Page:Cyclopedia of Western Australia, volume 1.pdf/45
The alluvial deposits are the most important of any yet opened out in the State, having yielded 11,815.04 tons. They vary very largely in nature and range from an extremely hard ferruginous conglomerate to a stiff clay or loose sand or gravel. The tin stone in the first named is often extremely coarse, though generally one-tenth of an inch or less in diameter, while that in the softer material is almost uniformly fine. Assays of ten samples of this class of ore from the Greenbushes field varied from .9 up to 33.3 per cent, of the metal, the average being 10.1 per cent. The associated minerals are quartz, kaolin, limonite, ilmenite, tourmaline, tantalite, stibiotantalite, garnet, zircon, gold, magnetite, rutile, and topaz. On the Greenbushes field the alluvial deposits may be divided into two main groups, the older being the old river courses or deep leads and the newer being represented by the existing channels. In the latter the tin-bearing gravels often lie at from 10 to 40 ft. from the present stream bottoms. The older deep leads attain considerable depths, the deepest being 96 ft. The tracing of the course of these deep leads is a matter of considerable economic importance to the future of Greenbushes. The residual deposits are either lateritic ironstone or sands, clays, etc., derived from the decomposition in situ of igneous rocks. These are frequently stanniferous. The chief minerals accompanying the tin are limonite, quartz, tourmaline, clay, and mica.
Lava Flow, Bunbury
The tin-bearing granites consist of granite passing in places into a highly-foliated and highly-micaceous granite, with little or no felspar. This granite (greisen) contains tin, tourmaline, zircon, garnet, etc., as accessory constituents.
In many of the tin-bearing districts of the State the granites, gneisses, and allied rocks are traversed by numerous dykes of a pegmatite composed mainly of quartz and albite, with subordinate mica, garnet, and cassiterite. Considerable quantities of the stream and a little (lode) tin owe their origin to the disintegration of veins of this nature.
Tantalum.—Quite a number of different ores of tantalum have been recorded from various parts of the State, viz., tantalite, columbite, stibiotantalite, microlite, and euxenite. Tantalite was first discovered in Western Australia at Greenbushes in 1893, and since that date tantalum ore has been raised for the market at Wodgina and neighbourhood in the Pilbara goldfield and at Greenbushes, in the South-West Division. At Wodgina there is a number of pegmatite veins which pass from the granite into the schists and more solid hornblende rocks. Within the area of the latter the pegmatites contain ores of tantalum, mainly manganotantalite, and it is these dykes which constitute the tantalum lodes of the district. Large quantities of detrital ore have been recovered from the shallow surface soil in the immediate vicinity of the outcrops of the pegmatite veins, and more or less waterworn alluvial ore has also been won from many of the adjoining gullies.
At Greenbushes only 3.19 tons of tantalite have been raised. It is not by any means common, but is practically confined to the southern half of the district. The whole of the output is from alluvial deposits; the tonnage, however, is probably far short of the true total of the ore raised, for much may not have been officially reported to the Government.
There have been up to the close of 1910 89.24 tons of tantalite raised in the State, and valued at £13,486, and of this quantity only tons are from lodes.
Iron.—The ores of iron are very widely distributed throughout Western Australia, but, save in one or two instances, the area in which the exploitation of such deposits is actively prosecuted is very limited, such areas being, with the single exception of Koolan Island, Yampi Sound, at present confined to localities where ore used as a flux in copper- and lead-smelting can be readily obtained.
The large iron deposits of the State, some of which are probably equal in size to any others in the world, have not been worked, and are, owing to their geographical situation and the absence of suitable coalfields, under present conditions practically valueless. The iron deposits of the Murchison stand out prominently before any of the others so far discovered, and though at present neglected are destined to form a very important State asset. No detailed geological surveys have yet been made of any of the iron deposits of the State, hence even an inventory of the available iron ore supplies can be only a mere approximation.
The iron deposits of Western Australia, when viewed from the standpoint of their geological relation-