tons of pig iron were produced in 1922–35, and 9½ million tons of ore, with 35–55 per cent iron, remain. Subtropical weathering of basalt flows in the Kaeo region of Northland has led to the formation of the aluminium ore, bauxite. Some 20 million tons have been proved so far, with 37 per cent aluminium and 25 per cent iron oxides.
Over £50,000 worth of cinnabar, the heavy red ore mineral of mercury, has been obtained sporadically since 1918 from the active and extinct hot spring areas at Ngawha and Puhipuhi respectively, both in Northland. Small amounts have also been worked at Thames and Paeroa, but no mercury reserves are known.
Some metal ores, such as the manganese and copper of Northland, are associated with volcanic lavas that were erupted into soft sediments beneath the sea. Almost all the 26,000 tons of manganese ore mined in New Zealand has been produced from volcanic horizons in the greywacke rocks that extend from the small, closed mines of the Hunua Ranges, south of Auckland, to the Bay of Islands. Past experience suggests that, although additional deposits will be found in the future, they are likely to be small and of doubtful economic value. Copper was found in the Kawau manganese mine about 1840, and thereafter this mine, like that at Great Barrier, produced well over 2,000 tons of ore. Copper, which is known from marine volcanics in several other parts of Northland, was mined at Pakotai in 1947–51 and is being prospected at Parakao nearby. Much copper prospecting and mining took place in the mineral belt of Nelson, extending southwards from D'Urville Island. As elsewhere in the world, chrome ore (chromite) is found in serpentine rocks, and £38,000 worth was mined in Nelson's mineral belt in 1858–66 and 1900–02.
Arsenic worth nearly £2,000 has been obtained as a by-product of Reefton's gold mines. Antimony, which is widely used in alloys and drugs, was also found there, but was more extensively mined from the schist rocks of the South Island. The largest mine was in the Marlborough Sounds, and smaller mines were at Alexandra, Waipori, and Mt. Stoker in Otago. The schist rocks also yield scheelite, a white heavy ore mineral of tungsten. Farmer-miners still work the once important Glenorchy field. Mining there, and at Macraes Flat, together with smaller operations at Wakamarina in Nelson, and Hyde, Waipori, Stoneburn and elsewhere in Otago, has produced exports to the value of £875,000. Tin and another tungsten mineral, wolfram, have been recorded together from the area of the granitic rocks in Stewart Island, an association found elsewhere in the world. Only about a ton of tin ore (cassiterite) has been recovered from auriferous gravels there, and considerable prospecting has located no major field.
The world-wide search for uranium after the Second World War spread to New Zealand. The first discovery was made in the Hawks Crag breccia in the lower Buller Gorge in 1955, followed later by additional records in that formation in the Paparoa Range south of the Buller. Unfortunately, bulk samples have nowhere shown the presence of a workable deposit.