NOTE ON COPPER IN ANDESITE. 51 



A representative portion of the specimen was selected 

 for chemical analysis which was made in the University 

 Chemical Laboratory, and resulted as follows: — Copper 53J / 

 iron 7f°/°, sulphur 21J°/°, insoluble 16^°/°, unestimated l%°j°. 

 The 'unestimated' would include substances like Al 2 O g , 

 CaO, Na 2 0, K2O, Sb and Sn. 



If the insoluble portion be left out of consideration, 

 the copper forms about 64°/° of the soluble portion which 

 indicates a mixture of chalcocite (CugS) and bornite 

 (Cu 3 FeS 3 ). After arriving at this result, I examined half 

 a dozen sections of andesitic rock from the same district 

 (petrological note appended). In two of these I recognised 

 the same blue-black and velvety-brown minerals which 

 form the bulk of the specimen already described, with 

 this difference that they occur in minute cubes scattered 

 about sparingly like magnetite and pyrites in ordinary 

 rocks. I accordingly powdered about 5 grams of each of 

 these two rocks and mixed the powders. Then I took 5 

 grams of the mixture and digested it with aqua regia, pre- 

 cipitated the copper as sulphide and weighed it as oxide. 

 The weight recovered was '0017 gr. of OuO equivalent to 

 •034°/° of OuO. It is therefore evident that the andesites 

 of the Lautoka district are copper-bearing and that the 

 copper exists in them as sulphide. Both specimens of 

 cupriferous andesite come from Vilau (or Ni-Vilau) near 

 Lautoka. 



Judging by the microscopic appearance, not unlike that 

 of some alloys, the ore specimen might have originated 

 either as a segregation (which appears most probable) 

 formed during the cooling of the lava, or by the metaso- 

 matic replacement of a glassy base by copper sulphide, 

 during or immediately after consolidation, through the 

 agency of magmatic waters. The arguments in favour of 

 the supposition that the ore is a segregation are : — 



