OG13 RC 104-110

kudoucliff

According to a recent theory, Archean-age gold-quartz vein systems were formed more than 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.


这篇真的好难


104. The author is primarily concerned with

• Aadvocating a return to an older methodology
• Bexplaining the importance of a recent theory
• Cenumerating differences between two widely used methods
• Ddescribing events leading to a discovery
• Echallenging the assumptions on which a theory is based
  

105. According to the passage, the widely held view of Archean-age gold-quartz vein systems is that such systems

• Awere formed from metamorphic fluids
• Boriginated in molten granite-like bodies
• Cwere formed from alluvial deposits
• Dgenerally have surface expression
• Eare not discoverable through chemical tests
  

106. 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?

• ASurveying several sites known to have been formed more than two billion years ago
• BLimiting exploration to sites known to have been formed from metamorphic fluid
• CUsing an appropriate conceptual model to select a site for further exploration
• DUsing geophysical methods to analyze rocks over a broad area
• ELimiting exploration to sites where alluvial gold has previously been found
  

107. Which of the following statements about discoveries of gold deposits is supported by information in the passage?

• AThe number of gold discoveries made annually has increased between the time of the original gold rushes and the present.
• BNew discoveries of gold deposits are likely to be the result of exploration techniques designed to locate buried mineralization.
• CIt is unlikely that newly discovered gold deposits will ever yield as much as did those deposits discovered during the original gold rushes.
• DModern explorers are divided on the question of the utility of simple prospecting methods as a source of new discoveries of gold deposits.
• EModels based on the theory that gold originated from magmatic fluids have already led to new discoveries of gold deposits.
  

108. It can be inferred from the passage that which of the following is easiest to detect?

• AA gold-quartz vein system originating in magmatic fluids
• BA gold-quartz vein system originating in metamorphic fluids
• CA gold deposit that is mixed with granite
• DA gold deposit that has shed alluvial gold
• EA gold deposit that exhibits chemical halos
  

109. The theory mentioned in highlight text relates to the conceptual models discussed in the passage in which of the following ways?

• AIt may furnish a valid account of ore-forming processes, and, hence, can support conceptual models that have great practical significance.
• BIt suggests that certain geological formations, long believed to be mineralized, are in fact mineralized, thus confirming current conceptual models.
• CIt suggests that there may not be enough similarity across Archean-age gold-quartz vein systems to warrant the formulation of conceptual models.
• DIt corrects existing theories about the chemical halos of gold deposits, and thus provides a basis for correcting current conceptual models.
• EIt suggests that simple prospecting methods still have a higher success rate in the discovery of gold deposits than do more modern methods.
  

110. According to the passage, methods of exploring for gold that are widely used today are based on which of the following facts?

• AMost of the Earth’s remaining gold deposits are still molten.
• BMost of the Earth’s remaining gold deposits are exposed at the surface.
• CMost of the Earth’s remaining gold deposits are buried and have no surface expression.
• DOnly one type of gold deposit warrants exploration, since the other types of gold deposits are found in regions difficult to reach.
• EOnly one type of gold deposit warrants exploration, since the other types of gold deposits are unlikely to yield concentrated quantities of gold.