1、DIAMOND IN VENEZUELA AND GUYANAHISTORICAL REVIEW of DIAMOND GEOLOGY and MININGExchange 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-washing
2、s 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 sit
3、uated 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 ex
4、act 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 small
6、 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 sma
7、ller 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 of
8、 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 bott
9、om 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 sho
11、uld 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 abo
12、ut 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 t
13、o 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 ar
14、e 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 w
15、hich 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 somewh
17、at crude methods hitherto in use. The auriferous gravels of this stream consist of fragments of more or less decomposed diabase, pebbles of concretionary ironstone and angular quartz. They yielded at one time hundreds of very small diamonds, the majority of which were perfectly clear and colourless
18、octahedra, the remainder being of various shades of pink, green, and clear yellow.It is stated by Mr. G. F. Kunz that in Dutch Guiana also diamonds have been found for years past in the tailings of the gold-washings. They have been for the most part small and have attracted but little attention, the
19、 gold being the chief object sought for. One fine stone, however, is reported to have been found about the year 1890 and to have been cut in the United States.Diamond Geology 1 India 3 4 5 6 7 8 Brazil 10 11 12 13 14 15 16 17 18 19 20 Borneo 22 South Africa 24 25 26 27 28 29 30 31 32 33 34 35 36 37
20、38 39 40 Venezuela, Guyana 42 Australia 44 Argyle Congo 46 47 48 49 50 51 52 53 54 55 Angola 57 58 59 Guinea Related links: Diamonds: Large and Famous Properties Geology and Mining Diamond Cutting Gem Cutting Diamond Trade Values of diamondsRecomend this page:DIAMOND IN VENEZUELA AND GUYANAThe Amazo
21、nian craton consists of the Guapore Shield in Brazil and the Guyana Shield in southern Venezuela, Guyana, Surinam, French Guiana and northern Brazil. It consists of several distinct geologic elements whose style of tectonism and age can be correlated with events in West Africa. Gneisses, amphibolite
22、s, itabirites and intrusive granites characterize the Imataca Complex in Venezuela. Pyroxene granulite facies rocks in the Imataca Complex have been dated between 3.6 and 2.7 Ga. Within Imataca Complex Gurian event was defined at about 3 Ga, which is equated to the Leonean in West Africa, and a late
23、r Imatacan event, dated at 2.7 Ga, is equivalent to the Liberian event in West Africa. The transamazonic event, dated at 2.0-1.7 Ga and characterized by felsic magmatism, correlates with the Eburnean event in West Africa. We can deduce that most of the Precambrian the Amazonian and the West African
24、cratons were a single unit or in relatively close proximity.A major feature of the Guyana Shield is a thick sequence of relatively horizontal Precambrian arenaceous and clastic sedimentary rocks forming Roraima Group. The contact between Roraima Group and underlying basement is unconformable. Extens
25、ive diabase dikes and sills penetrated Roraima Group about 1.54 to 1.71 Ga. Ripple marks, pebble imbrications and crossbedding are common features in Roraima Group units, which suggests dominant stream and deltaic sedimentation. The minimum age, based on 40Ar/39Ar data is 1.84 to 1.8 Ga for the Rora
26、ima sequence in Guyana, while Rb/Sr in pyroclastics in Surinam suggests 1.66 Ga for Roraima and 1.87 Ga for rocks upon which rests the Roraima Formation.Alluvial diamond deposits in Venezuela and Guyana are mainly associated with Roraima Formation, which suggest that sources are found within sedimen
27、tary sequences of it. All rivers and streams that flow along or across Roraima group contain diamonds. In Venezuela, terraces or recent gravels of Caroni, Paragua and Cuyuni Rivers and their tributaries have alluvial diamonds. In Guyana, Mazaruni, Potaro, Irang and Cuyuni Rivers were and are exploit
28、ed for diamonds. Gravel of most of tributaries, such as Ekareku, Meamu, Kurupung, Eping, Arnik, Echirak of these major Rivers are diamond bearing.To date, no kimberlite or lamproite indicator minerals, like pyrope garnet, Mg ilmenite, chromite or chrome diopside, have been found in heavy minerals su
29、its in the alluvial deposits of Venezuela or Guyana. The only exception, in Venezuela, is alluvial diamonds deposit found in Quebrada Grande tributary of Rio Guaniamo. This deposit, worked since 1969, is covering an area of 15 km in length by 6 km wide and produced since it discovery over 6 million
30、carats. Kimberlitic garnet, Mg ilmenite and chromite are found in heavy minerals of Guaniamo deposit.Essentially all of the diamonds in the Guaniamo placer deposits are of local origin, and has been derived from kimberlite sills, rather than having been recycled from ancient sediments. Variations in
