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Scientists searching for precious metals have turned to the ocean floor, where natural chimneys are spewing out a metal-enriched black dust containing particles of gold, silver and zinc.

Scientists   31   these hot springs are recreating the process which, billions of years ago, created   32   metal deposits now found on land.

The discovery is giving geologists a   33   into the earth’s early history and fuelling some new theories on origin of life.

It also has huge implications for   34   companies.

Geologists are just beginning to understand how these chimneys,   35   clusters of sulphur and minerals, are formed, and what makes them spew out the mineralized dust.

At present it is not commercially   36   for mining companies to operate beneath the sea, although some scientists believe the “black smokers”,   37   known as active mineralizing systems, will be a major—and renewable—source of metals in the next decade.

38   the meantime, mining companies are using the ocean-floor research to locate similar deposits on land.

“The hottest thing in the mining research game right now is the   39   within the past few years of mineral deposits currently forming—in front of our eyes—on the ocean floor,” said Dr. Joseph Fox, a Montreal (加拿大蒙特利爾) geologist.

Canada has mined some of the richest copper, zinc and gold   40   in the world. In the past year, mining companies have used knowledge about where mineral formations   41   on the ocean floor to find the deposits on land.

Geologists are excited because,   42   metal deposits on land, which are two or three billion years old, the undersea deposits keep   43   themselves.

“It’s really incredible to think that we have a renewable metal resource   44   we’ve been taught to think of metal resources as non-renewable,” Fox said.

The 30-foot-high (10-metre) chimneys or vents,   45   in 1979, are found along fractures in the ocean’s crust.

Scientists believe the deposits form when cold sea-water seeps into the fractures, leaving metals   46   it is drawn down.

As the water travels in the direction of the earth’s core, it   47   up. Eventually, the hot water rises, carrying with it the hot metal sulphide   48   the ocean floor.

When the hot sulphide meets the cold sea water, a thick black smoke-like substance is formed, spewing out of vents in built-up deposits of   49  .

The particles in the smoke eventually   50   on the ocean floor, forming vast solid sheets of metal sulphide.

31. A. believe B. thought C. uncover D. found

32. A. smooth B. tiny C. vast D. rust

33. A. chance B. revision C. weapon D. glimpse

34. A. metal B. mining C. alloy D. global

35. A. made of B. consisting in C. resulted from D. dealing with

36. A. periodic B. reliable C. comparative D. feasible

37. A. formally B. chiefly C. economically D. occasionally

38. A. At B. On C. In D. For

39. A. invention B. discovery C. findings D. theory

40. A. samples B. deposits C. mines D. fractions

41. A. range B. suffer C. occur D. form

42. A. unlike B. like C. as D. except

43. A. to renew B. renewing C. having renewed D. to be renewed

44. A. before B. until C. because D. when

45. A. broken B. fixed C. discovered D. originated

46. A. since B. as C. for D. whereas

47. A. speeds B. goes C. gives D. heats

48. A. from B. on C. toward D. beyond

49. A. sulphide B. substance C. deposits D. element

50. A. rely B. move C. turn D. settle

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