Page:Cyclopedia of Western Australia, volume 1.pdf/43
south-east direction. The geological formations in which the gold deposits of the State occur are among the most ancient in Western Australia, and possibly among the oldest of all the formations of the Australian continent.
The distribution of the major portion of the auriferous deposits is in relatively narrow parallel belts, having an almost identical trend, which coincides with that which make up the fundamental complex of the plateaux of the interior. Wherever the auriferous deposits have been examined it has been invariably found that granite or rocks allied thereto are of frequent occurrence, either immediately contiguous to the lodes or sufficiently near to have exercised some influence on the genesis of the gold. In some fields the auriferous veins are closely associated with, and appear to pass gradually into, pegmatite dykes or rocks of similar character.
In addition to the gold derived from quartz reefs, lode formations, etc., the conglomerates at the base of the Nullagine Series have been mined in two localities—Nullagine and Just-in-Time. The auriferous bands occur throughout a thickness of 300 ft. of grits, sandstones, and conglomerate, which form the lowest portion of the series. Those portions of the strata which have proved to be gold-bearing are those largely impregnated with the oxides and sulphides of iron which lie between a well-marked fault and a greenstone dyke. Mining operations have up to the present time been confined exclusively to the oxidized zone and to very limited and shallow depths. The gold in this deposit is a secondary and not an original constituent of the conglomerates.
The deepest gold mines in Western Australia are now more than 2,600 ft. below the surface, and show little diminution in the value or the size of the lodes. There are no reasons for believing that the lodes of the State have, even at the greatest depth yet attained, by any means reached the limits of ore deposition, or that the lodes will not prove productive in depth.
Copper.—The history of mining in Western Australia began with the discovery of the Waneranooka copper lode at Northampton in the year 1842. Since that date copper mining cannot be said to have made continuous progress, though the State's total production of copper ore amounts to 139,392.58 tons, valued at £849,966.
Copper ores, however, are plentifully distributed throughout the length and breadth of the State, but so far, on account chiefly of their not carrying any gold, have been worked only in a few districts, notably Marble Bar, West Pilbara, Ashburton, Lawlers, Nannine, Day Dawn, Yalgoo, Northampton, Yandanooka, Mount Morgan, Mount Margaret, Menzies, East Coolgardie, and Phillips River.
The principal sources of copper ore have been in the districts of West Pilbara, Mount Morgan, and the Phillips River; the largest output 57,308 tons being from Phillips River, 47,857 tons from Mount Morgan, and that of West Pilbara 32,806 tons.
The Phillips River field consists of a series of highly metamorphic sedimentary beds, associated with a complex series of crystalline rocks, the whole being uncomformably overlaid by a newer, though still old, series of sandstones, quartzites, and conglomerates. The crystalline rocks are of igneous origin and range from granite to serpentine with their foliated or schistose varieties.
The lodes of the field, which strike east-north-east and west-south-west, are of two types, the first being basic cupriferous dykes and the second siliceous and ferruginous deposits, which apparently fill leaching channels along the rock joints. Many of the deposits give promise of being of fair size but of low grade, while the remainder, which attain considerable dimensions, promise to yield a large quantity of lowgrade ore suitable for concentration. The field may be expected to produce a fair quantity of copper ore in the future as systematic mining operations proceed.
The Mount Malcolm Copper Mine at Eulaminna (late Anaconda), in the Mount Morgan district, has yielded nearly one-third of the copper production of the State. The whole of the mineral belt is greenstone, with quartz, jasper, and ironstone veins, which show little or no copper at the surface. The ore channel has been opened up at three points in a length of 1,300 ft. The ore has been concentrated into zones of enrichment, in reality bonanzas, three of which have been worked at the surface by open-cuts for lengths of 130, 110, and 200 ft. and to depths of 30, 20, and 180 ft. in width, and in value from 5 to 70 per cent. The oxidized zone, rich in carbonates, extends downwards to the ground water-level, which varies from 90 to 110 ft., and below this the ore changes into black sulphides with iron pyrites. Nothing very definite is known of the exact geological relationships of this important copper deposit.
The West Pilbara field has been a large producer of copper, which, however, has been obtained principally from one property—the Whim Well Copper Mine. This copper lode has proved to be the largest and richest ore body yet discovered in Western Australia in the oxidized zone. The country rock in which the deposit is situated is a weathered talcose schist, the foliation of which runs a little north of west and south of east. The ore deposit is a very flat-lying lode, conforming to the bedding of the enclosing schist country. The lode is traceable for at least 50 chains in length and has been cut into by a number of open-cuts and workings from shallow tunnels. These have