170 



UNITED STATES MINERAL RESOURCES 



Lowell and Guilbert (1970, p. 403-404) described 

 the hydrothermal alteration of a typical deposit 

 as forming in response to a "physical-geochemical 

 continuum from low-temperature magmatic to 'con- 

 ventional' hydrothermal conditions" in which hydro- 

 gen ion concentrations increase outward or with 

 time. Where the country rock is homogeneous, a 

 series of symmetrical, concentric shells of distinct 

 hydrothermal mineral assemblages may be pro- 

 duced around the igneous intrusive. From the in- 

 terior of the deposit outward, these are the potassic 

 (quartz-K-feldspar-biotite), the phyllic (quartz- 

 sericite-pyrite), the argillic (quartz-kaolin-chloride), 

 and the propylitic (chlorite-epidote-carbonate- 

 adularia and albite). 



Many factors limit or disrupt the symmetry of 

 the model porphyry copper deposit. Lowell and Guil- 

 bert listed (1) a strong regional structural fabric, 

 (2) heterogeneity of preore host rocks, (3) postore 

 faulting and intrusion, and (4) different levels of 

 erosion exposing the deposits. 



Titley (1972) drew attention to the profound 

 effect of the country rock on the alteration min- 

 eralogy and on the form of the ore bodies. He dis- 

 tinguished "intrusion porphyry copper deposits" in 

 which the ore body is mostly within the intrusive 

 porphyry, from "wall rock porphyry copper de- 

 posits" in which the wallrock near a porphyry in- 

 trusion is altered and mineralized. Composites of 

 these types are common. Titley emphasized that ore 

 bodies in wallrocks may be just as valuable as those 

 in intrusions and should not be overlooked because 

 their alteration pattern does not follow the ideal 

 model. 



Porphyry copper deposits of the Pacific and Ca- 

 ribbean islands exhibit alteration patterns that di- 

 verge sharply from the Lowell and Guilbert model. 

 Biotite is the most common mineral in the potassic 

 alteration facies, and K-feldspar is commonly lack- 

 ing, possibly because of the low availability of po- 

 tassium in the intrusive quartz diorite and diorite 

 porphyries and surrounding andesitic volcanic rocks. 

 Propylitic alteration is widespread in these deposits, 

 and in Puerto Rico it appears to follow an early 

 period of biotitic alteration (Cox, 1971). 



Figure 22 shows the size and grade characteristics 

 of various porphyry copper deposits. The Bingham 

 deposit in Utah is the largest in the United States, 

 with a production since 1904 of over 14 million tons 

 of copper (Anaconda Copper Corp., annual reports). 

 The field of 22 U.S. deposits in production in 1960 

 has its center point at about 100 million tons at 

 0.8 percent copper or about 800,000 tons of copper 

 metal per deposit. 



A second field of eight U.S. and Canadian deposits 

 brought into production in the last 10 years shows 

 a downward trend in grade. At Sierrita, Ariz., the 

 mining of ore averaging 0.34 percent copper is 

 made possible by lower cost mining and processing 

 methods. These new deposits contain on the average 

 about 2 million tons of copper each. Likewise, por- 

 phyry deposits with byproduct molybdenum discov- 

 ered in other parts of the world are being brought 

 into production at average copper grades as low as 

 0.18 percent (Brenda, British Columbia) and 0.3 

 percent (Casino, British Columbia) (Peters, 1970). 

 La Caridad, Mexico (Peters, 1970) and the com- 

 bined San Manuel-Kalamazoo deposits, Arizona 

 (Peters, 1970; Thomas, 1966, p. 139) are the larg- 

 est of the new deposits, with about 7 million tons 

 of copper each. 



Overshadowing these are the great Andean por- 

 phyry deposits of Chile and Peru. These deposits 

 were brought into production with average grades 

 of 1-2 percent, and the average of 11 deposits 

 plotted is about 4 million tons of copper each. The 

 El Teniente and Chuquicamata deposits in Chile, 

 which would lie on the right side of this field in 

 figure 22, are larger by at least an order of magni- 

 tude than the majority of North American porphyry 

 deposits. It is obvious that the Andes represent a 

 copper province of great wealth and that numerous 

 other lower grade deposits await development. 



The principal byproduct metals of porphyry de- 

 posits are molybdenum, gold, and silver. King (see 

 "Molybdenum" chapter) gives a range of M0S2 

 values of 0.015-0.1 percent for porphyry copper de- 

 posits; Simons and Prinz (see "Gold" chapter) give 

 a range of 0.001-0.025 ounce gold per ton in copper 

 deposits in general. An average silver value of 0.06 

 oz per ton is given for 14 Arizona porphyry deposits 

 by Heyl and others (see "Silver" chapter). Kesler 

 (1971) pointed out that continental porphyry de- 

 posits have higher molybdenum values and that 

 island arc deposits have higher gold values and 

 lower molybdenum values. Gold content of the 

 Panguna deposit in Bougainville is 0.018 oz per ton; 

 the Marcopper deposit in the Philippines contains 

 0.001 oz per ton (Espie, 1971; Loudon, 1972). Mo- 

 lybdenum values in these deposits are negligible. 



Economically important trace amounts of selen- 

 ium, tellurium, and rhenium are derived from cop- 

 per and molybdenum concentrates from porphyry 

 deposits. Pyrite concentrates are recovered for their 

 use in sulfuric acid manufacture, and magnetite 

 concentrates are recovered from some porphyry de- 

 posits such as Atlas in the Philippines (Madamba, 

 1972). 



