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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.
The passage implies that which of the following steps would be the first performed by explorers who wish to maximize their chances of discovering gold?
选项 A、Surveying several sites known to have been formed more than two billion years ago.
B、Limiting exploration to sites known to have been formed from metamorphic fluid.
C、Using an appropriate conceptual model to select a site for further exploration.
D、Using geophysical methods to analyze rocks over a broad area.
E、Limiting exploration to sites where alluvial gold has previously been found.
答案C
解析 一个想最大可能发现金矿的人，第一步应该：A．探测20亿年以上的地点。无。B．探测从变质流体演化生成的地点。文中未直接说这会发现金矿。C．正确。用正确的模型来选定继续探测的地点。见原文L36—43，要注意矿化地区，而矿化地区要靠模型来确定。D．地质物理法分析岩石。见L30—33，但文章后面指出这是有局限性的。未矿化地区探测不出来。E．alluvial gold和正式探矿无关。
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