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  3. According to a recent theory, Archean-age gold-quartz vein systems were form

According to a recent theory, Archean-age gold-quartz vein systems were form

游客2024-01-13  25
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 granite-like bodies deep beneath the surface of the Earth. This theory is 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. The recently developed theory has considerable practical importance. Most of the gold deposits discovered during 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 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 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 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 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 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. [br] Which of the following statements about discoveries of gold deposits is supported by information in the passage?
选项 A、The number of gold discoveries made annually has increased between the time of the original gold rushes and the present.
B、New discoveries of gold deposits are likely to be the result of exploration techniques designed to locate buried mineralization.
C、It is unlikely that newly discovered gold deposits will ever yield as much as did those deposits discovered during the original gold rushes.
D、Modern explorers are divided on the question of the utility of simple prospecting methods as a source of new discoveries of gold deposits.
E、Models based on the theory that gold originated from magmatic fluids have already led to new discoveries of gold deposits.
答案 B
解析 哪一个关于金矿发现的信息在文中有?A．最初淘金热到如今的金矿数目。文中未提。B．正确。新金矿发现是对确定地下矿化层的探测技术的成果。文中最后两段都提到，要发现一个矿床，先要确定矿化地区，而文中所说的模型就起到此作用。另外，文中L19—23，现在大部分金矿未被发现，因为其深埋地下，没有地表特征，也可推知，新发现金矿需满足本选项条件。C．产量问题。文中未说。D．简单探矿法能否用于探新矿，现代探矿者有分歧。未提。E．已发现新矿床。无。文中只说“considerable practical importance”。
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