514 THE ORE-DEPOSITS OF BORAH CREEK, 



the other, but occasionally divided, and adhered, in part, to both 

 walls. This open< d the way for the deposition of the ores. 

 Previously the solutions arising along the crushed zone were 

 slow-mo\ing, and chilled rapidly with ascent, thus losing, at the 

 same time, their solvent power. With an open fissure, however, 

 the ascending solutions could maintain a relatively high temper- 

 ature, and so carry the metallic subst;inces towards the surface. 

 When tlie temperature fell, the most insoluble minerals, under 

 the conditions of solution, were naturally the first to precipitate. 



Change in the nature of the solution, due to minerals precipi- 

 tated from it and access of new solution, doubtless complicates 

 the problem, l)ut several well marked phases of deposition may, 

 nevertheless, be distinguished. Arsenopyrite is found forming 

 as the outside zone, and is, consequently, the earliest deposited 

 mineral. It is, as a rule, fairly well marked-off from the spha- 

 lerite band, which lies adjacent to it on either side of the lode. 

 In many places, a good deal of pyrite is associated with the 

 arsenopyrite, and seems to be partly later, and partly of con- 

 temporaneous origin Following the deposition of sphalerite, 

 came the formation of admixed copper pyrites and sphalerite. 

 Accompanying this phase of the deposition, stannite is to be 

 found in the vein. This mineral does not usually occur as a 

 distinct band or vein,* but in bunches of irregular shape and 

 size, situated between the zone of sphalerite and the inner 

 metallic zone, which is galena. The last deposition filled the 

 centre of the fissure with comb-quartz. 



The presence of numerous vughs in the mine, with their linings 

 of quartz-crystals, and occasional fillings of hydrous iron-oxides, 

 suggests the following explanation. As the fissure became filled 

 by deposition from solution, some irregularity in the filling might 

 well be conceived; and, hence, the possibility of certain spaces 

 being cut off from the main body of solution, may be readily 

 admitted. Given an isolated body of solution, crystallisation 



* In one place stannite was observed as a very distinct band in the lode. 

 In this case, it occupied a position between the arsenopyrite and sphalerite 

 bands. 



