《DIAMONDINVENEZUELAANDGUYANA.doc》由会员分享，可在线阅读，更多相关《DIAMONDINVENEZUELAANDGUYANA.doc（8页珍藏版）》请在金锄头文库上搜索。
1、DIAMOND IN VENEZUELA AND GUYANAHISTORICAL REVIEW of DIAMOND GEOLOGY and MINING Exchange rate between the old and present value is: 1 = US$140.In 1890, diamonds have been found in another part of South America, about 2,000 miles northwest of the famous Brazilian localities, namely, in the gold-washin
2、gs on the upper course of the River Mazaruni in British Guiana. The discovery was made accidentally by Edward Gilkes, who, while prospecting for gold along the Putareng creek, a tributary of the Mazaruni, found a few diamonds in the batea he was accustomed to use for gold-washing. The locality is si
3、tuated in latitude 6 14 N. and longitude 60 18 W., about 150 miles above the town of Bartica(巴蒂卡) on the confluence of the Mazaruni and Essequibo Rivers. It was reached after a twelve to twenty days journey, according to the state of the river, which has many falls and rapids, from Georgetown. The e
4、xact spot is situated about four miles from the Mazaruni, and is reached by a narrow trail across swampy land and through tropical jungle, everything having to be carried on the heads of Indians.The rocks of the Mazaruni valley are largely gneisses and granite traversed by dykes of diabase and other
5、 similar rocks. The diamond-workings at present under consideration are situated on the side of a hill, and penetrate(1) 18 inches of pure, white quartz-sand,(2) 18 inches of yellowish sandy clay, with fragments of quartz and portions of sand and gravel cemented together by iron oxide, in which smal
6、l diamonds are occasionally found,(3) 7 feet-the present limit of working of clay, which becomes more and more gravely and the constituent fragments larger and more frequently cemented together with iron oxide, as greater depths are reached. Some of the pebbles are much rounded, and have sand and sm
7、aller pebbles attached to them by a felspathic cement, while others are sharp and angular. Some consist of felsite and concretionary ironstone, but most are of quartz. Associated with these pebbles are grains of ilmenite and small rounded pebbles of black tourmaline and pleonaste, and occasionally o
8、f topaz and corundum.When dug out, the gravel is carried in wooden dishes to a little creek hard by, where it is washed in sieves of one-sixteenth inch mesh, the residue being picked over while wet.The diamonds, originally found by Mr. Gilkes in 1890, were obtained at the foot of the hill in the bot
9、tom of the valley. The diamantiferous gravel here contains many crystals of quartz, and rests upon a bedrock of kaolin, differing in both these respects from that which lies on the hillside.Up to 1,900 diamonds had been found only over an area of country measuring 200 yards in length by about 100 in
10、 breadth, but it is probable that the diamantiferous gravels are much more widely distributed. The mode of occurrence of the diamond in the gravels and the minerals with which it is associated are very much the same as in Brazil. The diamond has not yet been found here in its mother-rock; if this sh
11、ould at any time be discovered, British Guiana may become an important diamond-producing country.As yet there have been found only some few thousands of small diamonds, which have the form of octahedra, and are exceptionally white and brilliant. The smallest are of very small size and the largest ab
12、out 1 1/2 carats, but there are very few exceeding a carat in weight. During the ten years between 1890 and 1900 between 2,000 and 3,000 diamonds were found, while, according to customhouse returns, the total export of diamonds up to January 28, 1902, amounted to 10,000. A parcel of 282 stones sent
13、to London during the year 1900 was valued at 2 8s. per carat. During a period of six weeks in the following year, a New York company obtained 8,227 small diamonds with an aggregate weight of 767 carats, which were valued at 1,920 or 2 l0s. per carat. A dozen companies have since been organized and a
14、re now at work, and fresh ground is constantly being opened up, so that the diamond-mining industry of British Guiana is likely to develop rapidly.We are indebted for the above account of the occurrence of diamonds on the upper Mazaruni River to Mr. G. F. Kunzs Annual Reports on Precious Stones, in
15、which is brought together much information from various sources.Professor J. B. Harrison, the Government Geologist of that colony, reports another occurrence of diamond in British Guiana. This is in the gold-washing claims of the Omai creek, this stream being a small tributary of the Essequibo River
16、, which it joins at a spot about 130 miles above the mouth of the latter. From a part of the bed of Gilt creek, one of the tributaries of this stream, measuring about 500 feet in length and 50 in breadth, some 60,000 ounces of gold and some hundreds of small diamonds have been recovered by the somewhat crude methods hitherto in use. The auriferous gravels of this stream consist of fragments of more or less decomposed diabase, pebbles of co
