DIAMONDS IN AUSTRALIA.
HISTORICAL REVIEW of DIAMOND GEOLOGY and MINING
In the year 1851 diamonds were discovered in one or two of the Australian gold-fields, and later on in a few of the stanniferous gravels of the same continent. They are present in not altogether insignificant numbers, and up to the year 1890 a total of 50,000 diamonds had been found. New South Wales has up to the present time furnished the greater part of the yield, but a few stones have been found in Victoria and Queensland as well as in South Australia and Western Australia.
Australian diamonds are decidedly small, the largest stone yet found, which came from New South Wales, was an octahedron, and weighed 5 5/8 carats; an octahedral crystal from South Australia weighed 5 5/16 carats. The average weight of diamonds from New South Wales compared with which the yield from the other States is negligible, is only 1/4 carat; the great majority of stones vary in weight between 1/8 and 1 1/2 carats. According to the statements of diamond-cutters, Australian diamonds are harder than the majority of stones from other parts of the world, and can only be cut with their own powder; they have a peculiarly strong surface lustre, and in spite of their extra hardness are usually much water-worn.
In New South Wales there are two principal diamond districts (See above Fig.). One is a stretch of country extending to the northwest of Sydney, as far as the Cudgegong River, and to the west of Sydney, as far as the Lachlan River.
The other diamond district is in the northeast corner of the State, in the district of the Gwydir River, a tributary of the Darling; it embraces the neighborhood of Inverell and Bingera, and extends to the east of these townships into New England. In these districts the diamond occurs in sands together with gold and tin (cassiterite), and with one possible exception it has never been found in the solid rock; it is therefore impossible to make any suggestion as to the nature of the rock in which the diamond was formed.
In the southern diamond districts the diamond-bearing debris is mainly confined to ancient watercourses of Pliocene (a subdivision of Tertiary) age. When the precious stone is found in the beds of recent rivers and streams, it is always associated with material derived from these older deposits, which has been re-deposited by natural agencies or during the process of gold-washing, etc. In this district the diamond is invariably accompanied by gold and it was in the gold-washings that the first discovery of diamonds was made. The diamantiferous gravels and sands of these ancient river deposits, which are always above, and often far above, the present water-courses, are very frequently overlain by a sheet of compact basalt, which must be penetrated before the diamond and gold-bearing stratum can be reached. Re-deposited masses of material, containing both gold and diamonds, often lie on the basalt, having been washed down from the upper part of the valleys.
The first discovery of diamonds in Australia was made in this State in l851; the stones were found in Reedy Creek, a tributary of the Macquarie River, sixteen miles from Bathurst a few were found in the same district in 1852, in Calabash Creek. In 1859 a few stones, having the form of triakis-octahedra, were found in the Macquarie River, near Suttor's Bar, and at Burrandong; in the same year a hexakis-octahedron, weighing 5 1/8 carats, was found in Pyramul Creek. These places are all situated in the same district, and at none of them were more than a few stones found.
In 1867, however, diamonds in greater number were met with near Warburton, or Two Mile Fiat, on the Cudgegong River, nineteen miles northwest of Mudgee; and in 1869 the systematic working of an area of about 500 acres in this district was commenced. The working, which was not very profitable, was carried on at Rocky Ridge, Jordan's Hill, Horse Shoe Bend, and Hassalt Hill, as well as at the places already mentioned. The ancient river-deposits in which the diamonds are found lie under a capping of columnar basalt, and occur at isolated spots along the course of the Cudgegong River, more or less distant from the present river course, and at heights up to 40 feet above the present high-water level. They rest on the eroded edges of perpendicular sedimentary strata, which are interbedded with compact greenstones, and probably belong to the period of Upper Silurian deposits. The diamond-bearing debris consists of coarse sand and mud intermingled with pebbles of quartzite, sandstone, clay-slates, and quartz-slates, accompanied by water-worn grains and crystals of quartz, jasper, agate, silicified wood (this in large amount) and other siliceous minerals, also cassiterite (the "wood-tin", variety), topaz, common corundum (sometimes of a lavender-blue colour), sapphire, ruby, a peculiar variety of corundum called barklyite, zircon, garnet, ruby-spinel, brookite, magnetite, ilmenite, tourmaline, magnesite, nodules of limonite, grains of iridosmine, and, of special importance, gold. The quartz pebbles are frequently encrusted with oxides of iron and manganese. The whole mass of diamantiferous debris is in some places loose and incoherent, and in others bound together to form a solid conglomerate, the cementing material being a green, white, or grey siliceous substance, or a brown or black ferruginous or manganiferous substance. The deposit in places attains a thickness of 70 feet; the diamonds, which are of small size, are scattered through it so sparingly and irregularly that the working of it cannot be profitably prolonged for any length of time.
In spite of the poor character of the deposit, 2,500 stones were found during the first five months of work. All were small, the largest being the octahedron of 5 5/8 carats mentioned above, which, when cut, formed a beautiful colourless brilliant, weighing 3 5/16 carats. The stones average in weight about 1/4 carat, and vary considerably in colour, passing from perfectly water-clear through various shades of yellow, pale green, and brown to almost black; a twinned octahedron of a beautiful dark-green colour was once found. The commonest crystalline forms are the octahedron, which occurs both as simple and twinned crystals, the rhombic dodecahedron, triakis-octahedron, and hexakis-octahedron; one crystal with the form of a deltoid dodecahedron has been found. The crystals are, as a rule, much water-worn; when this is the case their surfaces are sometimes smooth and bright, at other times rough and dull. No spheres of bort, such as are found in Brazil and South Africa, appear to occur in Australia.
Solitary specimens of diamond have been found at many other places in this district. At Bald Hill, near Hill End, on the Turon River, a stone of 5 1/8 carats was found, and a number of diamonds, which though of small size were of excellent quality, were met with in the old gold mines of Mittagong. Again, near Bathurst, a black diamond, the size of a pea, and having the form of an almost spherical hexakis-octahedron, was found. Diamonds have also been collected from the gravels underlying the basalt at Monkey Hill and Sally's Flat, in Co. Wellington, just as they occur at Mudgee. Uralla, Oberon, and Turnkey are other localities at which more than solitary specimens of diamonds have been found.
The occurrences mentioned above were all in ancient river gravels among existing watercourses in which diamonds have been found might be mentioned the Abererombie, Cudgegong, Macquarie, Brook's Creek, Shoalhaven, and Lachlan Rivers. The stones found in existing streams are much worn, and many are broken; from this, and also from the fact that the minerals forming the gravels of these watercourses are identical with those of the ancient river deposits found underlying the basalt, we may conclude that the gravel of the present rivers is re-deposited material derived from the ancient river-beds.
The mode of occurrence of the diamond in the north of New South Wales, especially in the district of the Gwydir River, in the neighbourhood of Bingera and Inverell, is of some importance.
Diamond Geology [
Rafal Swiecki, geological engineer email contact
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