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## OG Archean-age gold-quartz vein systems

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#### OG Archean-age gold-quartz vein systems

Sun Sep 17, 2017 3:07 pm
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
(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.

Q104:B
Q105:A
Q106:C
Q107:B
Q108:D
Q109:A
Q110:C

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