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JOURNAL ARTICLE
Porphyry Copper: Circumstantial evidence suggests that porphyry-related deposits, currently the world's principal source of copper, may have formed as by-products of volcanic activity
Spencer R. Titley
American Scientist
Vol. 69, No. 6 (November-December 1981), pp. 632-638
Published by: Sigma Xi, The Scientific Research Honor Society
https://www.jstor.org/stable/27850714
Page Count: 7
A volcanic origin?
From the foregoing discussion, it is apparent that porphyry copper deposits came into being as a result of special geological conditions. Much evidence has now been accumulated that leads us to suspect that the deposits were formed as lay-products of volcanic activity where deep-seated magma bodies had suitable compositions and properties and where crustal characteristics permitted those bodies to rise rapidly to shallower levels. The evidence is indirect, and hence inconclusive, however, because the volcanic structures that may once have existed above the deposits would have been topographically and compositionally unstable and thus would have been removed rapidly by weathering, and because a large percentage of known deposits are now exposed as a result of millions or tens of millions of years of erosion.
Nevertheless, a volcanic origin for many porphyry copper deposits is consistent with the following facts. First, by far the greatest number of deposits are of Mesozoic age or younger—less about 230 million years old—and are located in either the “ring of fire,“ the belt of volcanoes and volcanic rocks that encircles the Pacific Ocean, or the “Tethyan” belt, the east-west band of volcanic rocks and mountains that extends throughout the southern Soviet Union and into the Middle East (Fig. 5). The association of the deposits with volcanic rocks and the style of occurrence are much the same in both regions. Moreover, both these parts of the world are currently the sites of much earthquake activity and volcanism, which are believed to be manifestations of the same sorts of events as are known to have occurred in or near these regions ever since about 230 million years ago.
Second, although most of the rocks that existed at the time the ore was formed have been removed by erosion. there are a few deposits where remnants of volcanic rocks about the same-age as the porphyries (up to 200 million years old) are found, and where porphyries containing copper deposits are present as intrusions in the volcanic rocks (Fig. 6). This coincidence of porphyries and volcanic rocks in both time and space is strong evidence for a volcanic origin for the deposits.
Third, the theory of the origin of porphyritic textures described previously involves two stages of cooling: an early stage, during which crystals of silicate minerals started to form, and a late stage, in which the residual magma was quenched to form a fine-grained matrix. The late stage of cooling was thought to be the result of magma rising rapidly to the cooler environment of the shallow crust. According to the volcanic genesis theory, the porphyry intrusions, which have compositions similar to those of many volcanic rocks, formed when magma rose rapidly toward the earth’s surface through the core of a volcano.
第二段例子讲的是（大概是p-copper附近的）一个石头还是什么的（印象里c打头，不是很确定）的形状是vertical的，这种vertical的形状与earth surface破裂/冲破有关，而earth surface 冲破与火山有关
P3：第一个间接证据：说可以通过一种另外的B body 来确认一种opening,这个opening 在current volcano systems有，这样就有关系了嘛~
Fourth, a large percentage of porphyry copper deposits contain bodies of breech—angular fragments of rock, which, in some instances, are cemented with fine crystalline volcanic-like rock or with rock “flour" derived from the mutual abrasion of porphyry fragments (Fig. 2). Such bodies are commonly irregular, essentially vertical columns, or “pipes,” with round or elliptical cross sections; less commonly, they assume a tabular form, such as that of a vertical sheet, or “dike.” The transportation and pulverization of rocks in breccias, as well as the geometry of the breccia bodies, suggest that there may have been an opening to the surface at the time that they formed. Moreover, breccias that look like these are present in active volcanic systems, where openings clearly exist.
Fifth, the distribution of deposits in well-explored regions of the Pacific rim shows tendencies toward clustering, as in southwest North America. where clusters occur in areas about 30 km by 30km (Fig. 7), and toward formation along lines on the earth's surface, as in the Andes. Volcanoes show similar tendencies to cluster or form along lines. This similarity in distribution of volcanic centers and porphyry ore deposits is not in itself compelling evidence for an association, but taken together with the other pieces of evidence, it strengthens the case.
The inferred link between circum-Pacific volcanism and the formation of porphyry ore deposits is extremely important, because it establishes a rational basis for searching for such deposits. Moreover, the high success rate—the discovery of more than a hundred new deposits—over the past two decades of searching premised in part on the assumption that the theory is correct attests in some measure to its validity.
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