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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  56
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] The author is primarily concerned with
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 主题题型：A．主张返回老方法。显非。B．正确。解释一最近理论的重要性。本文全在探讨新理论带来的研究方式的变化。C．列举两方法差异。无。D．描述导致发现的事件。无。E．挑战一理论的前提假设。无。
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