ECONOMIC GEOLOGY. 



By J. A. IVA TT, iM.A., B.Sc. 



A.— GOLD. 



The highly metamorphic character of the Pre-Cambrian rocks of the 

 McDonnell Ranges, their greatly disturbed state, their extensive development, 

 and lastly the presence of ninnerous intrusive masses varying much in composition, 

 are all circumstances favourable to the development of mineral deposits in them. 

 Gold is the only mineral that has been found in payable quantities in these I'anges, 

 and that only in a very limited area of about fifty square miles, situated seventy 

 to eighty miles E.N.E. of Alice Springs, on the Arltunga or Paddy's Hole gold- 

 field. Although, as just stated, gold in payable quantities has been found on the 

 above-mentioned gokltield, yet alluvial gold in small quantities has been found also 

 near Winnecke's Depot, Bald Hill, and in some of the gullies in the Georgina 

 Range. 



The greater portion of the alluvial gold has been obtained from a red clayey 

 washdirt occurring in the narrow gullies which take their rise in a low range. 

 The sinking is exceedingly shallow over the entire field, ranging from a foot or two 

 up to eight or ten feet, the average being about four to five feet. The bottom is 

 often very soft, being usually formed by gneiss, which is in a very weathered state, 

 and therefore soft and friable. The concentration of the gold in the washdirt has 

 not been satisfactorily accomplished, necessitating in many instances the washing 

 of the greater part of the alluvium, as the overlying alluvial detritus is often as 

 rich as the so-called washdirt. Much of the gold won from the gullies is very fine, 

 a character shared by iimch of the reef gold also, and consequently diflicult to 

 save. The largest nugget found on the field was taken from Red Gully, and 

 weighed two ounces three pennyweights. " Surfacing " has been attempted in a 

 few places, notably on the west side of Kangaroo Creek, where the surface was 

 removed to a depth of six to eight inches. Owing, however, to the scarcity of 

 water, and the heavy expense of carting it, when obtainable, to the claims, this 

 work had to be abandoned as unprolitable, in spite of a fair yield of gold. 



The most important auriferous quartz reefs have a prevailing due north 

 and south trend, and their gold contents show a remarkable uniformity. The 



