Mineral Wealth of New Zealand.

New Zealand Graphic, Volume XL, Issue 6, 8 February 1908, Page 45

Mineral Wealth of New Zealand.

Paper by

Dr. Bell.

A valuable paper was recently read before the Royal Colonial Institute, London, on “ The Mineral Wealth of New Zealand,” by Dr. Janies Mackintosh Bell, M.A., Director of the Geological Survey of New Zealand, who was visiting the Old Country on holiday leave. The Right Hon. the Earl of Ranfurly, ex-Governor of New Zealand, presided. Dr. Bell read his paper as follows: — The three principal islands of the Dominion of New Zealand, known respectively as the North, South, and Stewart Islands, are situated in the South Pacific Ocean, between latitudes 34 deg. and 48 deg. Considering their relatively small area, they present an unusual variety of geological phenomena. In the North Island is a district exhibiting over a large area thermal springs, geysers, fumaroles, solfataras, and other evidences of expiring vulcanism. In the South Island is the snow-clad chain of the Southern Alps, containing extensive snowfields, from which flow glaciers in many cases rivalling and even surpassing in size those of the better-known glacial centres of the Swiss Alps and the Caucasus. In addition to these, and many other features of chiefly scientific interest, the Colony contains mineral wealth widely distributed and of varied mineralogical character. It is the purpose of this paper to give a brief summary of the mineral deposits, their distribution and the broader features of their occurrence, without attempting to consider the details of their geological occurrence. As no doubt most of you know, the North Island is divided into the provincial districts of Auckland, Taranaki, Hawke’s Bay, and Wellington, and the South Island into the provincial districts of Nelson, Marlborough, Canterbury, Westland, Otago, and Southland. DESCRIPTION OF THE CHIEF PHYSIOGRAPHIC FEATURES OF NEW ZEALAND. Much of the North Island is of a broken and rugged description. It is traversed from north-east to south-west by several parallel ranges of mountains of inconsiderable altitude. There are, however, several areas of flat land, the most extensive being the coastal plain east and south of the splendid volcanic cone of Mount Egmont, near the west coast. At the north-eastern edge of this plain are the lofty volcanic cones of Ruapehu, Tongariro, and Ngauruhoe, the latter still active. North of these peaks is the broad volcanic zone of Taupo, exhibiting the varied thermal phenomena already mentioned. At the northern part of the North Island are two peninsulas of broken country —the western and much the mor: prominent being the North Auckland Peninsula, and the eastern being the Hauraki Peninsula. By far the greater part of the South Island is mountainous. The central axis is composed of the Southern Alps, with their continuation, the Spencer and St. Arnaud Mountains. From this principal axis radiate subsidiary ranges. In addition, there are several high parallel ranges. The Canterbury Plains, which border the coast on the east, form by far the greatest extension of flat land in the South Island, but there are minor stretches of level country at low altitudes in the west, in the north, and in the extreme south of the island. Stewart Island, much smaller than either the North or South Island, exhibits an area of broken, rugged hills, which are in general densely wooded. RESUME OF NEW ZEALAND STRATIGRAPHY. The oldest rocks in the North Island are stratified Paleozoic or very early Mesozoic sediments, which compose the main massif of the mountain chains. Overlying these in various localities are late Mesozoic and Tertiary sediments. Much of the northern and central part of the island is composed of volcanic rocks—both tufa and lava of Tertiary and Pleistocene age. Apparently, the oldest of these volcanics are intermediate in basicity, while the next in age are decidedly acid, and the most recent, basic.

