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  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 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.
According to the passage, the widely held view of Archean-age gold-quartz vein systems is that such systems
选项 A、were formed from metarnorphic fluids.
B、originated in molten granite-like bodies.
C、were formed from alluvial deposits.
D、generally have surface expression.
E、are not discoverable through chemical tests.
答案A
解析 普遍认为古代金矿石系统是：A．正确。从变质流体中形成，见原文L9。B．从熔化花岗岩状物体中生成。这是现在观点。C、D、E和第一段叙述无关。
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