Chapter VI.—The Vadose Region.

Prof. Posepny^* advances the sweeping proposition that the formation of ore-deposits could have taken place by descension and lateral secretion in the vadose region of circulation only, and must have been, in the deep region, the product of ascending currents. This distinction is, perhaps, too sharply drawn by him. It seems to the writer that the lateral-secretion theory

^* Op. cit., Trans., xxiii., p. 262.

page 33 can scarcely be put out of court by assuming a lateral-secretion to be impossible below the ground-water level. Yet the marked difference everywhere observed in the contents of auriferous lodes above and below that level required that the rocks of the two zones should be distinguished, and separately analyzed in this investigation.

From an economic standpoint, this difference is expressed by the almost universal experience in the Australasian gold-fields, that the average yield of gold is much smaller below the water-level than near the surface. This statement, which will doubtless be controverted in some quarters, is based on the concurrent testimony of a large number of mine-managers and others, having long experience in the auriferous deposits of Australia and New Zealand. The almost unanimous evidence is in favor of the greater richness of vadose deposits. Several men of great experience have even given the opinion that, for ounces per ton above the ground-water level, only pennyweights per ton have been found below it.

This important economic question is naturally' discussed in treatises on ore-deposits. Phillips,^* for example, gives a number of reasons why the results in the vadose region may seem to be, while they are not really, higher than those of deep levels. Even after taking these considerations into account, however, the evidence of greater richness in the vadose region in Australia seems overwhelming.

In this connection, reference may be made to the exhaustive work of Mr. R. Brough Smyth,^† and to a very interesting little work,^‡ dealing with the yield of the auriferous deposits of Victoria from 1851 to 1862. The reader of the latter book may suspect the anonymous author of overstating the facts; but a comparison of the average yields noted in it with those of mines now working in the same reefs, with the aid of the latest gold-saving appliances, can hardly fail to carry conviction, even to those who, permitting "the wish" to be "father to the thought," deny the impoverishment of auriferous lodes in depth. ^§

^* A Treatise on Ore-Deposits, by J. A. Phillips, London, 1884, pp. 60-62.

^† Gold-Fields of Victoria, Melbourne, 1889, pp. 233-281.

^‡ The Gold-Fields of Victoria in 1862, by a Special Reporter of the Argus, Melbourne, 1863.

^§ The gold-field of Bendigo is often cited as an instance to the contrary, good yields having been there obtained in some cases from great depths (2000 to 2800

page 36 nary temperatures. We know, however, that it is produced by the action of hydrochloric acid on the higher oxides of manganese, or by the action of sulphuric acid on the same oxides in the presence of chlorides.

The question whether agents for the re-solution of gold exist in the vadose region is thus practically narrowed to a search, in the waters and rocks of that region, for (1) free hydrochloric acid; (2) free sulphuric acid; (3) the higher oxides of manganese; and (4) ferric chloride and ferric sulphate.

It was desirable, at the outset, to determine the most dilute solution of hydrochloric acid which will, in the presence of the higher oxides of manganese, liberate sufficient chlorine to be detected by ordinary tests. Experiment showed that one part of hydrochloric acid of 1.16 sp. gr. in 2500 of water would give a distinct chlorine reaction, while one part of the same acid in 1250 of water produced chlorine enough to dissolve an amount of gold appreciable by delicate tests. As the proportion of pure HCl to water is in the first case only about 1 to 8000, and in the second case 1 to 4000, it is evident that extremely dilute acid will, in the presence of manganese oxides, dissolve gold.

Cause of Acidity in Mine-Waters.—The chief cause of acidity in mine-waters (see examples below) is without doubt the oxidation of pyrite, which yields ferric sulphate and sulphuric acid. The latter, acting on the chlorides, which are always present to greater or less extent in mine-waters, frees hydrochloric acid. The writer has never found a water containing free acid in which there was not also a large percentage of ferric salts.

The Occurrence of Oxides of Manganese in Mining Districts.—In some mining districts (notably in Karangahake, in the Thames gold-field) the oxides of manganese often form a great part of the lode-filling. While this, however, is exceptional in Australia and New Zealand, the presence of the higher oxides of manganese in the ferric oxides of the vadose circulation is surprisingly general. Twenty analyses of such material from various localities showed in 17 cases manganese, representing from 0.012 to 43.59 per cent, (reckoned as Mn³O⁴). To one sample, containing only 0.38 per cent, of Mn³O⁴, dilute hydrochloric acid and precipitated gold were added, and gold was found to be dissolved.

If, therefore, the vadose mine-waters are found to contain free hydrochloric acid, it is evident that agents for the re-solution of gold in that zone are not lacking.

The Acidity of Vadose Mine-Waters.—An acid reaction with test-paper does not prove the presence of free acid. Every water examined which contained an appreciable quantity of ferric salts gave a distinct acid reaction, though in a number of cases examination proved the absence of free acid.

Seventeen samples of vadose waters were examined for free acid; care being taken to collect the water as it ran from the rock or vein, before any considerable exposure to oxidizing agencies other than the oxygen held in solution by the water itself.

In calculating the results from those samples which carried much free acid, if both sulphates and chlorides were present, and the amount of free acid exceeded the amount represented by the chlorine radical in the water, the whole of the chlorine radical was taken as combined with II to form free hydrochloric acid, and the remainder of the free acid found was reckoned as sulphuric acid. The results are shown in Table XIX. The amount of ferric chloride and sulphate can be approximately calculated from the proportion of iron present as ferric salts. Even after complete oxidation by exposure to the air, the total weight of ferric salts could never exceed 12 grammes per liter. For this reason, in the experiments previously described, (see p. 35), I did not use solutions of ferric salts containing more than 20 grammes per liter.

Table XIX. shows the considerable increase in acidity caused by exposure to the air. It is noteworthy that all the samples marked*, when taken from the mine, precipitated gold from solution, but that the same waters, after thorough oxidation, dissolved metallic gold when the higher oxides of manganese were added to them.

The results shown in Table XIX. point to the following conclusions:

1.	In districts like the Thames, X. Z., where the country-rock is highly charged with sulphides, the vadose water may often contain free hydrochloric acid sufficient (when the higher oxides of manganese are present) to re-dissolve gold. Though the Thames samples were incapable of holding ordinary salts page 38 of gold in solution, they acted as solvents of gold when they were thoroughly oxidized and manganese oxides were present
2.	The great majority of the mine-waters analyzed contained no free acid which could liberate chlorine by acting on the oxides of manganese that are abundant near quartz reefs.
3.	The higher salts of iron are not present in any samples of water analyzed by me, in sufficient quantity to dissolve gold at ordinary temperatures. (Stronger solutions of these salts failed to dissolve gold.) It may be added, that in every case in which much iron was present, free acids were also found; so that in any solution of gold that might be effected, the more powerful solvent, chlorine, might also be acting.

Notwithstanding these conclusions, I must point out that the re-solution of gold has probably gone on, and is still going on, in the vadose region, even where the vadose waters contain neither free hydrochloric acid nor notable quantities of ferric salts. The analyses of samples from the vadose regions of Walhalla and Ballarat (see Tables XX. and XXI., and Diagram 8 and 9) the vadose waters of which contained no free acid, and were very poor in dissolved minerals, show that such re-solution has probably been considerable, though we find no agenda now existing which would account for it.