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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  35
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] It can be inferred from the passage that the efficiency of model-based gold exploration depends on which of the following? I . The closeness of the match between the geological features identified by the model as critical and the actual geological features of a given area. II. The degree to which the model chosen relies on empirical observation of known mineral deposits rather than on theories of ore-forming processes. III. The degree to which the model chosen is based on an accurate description of the events leading to mineralization.
选项 A、I only
B、II only
C、I and II only
D、I and III only
E、I , II and III
答案 D
解析 基于模型的探金矿有赖于哪些点?典型assumption的题目。I．模型所确定的地质特性要和某处实际特性吻合。L48—51，这一点对确立矿化结构很重要。正确。Ⅱ．依赖实践经验而不是成矿理论的程度如何。不符原文L44—47，两者都是模型的组成部分。Ⅲ．被选模型有多大程度真实地描述了矿化情况。选择此模型的目的就是为了探测矿化情况，此点不成立，当然无法探测。正确。∴D．I和Ⅲ。
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