Passage 32
According to a recent theory, Archean-age gold-quartz
vein¹ systems were formed over two billion years ago from
magmatic fluids that originated from molten granitelike
bodies deep beneath the surface of the Earth. This theory is
 (5) contrary to the widely held view that the systems were
deposited from metamorphic fluids, that is, from fluids that
formed during the dehydration² of wet sedimentary rocks.
he recently developed theory has considerable practical
importance. Most of the gold deposits discovered during
(10)the original gold rushes were exposed at the Earth’s surface
and were found because they had shed trails of alluvial³
gold that were easily traced by simple prospecting⁴ methods.
Although these same methods still lead to an occasional
discovery, most deposits not yet discovered have gone
(15) undetected because they are buried and have no surface
expression.
The challenge in exploration is therefore to unravel⁵ the
subsurface geology of an area and pinpoint⁶ the position of
buried minerals. Methods widely used today include
(20) 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
(25) envelop⁷ mineralization. However, none of these high-
technology methods are of any value if the sites to which
they are applied⁸ have never mineralized, and to maximize
the chances of discovery the explorer must therefore pay
particular attention to selecting the ground formations most
(30) 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
35) 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.
 
198. The author is primarily concerned with
(A)   advocating a return to an older methodology
(B)   explaining the importance of a recent theory
(C)   enumerating⁹ differences between two widely used methods
(D)  describing events leading to a discovery (B)
(E)   challenging the assumptions on which a theory is based
 
199. According to the passage, the widely held view of Archean- age gold-quartz vein systems is that such systems
(A)   were formed from metamorphic fluids
(B)   originated in molten granitelike bodies
(C)   were formed from alluvial deposits
(D)  generally have surface expression (A)
(E)   are not discoverable through chemical tests
 
200. 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?
(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 analyze¹⁰ rocks over a broad area (C)
(E)   Limiting exploration to sites where alluvial gold has previously¹¹ been found
 
201. Which of the following statements about discoveries of gold deposits is supported by information in the passage?
(A)   The number of gold discoveries made annually¹² has increased between the time of the original gold rushes and the present.
(B)   New discoveries of gold deposits are likely to be the result of exploration techniques designed to locate buried mineralization.
(C)   It is unlikely that newly discovered gold deposits will ever yield as much as did those deposits discovered during the original gold rushes.
(D)  Modern explorers are divided on the question of the utility of simple prospecting methods as a source of new discoveries of gold deposits. (B)
(E)   Models based on the theory that gold originated from magmatic fluids have already led to new discoveries of gold deposits.
 
202. It can be inferred from the passage that which of the following is easiest to detect?
(A)   A gold-quartz vein system originating in magmatic fluids
(B)   A gold-quartz vein system originating in meamorphic fluids
(C)   A gold deposit that is mixed with granite
(D)  A gold deposit that has shed alluvial gold (D)
(E)   A gold deposit that exhibits chemical halos
 
203. The theory mentioned in line 1 relates to the conceptual models discussed in the passage in which of the following ways?
(A)   It may furnish a valid¹³ account of ore-forming processes, and, hence, can support conceptual models that have great practical significance.
(B)   It suggests that certain geological formations, long believed to be mineralized, are in fact mineralized, thus confirming current conceptual models.
(C)   It suggests that there may not be enough similarity across Archean-age gold-quartz vein systems to warrant the formulation of conceptual models.
(D)  It corrects existing theories about the chemical halos of gold deposits, and thus provides a basis for correcting current conceptual models. (A)
(E)   It suggests that simple prospecting methods still have a higher success rate in the discovery of gold deposits than do more modern methods.
 
204. According to the passage, methods of exploring for gold that are widely used today are based on which of the following facts?
(A)   Most of the Earth’s remaining gold deposits are still molten.
(B)   Most of the Earth’s remaining gold deposits are exposed at the surface.
(C)   Most of the Earth’s remaining gold deposits are buried and have no surface expression.
(D)  Only one type of gold deposit warrants exploration, since the other types of gold deposits are found in regions difficult to reach. (C)
(E)   Only one type of gold deposit warrants exploration, since the other types of gold deposits are unlikely to yield concentrated quantities of gold.
 
205. 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
(A)   I only
(B)   II only
(C)   I and II only
(D)  I and III only (D)
I, II and III

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