Diamond Placer

Occurrence. The diamonds occur in the gravels and old marine deposits of the Colony, and are known to the trade as "sand diamonds", and average from 15 to 25 stones to the carat, although larger stones from 1/2 to 1 carat are occasio­nally found.

The diamondiferous gravel consists of pieces of quartz - sometimes well rounded but usually quite angular - embedded in a clayey matrix, containing pieces of staurolite, black garnetiferous sand, and small grains or nuggets of gold. The gravel, which varies from 2 to 6 feet in depth rests on a decomposed schistose bedrock usually of a yellowish or bluish green color, and is overlain by a variable thickness of overburden, consisting of a clayey alluvium.

Three types of gravel are commonly encountered:

a) a red gravel of lateritic nature, poor in diamond values, usually forming the upper reaches of the valleys.

b) a sandy clayey gravel, blue in color.

c) a blue clayey gravel, rather stiff, found bordering and extending outwards for considerable distances from the present river beds. Both (b) and (c) usually carry good diamond values. The crystallized diamonds are generally octahedra and dodecahedra, but muckles and borts are also fairly common, as are also cleavage flakes.

COMPREHENSION CHECK

Exercise 8. Complete the sentences using the information from text 2:

1. Gold placers result from ...

2. The best conditions for the concentration of gold into placers are found in ...

3. Because gold is six or more times heavier than ordinary rock, the liberated particles of gold will concentrate along …

4. Crystallized gold which is sometimes found in placers indicates …

5. The diamondiferous gravel consists of … .

Exercise 9. Find the English equivalents of the words and word combinations in the text:

золотоносный; золотосодержащие выходы; русло реки; жила замещения; чешуйчатое золото; коренная подстилающая порода (плотик); самородок; наносные образования, аллювий (россыпь); карат; алмазоносный песок.

LANGUAGE REVIEW

Exercise 10. Rewrite the sentence as in the example:

Example: Ancients used gold to decorate their tombs and temples. - Gold was used to decorate tombs and temples by ancients.

1. They valued the largest ever gold nugget at $43,534 at the time of its discovery.

2. A series of sieves and riffles separates any gold from unwanted minerals or materials.

3. Specialists predict that the deepest shaft of the mine will be able to access over 12.5 million ounces of gold.

4. Geochemists analyzed large number of samples for tracing elements incorporated in igneous rocks.

5. Special computer program determines coal seam thickness, and the nature of disturbances.

Exercise 11. Choose the right form of the adjective:

1. (The most popular/more popular) methods of obtaining gold during the gold rush were by panning and sluicing.

2. (The deepest/deeper) gold mine in North America is located in Quebec, Canada.

3. (The latest/later) innovation in stripping is the use of high-speed, rubber-tired two- and four-wheeled tractors for long-haul work.

4. New technologies lead to a rapid growth in labor productivity and make the miners' work (more safer/safer).

5. Gold is six or more times (heavy/heavier) than ordinary rock.

SPEAKING

Exercise 12. Complete the sentences based on the information in the text:

1. Gold placers or deposits such as gravel and sand al1 result from ...

2. Gold may be dissolved and carried below by means of natural chloridation processes that are established when ...

3. As the rocks of a region break up and decay under weathering, rainfall ...

4. Most of the gold in a placer usually rests ...

5. The high insolubility of gold in most surface waters is demonstrated by ...

Exercise 13. Discuss the questions with your partner:

1. The best conditions for the concentration of gold into placers are found in moderately hilly regions where deep secu­lar decay of the rocks has been followed by slight uplift. Why?

2. Among the best types of bedrock are compact clays, somewhat clayey decomposed rock and slates or schists. Why?

3. There are three types of diamondiferous gravel. What are they?

Exercise 14. Answer the following questions based on the information found in texts 1 and 2 and on your own experience.

1. What is the origin of gold placers?

2. What are the main features of gold placers?

3. What are the best conditions for the concentration of gold into placers?

4. Where are the beat placer concentrations?

5. How is gold distributed in a placer?

6. Is gold high insoluble? How can we demonstrate this fact?

7. How can gold be dissolved?

8. What does the diamondiferous sand consist of?

9. What type of gravel is rich in diamonds?

VOCABULARY

sо far - до сих пop

go gopher - рыть, производить бессистемные разведки

crude - сырой, необработанный

unsteady - неустойчивый, неравномерный

decline - спад

precipitous - крутой, отвесный

overlain - перекрывать

slope - склон

abrupt - резкий

descent - спуск

perennial - круглогодичный, вечный

scanty - скудный

exhaustion - истощение

to settle down - оседать

bar (of gold) - самородок (золота), нанос песка

gorge - затор, нагромождение

adobe - кирпич воздушной сушки

gulch - узкое глубокое ущелье

traverse - поперечина, перекладина

boulder - валун, галька

wash - песок, гравий, аллювий, старое русло реки, овраг, балка

to cling - прилипать, цепляться

lack - отсутствие, недостаток

Exercise 15. Find the meaning of the word combinations and make up sentences with five of them:

crude expedient methods; seasonal rainfall; costs of operation; above sea level; coarse outwash; perennial streams; to descent abruptly; branching; scanty water supply; natural tanks; higher-grade-placers; apparent exhaustion; gold-bearing quartz veins; exposures of bedrock; a quarter-inch screen; rough and angular gold

READING

Exercise 16. Read the text below looking for the main ideas concerning location, history and the current operation of the La Paz placers: