请帮忙分析一下这篇文章的逻辑脉络与结构框架

红红的云

非常感谢。看懂了意思，弄不懂结构。

og－32－43

passage 32

    

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.

[此贴子已经被作者于2009-6-16 21:24:02编辑过]

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