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 contrary to the widely held view that the systems were deposited from metamorphic fluids, that is, from fluids that formed during the dehydration3 of wet sedimentary rocks. he 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 alluvial4 gold that were easily traced by simple prospecting5 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 unravel6 the subsurface geology of an area and pinpoint7 the position of buried minerals. Methods widely used today include analysis of aerial images that yield a broad geological overview8; 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 envelop9 mineralization. However, none of these high- technology methods are of any value if the sites to which they are applied10 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.
1. The author is primarily concerned with
advocating a return to an older methodology
explaining the importance of a recent theory
enumerating11 differences between two widely used methods
describing events leading to a discovery
challenging the assumptions on which a theory is based
2. According to the passage, the widely held view of Archean- age gold-quartz vein systems is that such systems
were formed from metamorphic fluids
originated in molten granitelike bodies
were formed from alluvial deposits
generally have surface expression
are not discoverable through chemical tests
3. 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?
Surveying several sites known to have been formed more than two billion years ago
Limiting exploration to sites known to have been formed from metamorphic fluid.
Using an appropriate conceptual model to select a site for further exploration
Using geophysical methods to analyze12 rocks over a broad area
Limiting exploration to sites where alluvial gold has previously13 been found
4. Which of the following statements about discoveries of gold deposits is supported by information in the passage?
The number of gold discoveries made annually14 has increased between the time of the original gold rushes and the present.
New discoveries of gold deposits are likely to be the result of exploration techniques designed to locate buried mineralization.
It is unlikely that newly discovered gold deposits will ever yield as much as did those deposits discovered during the original gold rushes.
Modern explorers are pided on the question of the utility of simple prospecting methods as a source of new discoveries of gold deposits.
Models based on the theory that gold originated from magmatic fluids have already led to new discoveries of gold deposits.
5. It can be inferred from the passage that which of the following is easiest to detect?
A gold-quartz vein system originating in magmatic fluids
A gold-quartz vein system originating in meamorphic fluids
A gold deposit that is mixed with granite2
A gold deposit that has shed alluvial gold
A gold deposit that exhibits chemical halos
6. The theory mentioned in line 1 relates to the conceptual models discussed in the passage in which of the following ways?
It may furnish a valid15 account of ore-forming processes, and, hence, can support conceptual models that have great practical significance.
It suggests that certain geological formations, long believed to be mineralized, are in fact mineralized, thus confirming current conceptual models.
It suggests that there may not be enough similarity across Archean-age gold-quartz vein systems to warrant the formulation of conceptual models.
It corrects existing theories about the chemical halos of gold deposits, and thus provides a basis for correcting current conceptual models.
It suggests that simple prospecting methods still have a higher success rate in the discovery of gold deposits than do more modern methods.
7. According to the passage, methods of exploring for gold that are widely used today are based on which of the following facts?
Most of the Earth's remaining gold deposits are still molten.
Most of the Earth's remaining gold deposits are exposed at the surface.
Most of the Earth's remaining gold deposits are buried and have no surface expression.
only one type of gold deposit warrants exploration, since the other types of gold deposits are found in regions difficult to reach.
only one type of gold deposit warrants exploration, since the other types of gold deposits are unlikely to yield concentrated quantities of gold.
8. 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
Ⅰonly
Ⅱ only
Ⅰand Ⅱ only
Ⅰ and Ⅲ only
Ⅰ,Ⅱ and Ⅲ
