Quartz Veins and Mineralised Masses. 187 



increased, having been added to by silica dissolved from the decomposing 

 rocks by percolating waters. At the same time as silica was thus 

 deposited a concentration and deposition of the gold contained in the 

 decomposing rocks took place, and thus the quartz veins became enriched 

 in this metal in proportions varying with those present in the country 

 rock, with the result that the veins found in decomposed epidiorite and 

 hornblende-schist are, as a rule, i-icher in gold than are those found in 

 decomposed acidic rocks. A vein at the Barima Mine was enriched so as 

 to yield gold at the rate of thirty-one ounces to the ton. The supposition 

 of secondary enrichment is supported by the fact that the very friable 

 secondary quartz which, not unfrequently, forms the hanging walls of 

 the true veins of quartz in the Arakaka district is, as a rule, richer in 

 gold than are the veins themselves. 



The veins of quartz which traverse the dark-red decomposition- 

 products of the basic rocks are frequently very rich in gold at and near 

 their outcrops, these parts being far more auriferous than are the 

 remaining portions of the veins. Similarly the talcose selvages of these 

 veins are often highly auriferous. 



Cuyuni River District. — Few quartz veins were noticed during the 

 examinations of the Cuyuni River district, and these were mainly thin 

 segregation veins in decomposed porphyry, porphyrite, and schists 

 derived from them. Narrow veins were seen in hornblende-schist at 

 the Arimu Cataracts and at the Arawak Matope Cataracts. Evidence 

 was obtained that those of the former carried gold in small amount, 

 and that some of those at Arawak Matope were magmatic quartz veins 

 derived from neighbouring granitic intrusions. 



Veins of quartz traverse hornblende-schist at Markabu Island, and 

 near the mouth of the Oko Creek, which yield gold in quantities varying 

 from traces to about fifteen grains to the ton of quartz. 



At Wariri somewhat auriferous quartz, in veins and as a large 

 lenticular mass, occurred in a red clay, the decomposition-product of an 

 araphibolite. Veins of quartz traversing quartz-diorite, near to Wariri, 

 yielded gold uj^on assay at the rate of fourteen grains to the ton of rock. 



Masses of quartz occur in the mineralised granitite-gneiss near 

 Paiyuka, but are practically barren. Between Paiyuka Cataracts and 

 Popekai Rapids the gneiss is traversed in places by pegmatite veins 

 which pass into magmatic quartz veins. Great masses of white quartz 

 also occur in this part of the river's course, and one at Quartz-stone 

 Island contains numerous nests and needles of schorl. They are, 

 so far as regards their outcrops, practically barren in gold. 



The MazanDii-Fiiruni District. — In the gneissose and granitic 

 rocks, through which the Mazaruni flows in the lowest parts of its 

 course, veins of pegmatite and of aplite passing into veins of quartz are 

 of frequent occurrence, but with the exception of a wide vein of 

 quartz near Kalacoon they are, as exposed, all practically free from 

 gold. The Kalacoon vein yielded upon assay at the rate of thirty-six 

 grains of gold to the ton of quartz. This vein appears to be an off-set 