The south Island shows a more complete geological section than the North Island, and a wider petrographical range of igneous rocks. The heart of the Southern Alps, as well as that of some of the parallel ranges, shows in places, ancient crystalline schists flanked with Paleozoic and Mesozoic strata belonging to various periods. Much of the rock underlying the Canterbury Plains and the flat land on the west eoast of the island, known as the Westland coastal plain, is composed of Tertiary strata, which is also prominent in the southern part of the island and in the north. A considerable portion of the lower country in the South Island is covered by comparatively recent gravels of varied origin—fluviatile, marine, and glacial. Relatively only a small part of the South Island is composed of igneous rocks, though they present a marvellous variety of petrographical forms. Bank’s Peninsula consists entirely of rocks of this origin, and they are also prominent near the city of Dunedin, in the granite buttress of the south-western corner of the island, and in the ridges of the same rocks stretching northward from this buttress parallel with the coast. The socalled mineral belt of the districts of Nelson and Westland is composed in the main of a series of parallel and disjointed sheets of dunite and other magnesian rocks, which have a very small lateral extension, though in longitudinal direction they appear at intervals for nearly 200 miles. Stewart Island is composed almost entirely of granites and allied plutonics. SALIENT FEATURES OF THE ECONOMIC GEOLOGY. GOAL. Distribution of Coal.—Perhaps in no country of the world is coal more generally’ distributed than it is in New Zealand, as it occurs in almost every part of the Dominion —a fact which makes up for the narrowness of the coal seams and the inextension of the basins in which they lie, as compared with coal deposits in other countries. The coal varies considerably in quality, both in regard to the amount of ash and the state of carbonation. Unlike the European coal and most of that of the United States and Canada, the New Zealand coal is not of carboniferous age, but of much lesser antiquity. The coals of Kaitangata, Shag Point, and Nightcaps, which form fairly wide seams in Tertiary rocks in the southeastern part of the South Island, are lignites of good quality. The seams which are so widely distributed in the hilly country west of the Canterbury Plains are all lignites, but not generally so highly carbonised as are the southern coals. The coals of Greymouth and Brunnerton, in the Westland district, and of Westport and Puponga, in the Nelson district, are bituminous coals of varying degrees of purity. The coals of the North Island, which are chiefly in the northern part, near Auckland and Whangarei, are in general intermediate in state of carbonation between the bituminous coals and the lignites. No extensive seams of true anthracite have as yet been found in New Zealand, thourh there is a verv small deposit in earlv Tertiary sediments at Cabbage Bay, in the Hauraki Peninsula, which owes its high percentage of fixed carbon to the metamorphism produced by the extrusion of flows of andesite. Some of the lignite seams of Canterbury also are said to be in part altered to anthracite, as a product of contact metamorphism. PETROLEUM. In many parts of the Dominion there is abundant evidence of petroleum, though in no locality can it be said to have been proven to exist in great quantity. It has been found and exploited to some slight degree at Kotuku, near Greymouth, in Westland, near Gisborne, in the Auckland District, and at New Plymouth in Taranaki, not far from the base of the lofty cone of Mount F.gmont. In the last-mentioned locality,

