5.2

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.


104. The author is primarily concerned with
(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

105. 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 granite-like bodies
(C) were formed from alluvial deposits
(D) generally have surface expression
(E) are 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?
(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

107. Which of the following statements about discoveries
of gold deposits is supported by information in the
passage?
(A) The number of gold discoveries made annually
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.
(E) Models 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?
(A) A gold-quartz vein system originating in
magmatic fluids
(B) A gold-quartz vein system originating in
metamorphic fluids
(C) A gold deposit that is mixed with granite
(D) A gold deposit that has shed alluvial gold
(E) A gold deposit that exhibits chemical halos


109. The theory mentioned in lines 1–5 relates to the
conceptual models discussed in the passage in which
of the following ways?
(A) It may furnish a valid 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.
(E) It 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?
(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.
(E) Only one type of gold deposit warrants
exploration, since the other types of gold
deposits are unlikely to yield concentrated
quantities of gold.