2. 外语
  3. According to a recent theory, Archean-age gold-quartz vein systems were formed over two billion years ago from magmatic fluids t

According to a recent theory, Archean-age gold-quartz vein systems were formed over two billion years ago from magmatic fluids t

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问题 According to a recent theory, Archean-age gold-quartz
vein systems were formed over two billion years ago from
magmatic fluids that originated from molten granitelike
bodies deep beneath the surface of the Earth. This theory is
(5) contrary to the widely held view that the systems were
deposited from metamorphic fluids, that is, from fluids that
formed during the dehydration of wet sedimentary rocks.
he recently developed theory has considerable practical
importance. Most of the gold deposits discovered during
(10)the original gold rushes were exposed at the Earth’s surface
and were found because they had shed trails of alluvial
gold that were easily traced by simple prospecting methods.
Although these same methods still lead to an occasional
discovery, most deposits not yet discovered have gone
(15) undetected because they are buried and have no surface
expression.
The challenge in exploration is therefore to unravel the
subsurface geology of an area and pinpoint the position of
buried minerals. Methods widely used today include
(20) analysis of aerial images that yield a broad geological
overview; geophysical techniques that provide data on the
magnetic, electrical, and mineralogical properties of the
rocks being investigated; and sensitive chemical tests that
are able to detect the subtle chemical halos that often
(25) envelop mineralization. However, none of these high-
technology methods are of any value if the sites to which
they are applied have never mineralized, and to maximize
the chances of discovery the explorer must therefore pay
particular attention to selecting the ground formations most
(30) likely to be mineralized. Such ground selection relies to
varying degrees on conceptual models, which take into
account theoretical studies of relevant factors.
These models are constructed primarily from empirical
observations of known mineral deposits and from theories
35) of ore-forming processes. The explorer uses the models to
identify those geological features that are critical to the
formation of the mineralization being modeled, and then
tries to select areas for exploration that exhibit as many of
the critical features as possible.
选项 A、advocating a return to an older methodology
B、explaining the importance of a recent theory
C、enumerating differences between two widely used methods
D、describing events leading to a discovery
E、challenging the assumptions on which a theory is based
答案B
解析 The best answer is B. In the first paragraph the author describes a recent theory concentrating the formation of Archean-age gold-quatz vein systems, and in the second paragraph this theory is said to have “considerable practical importance” (lines 8-9). The remaining paragraphs explain why such theories of ore-forming processes are important for explorers seeking to locate gold deposits. The older method of prospecting for gold is mentioned, but rather than advocating this method (choice A), the author explains why prospecting is no longer viable. The author neither discusses differences between widely used methods (choice C) nor describes the events leading to a discovery (choice D). Although another, widely held view concerning ore-forming processes is mentioned, the author does not challenge the assumptions on which this view is based (choice E).
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