the oil seeps out of the surface along the sea-shore, and at a few places in the interior, and gas issues at many points along natural crevices. Boring operations have been conducted in this locality for a number of years, and very recently have apparently met with success. An oil-bearing stratum has been struck at a depth of' 2,400 feet, which at present is said by the proprietors to give a steady flow under high pressure. The oil-bearing stratum consists of a loosely consolidated marine sand underlying hard argillaceous sandstone, above which marine sands and' clays, more or less hardened, extend to the surface. As far as is known all the beds are disposed in almost horizontal attitude, the lowest being probably of Miocene age, the upper apparently Pliocene. In the Gisborne district apparently the petroleum is even more widespread, and the prospects equally bright for the discovery of large wells. However, comparatively little exploration has been done within recent years in this locality. IRON ORE. There are in New Zealand two very interesting deposits of iron ore. The most important of these appears at Parapara, in the rugged Cape Farewell Peninsula lying west of Golden Bay, in the South Island, while the other lies on the shores of the Tasman Sea, near New Plymouth. Geological Character. —The rocks in which the Parapara ore occurs are metamorphic, and consist of much corrugated hornblendie and micaceous schists, sideritie limestones, and cherty quartzites. These have been tentatively classified as Silurian by Professor Herbert Cox, formerly a geologist on the staff of th? New Zealand Geological Survey. Unconformably above this series of Paleozoic rocks lies a considerable thickness of coal-b firing, early-Tertiary strata, which consists of limestones, sandstones, grits, and coal seams. Forming a mantle over mueii of the district are deposits of fluviatile and marine gravels. These usually contain more or less gold, and have afforded somj rich bonanzas in the past. Structurally the iron ore appears to occur on the eroded crest of an anticline., composed of sideritic crystalline limestone, which is often rusty, due to the oxidation of iron carbonate contained. The width of the actual ore mass visible on the surface varies greatly, but is in places quite SOO feet. Broken by s veral gullies the ore body outcrops in a northwesterly and south-westerly direction at intervals for over six miles. Mr. George J. Binns, formerly Inspector of Mines for the New Zealand Government, estimates that in the large ore body, near Parapara Inlet, there are not less than 51,835,600 long tons of iron ore in actual ou’erop, which could be removed in an open quarry. The ore is in general a high-grade hydrous haematite botryoidal mammillary, and often porous in character. Apparently manganese is a constant constituent. Phorphorus, sulphur, and titanium are in general low. though there are small quantities of other impurities, principally alumina and silica. In places the ore contains rounded quartz pebbles, often in sufficient abundance to constitute a highly ferruginous quartz conglomerate. This eon glomerate is apparently the result of fluviatile action, and occurs only where streams now or formerly existed. It represents soft ore, which has been broken up by the streams and mixed with quartz pebbles derived from the overlying Tertiary strata to be subsequently reeemei'ted into the conglomerate now observable in the outcrops. The conglomeratic portion of the ore is auriferous. the gold apparently bung derived from the same, source as that in the quartz pebbles. Origin. Tire origin of this extensive deposit of iron ore is of considerable interest. Briefly stated. the ore has resulted from the concentration of iron oxide by meteoric waters. The waters hail their burden of iron derived e’ther directly or indirectly from the oxidation, partly of iron carbonate in the crystalline limestone, but more especially of the sulphides pyrite and marcasite present in both the quartz conglomerate of the overb’ing Tertiary strata and in the quartzites and mica schists associated with the crystalline limestone. New Plymouth deposits. The deposits of iron ore occurring near New Plymouth in Che North Island, are of a very different character. They consist of magnetic iron sands, all more or loss titaniferous. which owe their concentration in thick beds along the sea shore and

inland for a mile or more to the action of the waves or wind. Beds of pure black sand of very line grain are iuterst rat died with beds of felspathic quartz sand, containing considerable t Uriniferous magnetite, and with others practically free from iron. Sometimes the demarcation between the various beds is very abrupt and decided, again much more gradual. Considered longitudinally the beds of pure black sand become lenticular, and are replaced by others in which the iron constituent is lacking or unimportant, the latter beds being in their turn replaced by pure black sand. The black sand is supposed to consist of an intimate mixture of magnetic and ilmenite. It occurs very widely distributed, and, there can be no doubt, in very large quantities. It appears at very frequent intervals along the coast on either side of New Plymouth, from the mouth of the Waitara River as far south as the mouth of the Patea. The black sand has apparently been derived in part from the disintegration of hornblende andesites and more basic volcanics—all rich in titaniferous magnetite, and in part from tufa, representing the comminuted equivalent of these volcanics. Volcanics and tufas carrying titaniferous magnetite have a wide distribution near New Plymouth, around the volcanic cone of Egmont. COPPER. Ores of copper are found in New Zealand at many localities, both in the North and South Islands. There has recently been discovered a. mineralised horizon of sulphides near the Whangaroa harbour, in the North Auckland peninsula. The country is but little explored, and the exact nature of the deposits has not yet been investigated, but apparently the ore consists mainly of sulphide, with which is associated native copper and other oxidation products. One of the best-known deposits of ores of copper occurs in a heavily mineralised zone stretching from D’Urville Island southward to the Aniseed River, in the neighbourhood of the town of Nelson in the South Island. The ore bodies lie in small, disconnected, and generally parallel lenses, disposed along the planes of stratification of the enclosing argillites, and occur close to the contact between the argillites and extensive sills or bosses of dunite. The most common ore is a cupriferous pyrite, in which the copper content is generally low. On the surface the oxidation products malachite, azurite, chrysocolla, cuprite, and native copper are occasionally encountered, and in one case (the Champion Mino) are important, giving very rich, though small, bodies above the water level. Enriched sulphides are to be seen in some of the lower work-

