HAVING alluded to all the different conditions under which minerals occur, either as reefs or stratified deposits, it is now proposed to devote a chapter to a description of those repositories of minerals known as alluvial deposits. All alluvial deposits have been formed by the degradation of the lodes or other ore bodies already described, with their accompanying rocks, or of rocks impregnated with mineral. It is possible that some of the larger nuggets of gold, however, have been formed in situ, the fact that many of them have been found at shallow depths, sometimes only a few inches from the surface, appearing to favour this view.
In studying the deposits as a whole, however, we can afford to overlook such abnormal conditions, and only consider the more common form of origin.
The study of alluvial deposits is of considerable importance for two reasons :
(1) That the greater quantity, both of gold and tin, that has been mined in the past was won from alluvial, and, although of late years the output from reefs is greater than from alluvial, large areas still exist where a judicious expenditure of capital on comparatively poor ground will be remunerative.
(2) A thorough knowledge of the history of alluvial deposits affords the best introduction possible to their prospecting, and a means moreover of tracing minerals to their parent ore bodies.
The only minerals of importance found in alluvial deposits are gold and the other noble metals ; tinstone, diamond, corundum, and the other gems, which, from their hardness and their power of resisting chemical change, are preserved in their original state, even when submitted for long periods to the action of the weather ; monazite, and occasionally cinnabar in small quantities. With these are associated ironsand and other hard and heavy minerals, which are worthless from an economic point of view.
The history of the alluvial deposits of any district is included in an account of the denudation of the rocks. The country in which we find ourselves may be barren and rainless, with no rivers flowing from it, and no signs of such having previously existed, or, on the other hand, high mountain ranges and a plentiful rainfall may feed numerous streams, which in their turn unite to form large rivers, and any conditions may be represented between these extremes.
Where rivers exist, alluvial deposits of all classes may occur along their course, and in describing the deposits it is simpler to commence with the outcropping rocks amongst the ranges, and follow the rivers down along their course.
Surfacing Deposits. On the ranges and intersecting the rocks of which they are composed reefs or other ore bodies may crop out. As soon as the rocks or reefs disintegrate, owing to changes of temperature by night and day, in winter and summer, the oxidising action of the air or the effects of wind and rain, the decomposed material is transported by water to a lower level if the district is a rainy one, and the outcrops exposed show little or no sign of decomposition.
If, on the other hand, the country is dry, and disintegration proceeds more rapidly than denudation, what are known as gossans and surfacing deposits are found. If the lodes contained pyrites the result of decomposition will be to form gossan. If, in addition, they contained copper pyrites, the copper will be leached out as sulphate, which may either flow away in the surface water or descend into the lode to redeposit the copper lower down, where other sulphides are met. If beyond this gold exists, most of it will remain as free gold associated with the ironstone cap.
A small rainfall will, by degrees, wash away the soft gossan, but leave the gold behind, owing to its higher specific gravity, and a very rich deposit will form in time under very little cover and near to its parent lode. Even wind may bring about concentration when the rainfall is too small for the purpose, and West Australia has afforded many instances of its operation. On the flanks of any outcropping veins is detrital material, which thickens slightly away from the outcrop, and with this any gold occurring is associated. The gold has seldom been carried far, and the deposit, therefore, becomes poorer at a relatively short distance from its source. In mining it is often difficult to determine by inspection where the detritus ends and the decomposed surface of the main rock begins. The gold, however, affords a guide, for it does not continue at any rate for more than a few inches below the line of junction.
Mount Bischoff, again, is an excellent instance of a surfacing deposit ; indeed, the greatest example on record of the disintegration of a network of tin-bearing veins traversing and impregnating rocks, which in this case are slates and quartz porphyries. The residual surfacing that has been worked has yielded from 60,000 to 70,000 tons of 70 per cent, black tin. Similar, but smaller, deposits have also been found in the Malay States.
It is a remarkable fact, whether with gold or tin, that rich surfacing is often found where the source from which it was derived will not pay to work. Often, again, the surface of the country is strewn with broken quartz, leading to the conclusion that it was derived from a large quartz reef, but, in some cases at any rate, it has come from veins and leaders only having resisted denudation on account of its hardness. Such has been the case in many of the West Australian deposits, as well as at Mount Bischoff.
Instances of similar deposits may be found in some of the so-called " leads " of Eastern Australia, such as Temora, where a channel with but slight fall, and filled largely with clay, demonstrates that the drainage was sluggish, and failed to carry the disintegrated rock for any great distance. At the bottom of the clay gold was found, resting on bed rock, generally nuggetty, but very unevenly distributed along the course of the " lead," and with considerable blanks between the rich parts. Quite as coarse gold was found near the lower end of the " lead " as higher up, and it seems obvious that these patches represent surfacing deposits from veins which are covered by the fine water-worn material referred to, and that certainly the gold has not been carried for any distance by running water.