PRECIPITATION OF SULPHIDES. 1109 



whether or not gold is also precipitated as a sulphide no definite statement 

 can be made. The majority of those who have most closely studied gold 

 deposits hold that the gold occurs either as free gold or as telluride. In 

 evidence of this they cite the fact that where gold occurs with pyrite, as it 

 so generally does, the microscopical study of the sulphides shows flakes of 

 free gold within the sulphide. Admitting these facts, it does not at all follow 

 that another portion of the gold is not present in the pyrite as a sulphide. 

 Indeed, since gold sulphide is known to be a definite compound, which can 

 easily be produced in the laboratory, and is produced by a metallurgical 

 process, it seems to me highly probable that some of the gold is precipitated 

 as a sulphide and in this form is associated with the sidphides of the other 

 metals. 



Precipitation of sulphides transported as such. Tile precipitation of tl'ailSpOrted Slll- 



phides in the form in which they are found in the ore bodies may be 

 accomplished either by simple dilution of the compounds, as pointed out 

 by Becker" for mercury sulphide, by decreasing pressure and tempera- 

 ture, or by both. These are the conditions in ascending solutions, hence 

 the transported sulphides are likely to be precipitated where the water is 

 rising in trunk channels. The precipitation of the sulphides may result 

 also from mingling of solutions. If, for instance, an acid, such as boric 

 or sulphuric acid, be added to the solutions of the sulphides carried in 

 alkaline carbonates and alkaline sulphides, the simple neutralization will 

 result in the precipitation of man)' of the sulphides. By such neutraliza- 

 tion the alkaline sulphide, the solvent, is destroyed. For instance, if sul- 

 phuric acid be added to a sodium sulphide solution the following reaction 

 takes place: 



Na 2 S+H 2 S0 4 =Na 2 S0 4 +H 2 S 



This equation shows that not only is the solvent destroyed, but a precipitat- 

 ing agent for the sulphides is produced. Under such conditions, there 

 can be no doubt that the sulphides will be rapidly precipitated. Also, 

 where the sulphides are transported in solutions of sodium sulphide and 

 sodium carbonate, the addition of hydrogen sulphide in excess will result 

 in the precipitation of many of the metals. 



"Becker, cit,, pp. 429^31. 