ings of the United Mine above the waterlevel, though the ore is more or less oxidised on the surface. The Mount Radiant copper field has been recdhtly discovered, and promises to be the most important, though as yet it has not been developed. The field lies close to the mouth of the Little Wanganui River, which enters Karamea Bight on the north western side of the North Island. The veins occur in granite, and bear in many ways a striking resemblance to those of Butte, Montana. The gangue material consists chiefly or quartz, but there is always more or less feldspar present, and it becomes the predominating mineral in places. The principal copper-bearing mineral is chalcopyrite, with which are associated iron pyrites, molybdenite, and a little arsenopyrite and bornite. Molybdenite is in some of the veins as common as chalcopyrite, and it may be mentioned that where this is the case, a great increase in feldspar is to be noticed. Some of the veins carry fail- values in silver. The veins are in places definite and persistent, and in other instances they are uncertain and variable both in length and breadth. The Maharahara copper deposit is situated in Paleozoic strata, argillites and grauwackes, near the town of Woodville in the centre of the North Island. The vein shows some rich ore —mainly chalcopyrite—but as far as at present known it does not occur in great quantity. GOLD. In the past most of the gold of New Zealand was derived from placer deposits in various parts of the South Island, but more especially in the districts of Westland and Otago. The bonanzas in general occurred where morainic gravels had been reassorted by fluviatile or marine action. The amount of gold derived from this source is now much reduced, and the supply of the precious metal in New Zealand is to a greater extent obtained from quartz reefs. All through the ancient crystalline schists and Paleozoic argillites and grauwackes, which form such an important part of the strata of the South Island, quartz veins are common. Generally these are disposed parallel to the stratification, but are sometimes in fissures transverse to the bedding planes. Although quartz veins of this class are almost innumerable, many of them contain g».M only in traces, and comparatively few yield it in payable quantities. The veins are in rocks, which exhibit physiographically mature and Bub-mature forms, and in consequence are in general merely the remnant of former veins; the upper, and probably richer, part having been removed in the extensive and varied denudation —sub-aerial and glacial—which the country has undergone. At present much the most important centre of quartz mining in the South Island is at Reefton, where a number of mines are now working—the Globe, the Progress, the Wealth of Nations, the Keep-it-Dark, etc. The ore filling the veins consists chiefly of quartz. This contains a great deal of pyrites, rarely chalcopyrite, and arsenopyrite, and often stibnite. In all of the mines the values are very irregularly distributed in the quartz gangue, even near the surface, where the yield is usually higher than from the lower levels. Often not onlymay the ore shoots terminate within the quartz mass in the direction both of the strike and dip of the vein, but the quartz gangue itself frequently disappears and is replaced by comminuted gouge, to reappear either barren or with shoots of rich ore in the continuation of the line of reef. This occurrence of the quartz in patches is one of the most characteristic features of the veins at Reefton. The highest values are almost always along slaty selvages, which are frequently rusty, and where the quartz is of the character known as “ magpie stone ” by- the miners. ‘‘ Magpie stone ” is simply quartz enclosing numerous small angular fragments of argillite or grauwacke, which are the prevailing country rocks of the district. There is a small reefing area near the head of the Wilberforce and Arahura rivers, known as the Westland Reefs district, in which a number of promising auriferous quartz veins have recently been discovered, but which has been as yet very little prospected. Quartz raining is carried on at Tnitapu, in the Cape Farewell Peninsula, in reefs very much like those at Reefton. Quartz veins are also being worked at Skippers, Arrowtown, Barewood, and various other places