31、 characteristics of the diamond crystals from various deposits suggest that some have been derived from kimberlites that remain to be discovered.Most mineral inclusions in the diamond at Guaniamo are of the eclogitic paragenesis, and were likely derived from peraluminous mafic rocks. Significant num
32、bers of inclusions of a peridotitic (lherzolitic and harzburgitic) association e.g., chromian pyrope, chromian spinel, olivine, also have been found. One inclusion of ferroan periclase may represent the superdeep paragenesis, derived from the lower mantle.Most of the diamond from an eclogitic associ
33、ation has isotopically light carbon (_13C from -10 to -25), whereas the diamond from a peridotitic association is isotopically heavier (_13C from -3 to -9). On this basis, we estimate that 93 2% of the diamond at Guaniamo belongs to the eclogitic paragenesis.P-T estimates on mineral inclusions sugge
34、st that most are derived from near the base of the lithosphere (T 1200-1300 C). This zone may contain a substantial proportion of eclogite formed by subduction of crustal material.The very high proportion of diamond from an eclogitic association in the Guaniamo deposits, and several features of the
35、mineral inclusions in the diamond, show striking parallels to the Argyle deposit of Australia; both deposits occur within cratons that have experienced extensive Proterozoic tectonothermal activity.The most probabl source of alluvial diamonds in Roraima Formation are layers of conglomerate found on
36、the top of thick (800 m.) red sandstone layers. In Guyana, some crystals of diamonds found in alluvial deposits of Mazaruni and Kurubrang Rivers and its tributaries have thin brown to green outside layer (skin). This is characteristic of diamonds exposed to radiation for a long period of time. In co
37、nglomerate layers of Roraima Formation, associated with diamonds, uranium and thorium are abundant. As result, many alluvial diamond deposits in Guyana have higher than background thorium radiation levels.The largest diamond found in 1942 in Venezuela at St. Elena, near Brazilian border, is 155 cara
38、ts rough. The diamonds obtained from alluvial deposits in Venezuela and Guyana are mainly between 0.1 to 0.2 carats. Less than 5% of alluvial diamonds are above 0.5 carats. Only some 3 to 5 carats stones are found. For example, a miners parcel of 800 carats of diamonds from Mazaruni River in Guyana
39、had four stones between one and two carats and two stones of 3.2 and 4.8 carats each. The size of diamonds reduces with distance from Roraima Plateau gorges. Largest percentage of large diamonds is found in Rivers and Streams flowing over Roraima Formation, like Ekareku, Meamu, Kurubrang or Kurupung
40、 and tributaries. The gem to industrial ratio in Venezuela is 47% gem, 43% industrial and 10 bort, while in Guyana the ratio is 66% gem, 26% industrial and 8% bort.In Venezuela the recorded production between 1950 and 1968 was in average 100,000 to 120,000 carats/year. The discovery of Guaniamo depo
41、sit increased the production of diamonds to 3.5 millions carats between 1970 and 1974. By 1979, 77% of diamonds produced in Venezuela came from Guaniamo deposit. The ratio of gem to industrial dropped to 27% gem 43 % industrial and 30% bort. In general paleo-alluvial deposits sources provides higher
42、 gem to industrial ratio than kimberlite or lamproite sources. This is a consequence of abrasion in a high-energy fluviatile transport. Diamonds with imperfections (flaws, fractures, etc) are reduced to diamond sand while diamonds without flaws are retained. In Mazaruni River some diamond bearing al
43、luvials have up to 10 carats per 1 m3 of diamond sand.In Guyana, the diamonds were mined from 1890 and the official production for 2003 is over 400,000 carats. This figure is from government records and represents an unknown percentage of actual production. The most significant fact is that in 2000
44、Guyana only produced around 80,000 carats, so the last 3 years jump in production reflects the contribution of Brazilian small miners that are recently moving into Guyanas diamond fields. It reminds me of January 1990, when first Brazilian crews arrived with their equipment to Guyana from Boa Vista
45、as result of my promotional visit there. With brazilians arrived first portable diamond jigs and brazilian made gravel pumps, which greatly contributed to change for the better diamond mining techniques in Guyana. Previously, only sluice boxes were used to recover diamonds. My repeated test demonstr
46、ated that sluice box recovers only up to 5% of diamonds below 1 carat and 0% of larger stones. Still Guyanese small miners were making money using sluices as diamond recovery system. It only indicates how rich some of alluvial diamond deposits are. In 2004, at Itabali Landing on Mazaruni River, I me
47、t one of the first Brazilians that arrived with me in 1990. Now, he was the owner of two brand new CAT excavators. a positive change since 1990.The average price for Guyanese diamonds is between US$100 to 120 per carat. The Venezuelan diamonds are of lesser quality and many visiting diamond dealers come to buy in Guyana, after visiting Venezuela and Roraima State in Brazil, to improve the quality and price of diamond parcels they will later sale in New York, London, Antwerp or Tel Aviv.
