196 The Geology of the Gold Fields of British Guiana. 



the rock are probably infiltration-products from the great masses of 

 diabase which once overlaid them, and which are now represented by 

 great depths of laterite in the forms of more or less auriferous red and 

 ochreous clays and concretionary ironstone, whilst the gold in the 

 granophyi-e may have been enriched by infiltration of some of that 

 originally present in the diabase, and by impregnation during its 

 extrusion. 



Mineralised masses occur at and near the Ironside and Providence 

 Placer workings in the Minnehaha Valley. These are mineralised 

 pDrphjrry, aplite and granitite, and in parts are very rich in the 

 precious metals. The mean contents of the samples examined 

 in the Government Laboratory were sixty-five pennyweights of gold 

 and twenty-three pennyweights of silver per ton of the ore, but the 

 proportions varied widely in different samples^ — ^from thirty-one grains 

 to three hundred pennyweights of gold, and from twelve grains to one 

 hundred and fifteen pennyweights of silver. 



Samples of minei'alised and weathered granitite from Providence 

 yielded from ninety-five to one hundred pennyweights of gold per ton 

 of the rock. 



The original mineralisation of these masses resembles that of the 

 Omai aplite, and is due to similar causes, and, in addition, the rocks 

 have undergone enrichment during the extrusion and the subsequent 

 decomposition of the vast sills of diabase through which the valley of 

 the Minnehaha have been eroded. 



In places in the Potaro gold district mineralised masses of basic 

 rocks occur. One of these is at the Growler Mine, situated about 07ie 

 thousand three hundred and fifty feet above the valley of the Mahdia, 

 on the Eagle Mountain range. It consists of epidiorite of varying 

 texture, passing to quartz-diorite. These rocks are near the contacts 

 of diabase with an enormous caught-up mass of granitite-gneiss. The 

 coarser-grained varieties consist of dark-green, green, and olive-green 

 hornblende, some biotite, a little muscovite and a few plates of augite : 

 patches of quartz, showing strain-shadows ; large plates of plagioclase- 

 feldspar, clouded with sericite ; with magnetite, sphene, a few pi'isms 

 of apatite, some chlorite, and a little epidote as accessories. The 

 finer-textured varieties of the rock contain numerous small cubes of 

 pyrites, and are auriferous, the samples examined in the Government 

 Laboratory yielding from seven to thirty-two pennyweights of gold, 

 and from three to twenty pennyweights of silver per ton of the rock. 



In the course of the Konawaruk River near Willis' Landing 

 epidiorite and hornblende-schist occur, which are auriferous, yielding, 

 upon assaying, gold at the rate of two ^pennyweights per ton of the rock. 



In the Cuyuni River district between Tinamu and Paiyuka 

 Cataracts an area of a more or less mineralised granitite-gneiss occurs, 

 the mineralisation of which is due to intrusions of diabase. The gneiss 

 is much altered by the diabase, its original ferro-magnesian minerals 

 are completely destroyed, and are now represented by patches of 

 minute granules of magnetite and augite, whilst the feldspars are 

 darkened by the development of innumerable exceedingly minute 



