50 ECONOMIC GEOLOGY 



In the detrital deposits of the Transvaal it is considered by some 

 authorities that by the absence of nuggets the gold must have 

 been deposited by subsequent infiltration into the conglomerate 

 with which it is associated. The arguments in favor of its de- 

 trital origin are: (1) Gold is restricted to the conglomerate; (2) 

 the coarser the conglomerate the richer it is in gold content; 

 (3) it was present in the conglomerate before much erosion took 

 place; (4) it is independent of dikes and faults; and (5) it is inde- 

 pendent of those sulphides which serve as precipitants for gold. 



In Australia the detrital gold deposits have furnished one nug- 

 get weighing 233 lb., but as a mass of gold weighing 140 Ib. has 

 been found in quartz reefs, the larger nugget may be of detrital 

 origin. However, through chemical affinity, small particles of 

 gold are often welded together. 



Platinum. Platinum is derived from two types of rocks, the 

 ultra-basic and the basic intrusives, while detrital gold is often 

 derived from both the acid and basic igneous rocks. The rare 

 metals of the platinum group as osmium and iridium are also 

 found in the platinum placers. The Ural placers are the richest 

 in the world in platinum, but placers containing both platinum 

 and gold are found in California. 



Iron. Iron occurs abundantly as beach placers in the United 

 States and New Zealand, but it is seldom present in considerable 

 quantity in other forms than oxides. The black sands of many 

 river valleys are auriferous but they consist essentially of mag- 

 netite derived from the basic intrusives associated with its genesis. 



Tin. The most important tin deposits of the world are de- 

 trital. The oxide of tin, cassiterite (SnO 2 ), stoutly resists dis- 

 integration. During the erosion of the granites, pegmatites and 

 contact metamorphics the tin finds its way toward the bottom of 

 the detritus. It has been found in beach placers in England and 

 in torrential placers in Bolivia. 



* The tin deposits of the Federated Malay States are the most 

 important in the world. The acid intrusives forming the high 

 lands have suffered decomposition and erosion and cassiterite 

 has become the chief detrital placer mineral. 



On the Islands of the Banka and Billiton the tin deposits occur 

 in two forms; (1), "shoad" deposits in which the ore is found near 

 the lodes from which it was derived; and (2), the deep lead placers 

 which may extend over considerable area and varying depths. 



Assorted Minerals. These are often of no economic significance 



