184 The Geology of the Gold F'lalrh of British Guiana. 



The Potaro Goldfield is situated in the \ alley of tlie Mahdia, a 

 fairly large tributary of the P(jtaro lii\er, ilowiiig- in a north-westerly 

 direction and joining tlie Potaro abo^e and at a distance of about three 

 miles soutli-west of the Pakatuk Palls. The ^'alley hes between two 

 convergent ranges of mountains, the higher 2>arts of each oi which show- 

 bare, more or less precipit(jus clilfs standing out from the surrounding 

 forest. The main masses of the mountains are of diabase, the western 

 one l)eing capped with sandstone. They are about two thousand feet 

 in altitude. The lower part of the eastern range consists everywhere 

 of diabase, but above the Sti-ong Hope Placers, at an altitude of about 

 one thousand three hundred feet, a mass of granitite-gneiss, some two 

 to three hundred feet in thickness, occurs, and is covered hy about six 

 hundred and fifty feet of coarse-textured dialiase. The contact-rocks 

 of the granitite-gneiss consist of epidiorite and of (|uartz-diorite of 

 ^•arying texture, resulting from the absorption of parts of the included 

 mass by the diabase. Near the contact of the gneiss with the diabase 

 the resultant rock is in places auriferous to a marked degree, the 

 samples I examined which were taken from tlie tunnel of the Growler 

 Mine, situated on the margin of the granitite mass, yielding from seven 

 to thirty-two pennyweights of gold per ton of the rock. The epidiorite 

 contains cupriferous pyrites in large quantities. 



The diabase of the district is intrusive over a large area, through 

 more or less schistose quartz-porphyries and porj^hyrites. In places 

 near the contacts of these rocks with the diabase they are minerahsed 

 to varying extents, and I have examined various samjDles taken from 

 near the Mahdiana Workings of the Inflexible Syndicate, which yielded 

 at the rate of from two to fifteen iDennyweights of gold to the ton of 

 the rock. Near Hope Placer and near the District Hospital a belt of 

 mineralised, coarse-grained augite-gi'anoph}'re occurs, which yielded, 

 upon assaying, gold at the rate of one and a hy If pennyweights per ton. 



The district to the east of the valley of the Mahdia, in which the 

 Konawak and Tiger Creeks Placers are situated, consists mainly of 

 (|uartz-porphyries and of porphyrites, through which great masses and 

 dykes of diabase are intrusive. The Tiger Creek crosses, at about a 

 mile above the Tiger Creek Falls, a belt of granitite-gneiss which 

 extends for about two miles to the Top Falls of that creek. A reef of 

 granular white quartz traverses quartz-por2)h3^ry near its contact with 

 diabase at the Garnett Syndicate Placers on the head waters of Tiger 

 Creek ; the quartz is auriferous, various specimens from it having 

 yielded to assay from two to sixty-nine jjennyweights of gold per ton 

 of the rock. The Mahdia valley is separated at the south by a ridge 

 of more or less compact quartz-porphyry and intrusive diabase from 

 the head of the valley of the Minnehaha Creek, a tributary of the 

 Konawaruk River, to which it flows in a south-easterly direction. 



This stream has many tributary creeks and its ^-alley rapidly widens 

 as it approaches the Konawaruk. Several shallow shafts and drifts 

 have been driven into the country-rock near to and at Ironside Placer 

 in this valley. Here the contact-rocks derived from quartz-porphyry 

 and dial)ase are mineralised, but to very varying extents, as upon 



