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Gold Mines.
Gold was discovered within less than three years of the foundation of the colony, in 1840, but it was not practically worked until 1852, when the mines at Coromandel first attracted attention to the same district that at the present time forms the chief seat of mining operations in New Zealand; but the yield from those mines has, up to the present time been small, when compared with the quantity of alluvial gold obtained in the South Island, subsequent to 1861, at which date the gold fields of Otago became prominently known. I have already alluded to the peculiarities of the manner in which gold is distributed in New Zealand, and taking these as a guide we naturally divide the gold fields into the following districts for convenience of description:—
- 1.
The Northen gold field.
- 2.
Nelson and Westland, or Western gold field.
- 3.
Otago, or Southern gold field.
The Northern gold field is situated chiefly on the west side of the Cape Colville peninsula, but gold has also been obtained in small quantities on the eastern side. The peninsula is a bold mountain ridge, having a mean altitude of 2000 feet above the sea level, composed of slate rocks interbedded with the eruptive rocks belonging to the Diorite series of the lower Secondary period. These rocks form at least three well-defined belts crossing the peninsula obliquely in a north-east direction, the depressions which separate them being occupied by tufaceous deposits of volcanic ash, intermixed with ejected fragments of rocks of various ages. These Tufas which envelope the older rocks to an altitude of at least 1400 feet above the sea level, were deposited in the Tertiary epoch, and form part of a formation that occupies a very extensive area in the Auckland province.
The district as yet tested by the miners is a very insignificant portion of the whole area of the country, and the auriferous reefs which have been discovered, are confined to a few isolated localities, in which the tufa presents a very marked difference from its prevailing character, being a fine-grained rock containing a large quantity of auriferous pyrites (Bisulphide of Iron) dispersed through its substance in minute grains and imperfect crystals. This rock, which from its proneness to chemical change decomposes freely, and frequently contains Gypsum (Sulphate of Lime), is the characteristic bed-rock of the auriferous lodes at the Thames and Coromandel. In all the auriferous areas Diorite dykes intersect the tufaceous rock, and it is not yet satisfactorily determined whether the auriferous tufas should be considered to form part of the Tertiary volcanic series, or a distinct formation intermediate in age between them and the slate rocks of the district. The auriferous reefs are rarely well defined, and consist chiefly of sub-crystalline quartz in which the gold is found distributed through the compact portions as minute specks, or entangled in a crystalline or dendritic form, where the quartz is open in its texture. On the whole, the reefs appear to be richest in the decomposed rock near the outcrop, which has frequently led to over-sanguine anticipations regarding some of the
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mines; but it is well established that the quartz continues to be auriferous after it is traced into the harder undecomposed rock.
The gold contains the proportion of silver which is characteristic of the compound of gold and silver known as Electrum, and is associated with native Arsenic, Sulphides of Copper, Lead, Zinc, Antimony, and other metals, but only in small quantities.
At Coromandel, where mines were first worked in this formation, the lodes have been “proved” to a depth of over 300 feet from the surface, but the best mines at the Thames have, as yet, principally been confined to the decomposed, and comparatively superficial rock. Veins have been discovered and gold obtained at all levels on the ranges, from the sea level to an altitude of 2000 feet. The quantity of gold that has been obtained from some of these quartz reefs is very great, and for considerable distances the quartz has yielded, pretty uniformly, at the rate of 600 oz. per ton. Such reefs are, however, very exceptional. The quartz reefs are not altogether confined to the fine-grained tufa rock, but when in other formations they are not so auriferous. Those in the underlying slate rocks however contain decidedly more gold than the reefs in the overlying Tertiary tufa, which seldom yield appreciable quantities.*
The amount obtained up to the present time (Dec. 1869) is 213, 489 oz., only a few hundred ounces of which have probably been obtained as alluvial gold, the remainder having been extracted from the rock matrix by true mining processes.
The development of this mining district must be looked on as hardly commenced, though unless new districts be discovered, the large yield in proportion to the labour employed, which has hitherto characterised the early days of the mines, cannot be expected to continue.
Before proceeding to describe the second group of gold fields I may mention shortly the character of the rock formation near Wellington, which is the only other locality in the North Island where gold has been obtained in appreciable quantities, though not as yet sufficient to entitle it to rank as a gold field.
The locality where the gold has been obtained is about twelve miles west of the City of Wellington, near Cape Terawiti, where the country is composed of abrupt ridges of Primary slates, shales, and sandstones; with intervening valleys, some of which are occupied by marine tertiary strata. The older rocks belong to two distinct periods, and though both are found generally in a vertical position, the line of junction between them is well marked. The district has been traversed by a series of dislocations, which cross the lines of stratification in oblique lines. The consequent displacement appears to have indurated the sandstones, and altered the shales, when in contact with them, into friable cherty slates of a deep blue colour, traversed by thread-like veins of quartz. The altered sandstone is slightly micaceous, and has been so infiltrated with silica, that hand specimens have been mistaken for granite.
Near these altered patches of rock, a small quantity of fine-grained but rough unworn gold is generally found, the total quantity obtained having been about eighty ounces. Well-defined reefs of quartz also occur in the district, especially in the sandstones, but none of these have yet proved auriferous, on being tested, and the evidence seems to indicate the veins in the blue cherty slate, as the probable source of the gold.
[Footnote] * For full particulars respecting these mines, see Report by Captain Hutton, “Geological Survey of New Zealand,” 1869.