Where To Look

Bill James (Australia) writes:

Wind, rain frost and chemical reaction with gases in the atmosphere, smash and decompose the hardest rocks. Running water is the world’s most energetic miner. It carries vast loads of material to sea with every rainfall. Rivers and streams carve away the faces of mountains, pounding rocks into pebbles and sand. Where the stream rushes swiftly, the bedrock is scoured; where it loiters, the gravels carried in the water are left in what are called alluvial deposits.

These deposits contain the heavy insoluble materials washed from the rocks, including gold and gemstones. Rain falling over a mountain range sweeps into the river systems of the area the debris of rocks already decomposed by exposure to air and the alternate effects of frost and sunshine. In this way rivers concentrate deposits of both gold and gems that are far richer than those contained in the original rocks.

But many things can happen to a river in a few thousand years. Most likely it will change its course several times, each time leaving gem gravels high and dry and creating fresh deposits in new places. Rivers have been gathering gold and gems from the rocks of Australia not for thousands of years but thousands of million of years. Time and again the landscape has changed. Where once a river flowed from east to west, it is diverted to flow from north to south. What was once a streambed becomes a mountain-top; sea invades the river valley, smashing the rocks with its waves. Or perhaps a huge glacier, most ruthless of nature’s jackhammers, grinds across the land, carving mountains to new shapes, leveling valleys, carting heaps of rock and gravel many miles to faraway places. Glaciers left their mark on Tasmania and New South Wales and scattered a thin trail of diamonds as they rasped across the highlands of north-eastern Victoria. In one place ice blocked the rivers; in another, fire. Lava from a volcano flowed over the diamond field of Copetown in New England some 40 million years ago burying a riverbed under up to 130 ft. of basalt.

Buried alluvial deposits are known as deep leads and some people believe there is a fortune in diamonds in the deep lead at Copeton, in the New England district of New South Wales, but the cost of finding out has made it too great a gamble for anyone to venture as yet. Copeton is unique in being the only place in Australia where a diamond has been found in its original volcanic vent. This was a dolerite (basalt) dyke at Oakey Creek. The vast changes that have taken place on the earth’s surface have given rise to two other types of rocks beside those of an igneous nature. Sedimentary rocks are derived from the weathered waste of older rocks, like sandstone or conglomerate, or precipitated from solution, as gypsum or limestone. Gemstones are sometimes found among the pebbles and rock fragments making up conglomerates.

Richer in ores than the water-formed sediments are the metamorphic rocks. These are created by the transformation of igneous and sedimentary rocks under extremes of heat and pressure. Under these circumstances granite become gneiss and limestone changes to marble. This is known as regional or dynamic metamorphism and may occur over large areas. Metamorphic rocks are a product of the tremendous convulsions of the earth’s surface that I have already mentioned. When magma forces its way into and through existing rocks, changes occur that are described as local or contact metamorphism.

In this, the superheated, concentrated solutions of magmatic water, with its chemical mineralisers, play an important part. As a result, sapphires and rubies, garnets and spinels among other gems occur in contact metamorphic deposits.

In some metamorphic rocks, crystals of mica, garnet, chlorite and other minerals have been lined up so that the rock will break easily along parallel surfaces. Such rocks are called schists. Turquoise is found in veins through some schists and slate —metamorphosed shale from Rockhampton to Brisbane in Queensland, and also at localities in New South Wales and Victoria. Among other gemstones produced from silica solutions are chalcedony, agate and Australia’s pride, the opal.

Opal comes in two forms, common and precious; with the common opal, known to the miners as potch, often acting as a signpost to the better stuff. Potch can be colorless, as hyalite, amber, milky, blue-gray or black, but it lacks iridescence and fire. Precious opal glows in red, blues, and greens, colors that flash and move as the stone is turned in the light, rainbow hues created by the breakdown of light through layers in the gem having slightly different refractive properties.

In Australia precious opal was first discovered as nodules and veins in cracks and cavities of basaltic lava and the quartz-rich igneous rocks called andesite or trachyte. But this mountain opal was disappointingly subject to cracking and crazing on exposure to dry air. The real opal country is the 50000 square miles of sandstone laid under the sea that spread over much of central Australia 120 million years ago. This dry and bare expanse of modest hills and sandstone ridges takes in 52 known mining localities in Queensland as well as White Cliffs and Lightning Ridge in New South Wales.

Mines at Andamooka and Coober Pedy are in sandstone and shale of a similar period and it is likely that important deposits of opal await discovery in the so-called desert sandstones of South Australia. The silica solutions which filled crevices and replaced other substances, such as wood and bone, in the rock also cemented the sandstone into what is called duricrust. As angel stone, shin-cracker and steelband, this hard rock is usually found in layers above opal.

A number of fossils have been turned to opals, including plants, shells and the bones of dinosaurs. Just before the First World War an almost perfect skeleton of a small dinosaur came to light at White Cliffs. Part of a crocodile’s jaw and teeth in blue black opal was dug up at Lightning Ridge, where the celebrated ‘nobbies’ are also of fossil origin. But this is not to say that the opal dates back to dinosaurs. The skeleton of a cat, buried in a miner’s hat, was found to have turned to pale pink opal. Gateposts buried 20 years were found to be opalised at the foot.

Where To Look: (continued)