Treasure Maps

Bill James (Australia) writes:

Most often used by rock-hounds is the inch to a mile scale military map, prepared by the Royal Australian Survey Corps and available from stationers and booksellers. This map gives accurate and detailed information on routes and camping places as well as indicating promising prospecting localities.

The military map is most useful in conjunction with one of the detailed geological maps produced by the Commonwealth Bureau of Mineral Resources or the Mines Department of the various States. When planning a prospecting trip, it is wise to have several alternatives in mind, so as to avoid disappointment. In a granite area, for instance, we may wish to try our luck on the rockface and talus, on mine dumps and also at what we work out as the most promising stretch of the river.

We require a good deal of equipment to tackle the rockface. A pick and showel, a light mineralogist’s pick-hammer, a striking hammer of about 3lb weight—one with a square face and a chisel peen is the most useful—and half-a-dozen rock gads of various sizes. A steel crowbar or prybar of the type known to American rockhounds as a pocket robber is invaluable. This tool, which varies from 18 in. to 45 in. according to taste, has a 30 degree bend at each end to provide powerful leverage.

A sledgehammer up to about 14 lb in weight may also be needed. The rockface offers three collecting areas—the face itself, the talus, and the eluvial deposits buried beneath the talus which have been concentrated by gravity. Both the last-named present few problems except hard pick and showel work, cracking any lumps of rock that look as if they could contain crystals. A quarter or eighth-inch sieve will be required to deal with the smaller gravel.

The rockface is another matter. Weathered rock can be a deathtrap even to a properly equipped and experienced climber. But you are neither experienced nor properly equipped. If the face is in any way difficult, you will be a fool try climbing and working on it. Leave it alone and concentrate on the easier stuff. That goes for quarries too, but there the owner will probably insist that you remain on terra firma, anyway.

It may happen that a pegmatite dyke or sill or a rock cavity occurs in a position where you can attack it safely. The finest gem crystals are often found in gas holes of pink or red granites and plants growing out of the rockface may indicate such a cavity.

In basalts, the steamholes or amygdales may be filled with opal or agate. Amethysts are found in quartz veins as a druse—a cavity lined with crystals of the same nature as the host rock, as distinct from a geode. Gem pockets are usually found in the midst of the thickest part of pegmatite dykes and sills. The rock should be tested at these points with the pick and will be found to break open comparatively easily over a pocket of crystals.

Cavities in the rock mostly have to be opened by splitting with a succession of gads and the prybar then used carefully to break off and bring out the crystals. Sometimes these cavities are filled with clay and the contents must be sieved to make sure no gems are lost.

Sieves, a bucket and shovel, with maybe a pick and a crowbar to shift the boulders are what we require to deal with river gravels. Some people use a miner’s pan but the best equipment for gemstones is two sieves which fit together. I prefer the half-inch and quarter-inch mesh in general, but a quarter-inch sieve and an eight-inch or even a one-sixteenth-inch are better if there is any chance of picking up diamonds.

Standing gumboot deep in a river agitating a couple of sieves becomes hard work very quickly. However, special aluminum sieves are now available to lighten the load for rockhounds. It is no use looking for gem gravels on the steep slopes where the river runs swiftly most of the time and torrentially in heavy rain. But as soon as the surface becomes more level or some obstruction checks the flow, the heavier part of the gravels will be discarded.

Deposits are formed on the inside of bends and on the downstream side of obstructions. Any gutters, hollows and crevices in the streambed become natural traps for gemstones. Boulders trundled along the river by floods pin down the gravels wherever they come to rest. Any gravel bar or dump of tailings from an alluvial dredge is worth a try on a river known to contain gemstones. The method is to fit the two sieves together with the wider mesh on top and then fill the upper sieve with gravel. Dip both sieves in the river so that the water covers the gravel and rotate to and fro vigorously. Continue this action until all the finer material has dropped into the lower sieve.

Carefully go over the pebbles remaining in the top sieve for any gem material before emptying it. Then take the lower sieve, immerse it until the gravel is just below water level and agitate thoroughly once more. The object is to bring all the heavier material, including gemstones, to the bottom of the sieve. Give the sieve a final to-and-fro jerk, take it smartly from the water and turn it over on the nearest handy piece of flat ground.

Treasure Maps: (continued)