MoiLiif Morijdii Gobi Mine. 345 



irregular manner distributed throuph the siliceoui? and kaolin 

 ore. The average contents of gold in this zone would probably 

 be only dwts. against ounces in the enriched zone. 



The oxidisation aaid leaching of this ore has extended from 

 180 feet from the sm-face to perhaps 300 feet in the deepest 

 part, and, as elsewhere, this action has prevailed less deeply 

 along the -walls than towards the centre of the mass of ore 

 There can be little doubt that so fas as leaching has proceeded 

 gold has been conveyed thence in solution and again deposited 

 in the enriched zone, and although the sulphides in their original 

 condition in the upper portion of the mine may have been in 

 no riclier gold ore than in the lower portion now being worked, the 

 extensive concentration from an enormous bulk of adjacent ore 

 might account for the wonderful accumulation of gold in the 

 enriched zone or secondary ore. 



Formerly the term tufa was applied to the cellular siliceous 

 ore of this zone, and certainly its appearance in the upper 

 workings was unusual, but now that it has been follow^ed down 

 to its roots there is no room for doubting that it is merely the 

 skeletonof silica, the sulphides having l)een oxidised and leached out. 



Eveiywhere in the mine as depth is attained this oxidised and 

 leached ore is found to give place to sulphide ore, (F) tlie un- 

 altered zone. The change is gradual in places, and first iron 

 pyrites is met with, then at lower depths copper pyrites is met 

 n4th associated with the iron pyrites. In the sulphide ores gold 

 is most irregularly distributed, and in the bottom levels the 

 average gold contents has fallen to as low as Ij dwts. of gold 

 per ton, but the copper contents have increased. In this con- 

 nection bulk, as against weight, has to be considered, for a ton 

 of sulphide ore would perhaps be only 10 or 11 cubic feet, while 

 a ton of the cellular siliceous ore would probably exceed 30 

 cubic feet, so that until careful experiments are made as to the 

 proportion of gold per cubic yard in the different portions of the 

 mine it would be unsafe to assume that there is an actual 

 diminution in the gold contents in the original ore as depth is 

 attained. 



Then outside the sulphide ore and skeleton ore from which 

 the sulphides have been removed are the walls of crystalline 

 igneous rocks (G), altered at the surface to kaolin, but in the 

 deeper levels unaltered and apparently diorite, etc. ; also a 



