200 The GcoJixjt/ of the Gold F'ipMs of Brifish Guiana. 



numerous instances of brf)a(l belts of granitic rocks intrusive through 

 the gneissose and schistose rocks of British Guiana, the neighl)ourlioods 

 of whicli are, so far as is known, free from placer deposits. Hence I 

 doubt whether in British Guiana the occurrence of l)elts of granitic 

 i-ocks intrusive in the gneiss or the schists ofiers any guide tft the 

 presence of payable jilacer deposits. 1 am, however, in full agreement 

 with Levat in his observations that the granitic areas are themselves 

 marked by the absence of placer-deposits upon them. 



On the other hand all my work in British Guiana tends to show 

 that the occurrence of placer deposits, of veins or reefs of auriferous 

 quai'tz, and of masses of mineralised rocks are, in districts where the 

 eflt'ects of d3^namo-metamorphic forces are strongly marked and are 

 governed by the presence of dykes of basic rocks, eitlier geohtgically 

 of very ancient origin, such as the more or less altered gabbros, 

 epidiorites, and hornblende-schists, or, of more recent origin, the 

 unaltered diabase. 



In districts where the nietamorphic forces have converted the felsites 

 and porphj^rites into well-defined schists, and the older basic rocks into 

 amplnbolites, epidiorites and hornblende-schists, and which have been 

 subject to later intrusions of diabase, the richest placer deposits of the 

 colony occur. Instances of this are the Omai, the Mahdia and the 

 Arakaka Goldfields. 



But the views of De Launay, Levat, Lungwitz, Du Bois, Braddon, 

 Perkins, and myself that the basic rocks are directly or indirectly the 

 main sources of the gold in the Guianas are not universally accepted. 

 For instance, in their work on Ore Deposits, Phillij^s and Louis state that 

 in British Guiana " Quartz veins occur mostly in metamorphic schists 

 and gneiss, and nearly all the streams and rivers that traverse regions 

 occupied by the above rocks or by granite are gold-bearing," whilst 

 J. E. Spurr, in his work on the " Ore Deposits of the Silver Peak 

 Quadrangle, Nevada," published in 1906, on pp. 149 to 151, critically 

 discusses the various views whicli have been published relative to the 

 source of gold in the Guianas, and arrives at the conclusion (p. 150) that 

 " in British Guiana the deposition of the gold ores represents one of the 

 closing phases of the gi'eat granitic intrusions, and that the basic dyke 

 rocks, with which the gold ores are associated, as well as the siliceous 

 dyke rocks, in connection with which they are also frequently found, 

 are representatives of the general process of granitic injection, earlier 

 than the veins, whilst subsequent to the main intrusion." 



In connection with the above theory of the source of the gold in 

 this colony it is important to note that the older gabbro, wherever 

 found unaltered, contains in greater or less abundance areas and 

 especially interstitial patches of a mici-o-pegmatite of feldspar and quartz, 

 and that by far the greater number of specimens of diabase which have 

 been examined show a similar structure, whilst in places the diabase 

 l>asses into a quartz-diabase or into an augite-granophyre, some samples 

 of which are gold-bearing. It is quite j^ossible that the occurrence of 

 gold in the basic rocks of the colony is more or less closely connected 

 ^\ith the presence of the quartz-feldspar micro-pegmatite and that the 

 micro-pegmatitic areas in the basic rocks may be exam2:iles on a minute 