in Central Otago, though none is of important proportions. The occurrence of the mineral scheelite in many of the quartz veins in Otago is interesting. The quartz with u'liidh'it is associated is nearly always somewhat auriferous, and in places payably so. The scheelite sometimes appears in a decided band running through the quartz, or again intimately intermixed, with the appearance of a pegmatite. At Barewood, Glenorclty, and Macrae’s, near Palmerston South, the mineral is found in sufficient quantity to pay for working. Much more prominent than any of the centres of quartz mining in the South Island, and of widely different character, are the goldfields of the Hauraki Peninsula of the North Island, which contain the mining centres of Coromandel, Thames, Karangahake, and Waihi, in addition to many less prominent mining localities. The output of gold from Coromandel is now comparatively sma.t. At Thames the Waiotahi Mine is at present yielding bullion to the value of £IB,OOO to £20,000 per month, but there is no other mine of importance. The site of the Waihi Mine, one of the greatest gold mines of the world, is a centre of immense economic importance, as well as of scientific interest. At Karangahake is situated the Talisman Mine, which is now the third gold producer in the Dominion. The oldest rocks of the Hauraki goldfield apparently consist of unfossiliferous argillites and grauwackes, which are probably very late Paleozoic or early Mesozoic. Unconformably- above these are late cretaceous rocks, containing unimportant coal seams, which occur as widely separated, isolated remnants in the northern part of the Hauraki Peninsula. On the denuded surface of these sedimentaries have been disposed thick and widely distributed volcanic deposits, flows, breccias and tufts of andesitic character, above which are others of rhyolitic character. By far the most important veins of the Hauraki Peninsula are in the andesitic flows or in the fine-grained andesitic tuffs. Veins occurring in the coarse andesitic agglomerates and tuffs rarely carry much gold, while those appearing in the rhyolites are decidedly unimportant as compared with those in the andesites. A few 1 relatively unimportant veins are found in the older sedimentary rocks. The andesites are often tremendously altered, especially near the veins. In fact, the rock in places has been changed to a mass of chlorite, epidote, sericite, calcite, quartz and pyrite, giving the so-called propylite. There were evidently at least two periods of vein disposition, much the more important apparently being the result of the andecite extrusion, while the second period of vein formation succeeded the outpouring of the rhyolites. Sinter deposits widely distributed throughout the Hauraki Peninsula testify to the enormous extent of hydrothermal activity in the past, and a few scattered hot springs show that it has not yet ceased. The veins are apparently mainly deposits by hot siliceous solutions, carrying a great deal of hydrogen sulphide in pre-existing fissures, greatly enlarged by replacement of the wall rock. Of the payable veins of the Hauraki goldfields there are two more or less distinct types—those which occur in decided, and definite veins in which the payable ore is contained chiefly in a well-demar-cated ore shoot with considerable horizontal continuation and with longitudinal extension from level to level, and those in which not only is the quartz of very irregular width, but the values are almost entirely in small disjointed patches, often of the enormous richness of “ jewellery shop” ores.’Of the former class much the most conspicuous example is given by the more or less connected reefs worked in the Waihi Mine. at Waihi, htou'git the Uiiion, Amaranth and Silverton veins, at the same place, and the Talisman vein at Karangahake are. of the same class. Of the “ jewellery shop” class of veins, the reef systems of the Hauraki, Kapanga, Royal Oak and Tokatea mines at Coromandel, and the reef systems of the Kurunui, Caledonian, Moanataiari and Waoitahi mines at Thames are good examples. Of these the. only one now giving heavy returns is the Waotahi. A bonanza of great richness was discovered recently in connection with this mine, which lias in consequence given a heavy output for the past year. The bonanzas of fine gold in the veins of the Waiotahi type are apparently always at points where the vein is heavily

