190 The Geology of the Gold Fields of British Guimia. 



Auriferous (juartz veins have been discovered in the Demerai'a Gold- 

 field at Kanaimapu, Appaparu, and Darina, in a district about three 

 to four miles north of the Great Falls at Ororu Marali, and have been 

 worked to some extent. The richest vein at Kanaimapu was in a 

 deep-red laterite, 23robably the decomposition-product of epidiorite or 

 possibly of diabase. Its average contents was about twenty-four 

 pennyweights of gold to the ton of quartz. The principal vein 

 occurred in a similar dark-red clay, and passed through an altered basic 

 rock into a more or less mineralised granitite, intersected by veins of 

 fine-grained quartz-porphyry. The quartz veins, where in the red clay, 

 were everywhere more or less auriferous, rich ore-pockets, some yielding 

 as much as four to five ounces of gold to the ton of quartz, occurring 

 here and there in them ; but the average contents of the princi^^al vein 

 was only about two and a half pennyweights to the ton of quartz. 

 The quartz of the vein near the contact of the basic and acidic rocks 

 was fairly rich in gold and in pyrites, assaying about thirty-seven 

 pennyweights of metal to the ton, but the vein in the acidic rock 

 proved to be practically barren, not yielding more than one pennyweight 

 of gold to the ton. 



The auriferous quartz veins at Darina are in epidiorite and horn- 

 blende-schist, whilst those at Appaparu traverse quartz-schist and 

 chlorite-schist. The country in which these veins occur is intruded by 

 numerous dykes of diabase. 



Gold-bearing Mineralised Masses. — The Essequibo-Potaro-Kona- 

 waruk Gold Fields are remarkable for the occurrence in them of 

 auriferous mineralised masses of acidic, and, in places, of basic rock. 



The principal of these at present known is the one at Omai, the 

 general geological structure of which has been already described. 



The mineralised rocks at Omai consist chiefly of aplite intruded 

 through a country of epidiorite, chlorite and sericite schists, these 

 being also mineralised to a greater or lesser extent. The mass of 

 aplite has been proved to a depth of nearly one thousand feet, 

 and in its widest part has a breadth of about five hundred yards. 

 It, in common with the country rock, is traversed by intrusive 

 dykes, sills and veins of diabase, which at one tiuie covered the 

 district, but has since largely been removed by decomposition and 

 detrition. 



The altered aplite has a specific gravity of from 2-69 to 2-75, and is 

 a light-greyish coloured, fine-grained granitic rock, containing veins of 

 quartz and of carbonates, with small crystals of cupriferous pyrites 

 generally, though very irregularly, distributed through them. It is 

 made up of very abundant clouded j^lates of plagioclastic feldspar, 

 largely albite, in places some small plates of orthoclase and a little 

 mici-opegmatite ; irregular patches of original quartz showing strain- 

 shadows which are, in places, moi'e or less granulated ; many inter- 

 stitial patches of secondary (juartz ; a very few small wisps of more or 

 less altered, generally chloritised, biotite ; and some grains of magnetite 

 and of titaniferous iron ore. Practically all the feldspar-plates contain 



