192 The Geology of the Gold Fields of British Guiana. 



The carbonates present in the first of these were separated and 

 were found to amount to 14-33 per cent, of the rock. They were made 

 up of 3-03 of iron carbonate, 8-89 of calcium carbonate, and 2-41 per 

 cent, of magnesium carbonate. 



The rock is essentially an aplitic phase of the widely distributed 

 rock named by J. E. Spurr "Alaskite," which is not unfrequently 

 auriferous. Probably the Arzruni quartz reef at Omai, which contained 

 gold, with tellurides, iron pyrites and scheelite, was an example of a 

 highly auriferous magmatic quartz vein traversing alaskite. 



Many samples of the mineralised aplite were examined in the 

 Government Laborator}', and, where free from veins of quartz, yielded 

 upon assay at the rate of from one to eighteen pennyweights of gold 

 per ton of the rock. The greater number of them, however, yielded 

 only at the rate of from oiie and a half to two and a half pennyweights 

 of gold per ton. 



I have made most careful microscopical examinations of many slides 

 cut from the aplite at Omai, and have failed to find in them any 

 indications that the aplite was enriched by injections of micropegmatite 

 after it had attained a pasty state ; but the numerous veinlets of quartz 

 which traverse it may be, in part at any rate, the final consolidation 

 products of the original intrusions, and as they are markedly richer in 

 gold than is the bulk of the rock, the residual quartz-liearing solutions 

 which gave rise to them may have carried the precious metal in higher 

 proportion than did the mass of the intrusion. 



The Omai aplite shows signs of two changes : — • 



{a) Metamorphism, resulting in the production of sericite in 

 abundance, and the almost total conversion of alkali-feldspar 

 into muscovite. This change appears to have been accompanied 

 by an infiltration of quartz and probably of part of its gold. 



{!)) Infiltration, probably from the decomposition of the diabase 

 above it, of the carbonates of calcium, magnesium and iron, 

 with silica and auriferous pyrites. This was probably accom- 

 panied by enrichment by gold derived from the diabase ; whilst, 

 as shown by Dr. Lungwitz, the undecomposed mass of aplite 

 was further enriched by downward infiltration of the metal 

 from the mass of aplite, now decomi^osed, which covered it. 



The quartz-diorite and epidioi'ite of the country, at Omai, generally 

 do not show signs of having been afl'ected to any marked degree by the 

 intrusion of the aplite, they are traversed, however, by thin veins of 

 quartz, and in places by white and yellowish-white veins of a coarsely 

 crystalline mosaic of zoisite and epidote, with some cak-ite and very 

 thin films of quartz. 



The epidiorites and quartz-diorite, probably, were derived from a 

 basic hornblende-porphyrite, some of which, in a slightly altered 

 condition, was traversed in the deeper parts of a boie-hole driven through 

 aplite into epidiorite at Omai. 



At some distance from the aplite the country consists largeh' of 

 chlorite and of chlorite-epidote schist. These are highly metamorphosed, 