mineralised with pyrite, with which is sometimes associated sphalerite, stibnite and chalcopyrite, and occasionally native arsenic. The bonanzas frequently have an intimate connection with the faulting of the vein. The faults, formed subsequent to the original period of vein deposition, may have determined the position of the bonanzas formed during a period of secondary enrichment of the vein, either by acting as barriers to migrating auriferous solutions, or by forming channels along which solutions came, reacting with those in the original vein channel. The great r <vaihi Mine, which is now working on the complex system of branching reefs —the Martha, ' Welcome, Empire, Royal and several smaller veins —had an output during the year ended December 31, 1905, of £712,000 sterling. This year its output will be even greater. The four weeks ending June 10, 1900, gave the record return of £02,470 sterling. Those reefs of the Waihi Mine system which approach the surface show an oxidised capping of moderate though not of bonanza richness, which gradually is replaced in depth by sulphide ores. The gold is very rarely visible to the unaided eye, though a considerable portion exists as free gold. One of the most characteristic features of the reefs of the Waihi Mine system is the continuity of the “ pay ore,” both in horizontal and vertical extension. If there was any enrichment of these veins subsequent to the original deposition of the quartz, which seems very likely, it was probably by secondary solutions ascending along the reef channels, excepting near the surface, where descending solutions gave a limited concentration. Some of the veins of the Waihi reef system are of immense size—the (Martha reaches a width of sixty feet on the surface, and at the levels at present being opened at about 800 ft. beneath the surface, it is, where widest, over twice that width". It is interesting to note that in general veins of the Hauraki Peninsula show a diminution in the amount of gold in the electrum in passing from tSie northern to the southern portion of the field. Very interesting evidence on the origin of gold and silver in quartz veins is given by certain hot springs in the centre of existing hydrothermal activity in New Zealand—the Taupo volcanic zone. Sinter'taken by the writer from the rim of a very ebullient spring at the Maori settlement of Whakarewarewa was found to contain on analysis, silver to the amount of 4oz. 18gr. per ton, and gold to the amount of Idwt. 4grs. per ton. The sinter analysed was stained with sulphur, but showed no visible evidence of any sulphides. Analysis made from the sinter deposited in a wooden trough, used to conduct water from the same spring at Whakarewarewa, gave the following result in the precious metals:—■ Gold, 12grs. per ton; silver, 15dwt. 3grs. per ton. The great geyser of Waimangu, which broke into action some years after the terrible Tarawera eruption of 1886, and remained active until November, 1904, deposited a blackish material, consisting chiefly of sulphides, but containing neither gold nor silver. Some mud obtained by Dr. Wohlmann, the Government balneologist, from a hot spring in the sanatorium grounds at Rotorua, gave the following somewhat remarkable analysis:— Silica .69.30 Alumina 4.52 Iron Oxides 2.00 Titanium Oxide 0.58 Lime 1.00 Magnesia ......,,,. • 0.10 Soda and Potash 1.30 Sulphur combined 1.40 Sulphur free 6.09 Organic matter ... t 10.01 Water 3.70 100.00 Microscopic examinations of the deposit showed that it consisted mainly of quartz and amorphous silica with a little felspar. The mud also contained 5 grains of gold and 6 dwt. 1 gr. of silver per ton. It is evidently not a deposit from the spring, but is merely a siliceous tufa impregnated .by the thermal solutions. PLATINUM. Platinum has been found at many points in the. South Island in the alluvial drifts, but never in sufficient quantities to be of any economic value, excepting where found with gold. Platinum has

been reported from the Queen of Beauty i quartz reef at Thames, and also from an occurrences of massive pyrites at Coromandel. During the past season a number of platiniferous quartz veins were discovered by the writer near the Teremakau River, in the district of Westland, in the South Island, in close proximity to sheets of altered magnesian eruptives — apparently originally dunite and situated parallel to the stratification of the enclosing phyllites. The quartz is somewhat vitreous, and in general very “hungry” in appearance. Iron pyrite is fairly common, and iron oxides derived from its alteration. In three analyses made of the platiniferous quartz, the platinum was found to occur associated with silver, and always in the approximate ration of seven parts of silver to one of platinum. The following is a characteristic result, showing the amount of platinum and silver: — Platinum 3cwt. 8 grs. per ton. Silver . .. ... loz. 4dwt. 9grs. per ton. It seems possible that the platinum and silver may exist in some mineralogical combination, though no data were obtained to substantiate this hypothesis. An interesting product of metamorphism of the magnesian eruptives, near which the platinum veins just described occur, is the mineral nephrite—the muchvalued Pounamu of the Maoris and the precious “greenstone” of commerce. This mineral, which is apparently a massive actinolite or allied amphibole, occurs as segregations from a few inches to several feet in width in a talcose matrix, also a product of alteration of the magnesian eruptives. Many analyses made of the magnesian eruptives failed to show any platinum actually existing in them. TIN. Cassiterite has been found in the form of “stream tin” in some of the streams amid the rugged hills of Stewart Island, and has been reported to occur actually; “in situ” in granite. Ores of antimony, lead, zinc, mercury, and' several other metals have been discovered in New Zealand, but scarcely in sufficient quantity to deserve special mention in this paper. Much of the wild, mountainous country of the South Island and of the thickly wooded central and northern parts of the North Island are still unexplored, and vast mineral wealth may yet be discovered in these areas of terra incognita.