Skptember 1, 1894.] 



KNOWLEDGE. 



203 



currents carrying the lower water to the upper portions of 

 the fissures ; other water must descend to take the place 

 of that so driven upwards, and thus a deep circulation 

 will be maintained, aided, no doubt, by inequalities of 

 pressure and capillary action. These water currents 

 would evidently suffice for the conveyance of dissolved 

 gold into the fissures, if we can account chemically for its 

 solution. It must not be forgotten that we have no actual 

 knowledge of the behaviour of substances under such con- 

 ditions of intense temperature and pressure as prevail in 

 the deeper regions of the earth, and that we are compelled 

 to reason from our laboratory experiments, which may, 

 however, be leading us directly away from the truth. 

 With this proviso, it is not difficult to imagine conditions 

 under which gold may pass into solution. The most per- 

 manent soluble salts of gold are the haloid salts, and it 

 has usually been assumed that it is in this form that gold 

 occurs dissolved in Nature. In the paper already referred 

 to, I suggested that it is far more probable that the gold is 

 dissolved in the form of an alkaline aurate. It is well 

 known that fused alkalies, in the presence of air or of an 

 oxidizing agent like nitre, attack gold, forming an aurate 

 soluble in water, and also that many natural waters are 

 strongly alkaline. Perhaps the strongest argument in 

 favour of this view is the invariable association of gold 

 with quartz, an association too invariable to be accidental ; 

 so that it seems natural to suppose that the solvent 

 that carried the quartz into the fissure carried the 

 gold also. I personally prefer this explanation, which 

 rests upon reactions and compounds that we know, to 

 imagining a hypothetical soluble silicate of gold that may 

 or may not exist. It presents, moreover, distinct analogies 

 to another mode of solution of gold, of which we have 

 positive evidence — its solution as a sulph-aurate in the 

 two localities already quoted, Steamboat Springs and 

 Sulphur Bank. Both these localities have been repeatedly 

 described by writers on the genesis of mineral deposits ; 

 nevertheless, a few lines about them, with reference more 

 particularly to the question we are now considering, may 

 prove of interest. The best account of both is to be found 

 in Becker's monograph on the geology of the quicksilver 

 deposits of the Pacific slope in the Uyiited States Geological 

 Survey, Vol. XIII. (1888) ; J. Le Conte has also written on 

 the subject in the American Journal of Science, Vols. XXIV. 

 and XXV. Steamboat Springs is the name given to a 

 small district in Washoe Coimty, Nevada, well known as a 

 thermal health resort, some six miles from the Comstock 

 lode. The underlying rock of the district is granite, 

 which is overlaid in many places by metamorphosed rocks 

 of the Jurassic system and by lavas. The springs issue 

 from a series of vents in a narrow valley between two 

 volcanic ridges, the waters being very hot, alkaUne, and 

 charged with carbonic acid and sulphuretted hydrogen. 

 Analysis has shown that these waters contain traces of 

 many metals, amongst them gold. The vents out of 

 which the water comes are the remains of a series of 

 extensive fissures, which are now nearly choked up by 

 having been filled with silica deposited by the springs 

 themselves. These deposits are " in many places stained 

 and clouded with metals. ... In places where there is 

 water still issuing slowly, silica is found in a gelatinous 

 condition. . . . Here then, undoubtedly, mineral veins 

 are now forming under our eyes, but their metallic contents 

 are in very small proportion " (Le Conte, loc. cit.). It is 

 noteworthy that cinnabar has been mined in the immediate 

 neighbourhood, and that the deposits of this mineral are 

 evidently closely connected with the springs themselves. The 

 phenomena at Sulphur Bank, California, closely resemble 

 the foregoing. The country rock consists of Neocomian 



sandstones and a series of metamorphic rocks, intersected 

 by eruptive basalts. There are numerous hot mineral 

 springs carrying alkalies and alkaline sulphides in solution, 

 associated with a series of deposits of cinnabar and silica, 

 crystalUzed and amorphous. These latter have been 

 mined, and streams of heated water and gases were met 

 with in the course of the operations. The deposits occur 

 both in the sandstones and the basalts. Other sulphides 

 are present besides cinnabar, and these have been shown 

 to contain gold. Most geologists who have studied these 

 deposits are of the opinion that their formation has not 

 yet ceased. Becker thus summarizes his views on both 

 these deposits : "I regard many of the gold veins of 

 California as having an origin entirely similar to that 

 of the quicksilver deposits. . . . The evidence is over- 

 whelming that the cinnabar, pyrite and gold of the quick- 

 silver mines of the Pacific slope reached their present 

 positions in hot solutions of double sulphides, which were 

 leached out from masses underlying the granite or from 

 the granite itself" [op. cit., pp. 449, 450). In both these 

 localities, therefore, gold has been pretty conclusively 

 proved to be in the course of deposition from hot alkaline 

 sulphuretted solutions, in which there is little doubt that 

 the gold is dissolved as a double sulphide of gold and an 

 alkali, or, in other words, as an alkaline sulph-aurate. 

 Apart, therefore, from the doubt thrown at the outset upon 

 any theories based upon our laboratory experience, there 

 seems to be little difficulty in accounting for the solubility 

 of gold at depths far beneath the earth's surface. 



The last stage of the process is also the most difficult of 

 explanation. Given that the gold is in solution, we have 

 to account for its deposition in the fissure under circum- 

 stances that do not admit of its re-solution in the same 

 water current. Perhaps the wisest course is simply to 

 confess our ignorance whilst suggesting tentatively a few 

 plausible hypotheses. Of course the solution, when it 

 enters the fissure, contains a number of other substances, 

 notably siUca, in solution besides the gold, and whatever 

 cause determined the deposition of these substances most 

 probably precipitated the gold also. For all we know, this 

 cause may have been some electrolytic action. Or again, 

 if temperature and pressure are requisite for the retention 

 in solution of the various substances, these latter would 

 naturally be deposited when the solution found its way 

 into the upper parts of the fissure, where the pressure 

 would be lower and the temperature would be gradually 

 decreasing as the solution rose upwards. It is easy to 

 imagine the mixture in the fissure of the original solution 

 with another which might contain dissolved in it various 

 reducing agents. Or, if the fissure already contained 

 particles of various metallic sulphides, these would 

 precipitate gold readily from such a solution as the one 

 described above. I have proved experimentally that 

 natural sulphides, such as galena and iron pyrites, pre- 

 cipitate gold readily from a solution of an alkaline 

 aurate. The simultaneous deposition of such a metallic 

 sulphide and of gold from a sulph-aurate would probably 

 account for the intimate state of combination of gold with 

 various " sulphurets," which, to his sorrow, the primitive 

 gold miner knew but too well, and to extract the precious 

 metal from which, all the resources of modern scientific 

 metallurgy are needed. 



Any of the above hypotheses are admissible as possibly 

 correct explanations, though it is hardly safe to say any- 

 thing more than this La their favour. It is more than 

 probable that no one simple reaction can account for all 

 the various occurrences of gold in veins, and that whilst 

 the above may all be valid hypotheses, there are probably 

 many other causes of precipitation besides those enumerated. 



