178 



UNITED STATES MINERAL RESOURCES 



11. Porous or chemically reactive strata, including 

 lava flovt's and pyroclastic rocks, within or 

 peripheral to rocks of the zeolite facies of 

 lov^r-grade metamorphism. Examples besides 

 the Nonesuch Shale are the Sangre de 

 Cristo Formation of Pennsylvanian and Per- 

 mian age in southern Colorado and the 

 Apache Leap Tuff at the Inspiration mine, 

 Arizona. 

 The environments described above are not all 

 mutually exclusive; therefore, a coincidence of sev- 

 eral may greatly increase the probability of find- 

 ing significant copper mineralization. 



SIZE AND GRADE CHARACTERISTICS 



Sedimentary deposits are diflScult to characterize 

 as to size. On the one hand, many separate mines 

 are not in separate ore deposits but in parts of a 

 continuously mineralized stratum, and on the other 

 hand, very large tonnage figures have been men- 

 tioned for beds continuously mineralized over wide 

 areas even though only a small part may be mined 

 economically. In figure 22, a field of seven deposits 

 in Zaire and Zambia has an average copper content 

 of 4 million tons each in ore averaging 3.5 percent 

 copper. Totals of production and reserves for both 

 the White Pine and Keweenaw Point districts in 

 Michigan amount to 7 or 8 million tons of metal 

 for each deposit but in lower grade ore. Estimates 

 for the Udokan deposit in the U.S.S.R. exceed 10 

 million tons contained copper. 



MASSIVE SULFIDE DEPOSITS IN VOLCANIC ROCKS 



Copper is more abundant in basalt and andesite 

 than in other igneous rocks. Under various condi- 

 tions, this copper content may be concentrated to 

 form an ore deposit of distinctly different form from 

 those associated with felsic intrusions. 



A small but locally important part of the world's 

 copper resources is found in lavas and pyroclastic 

 rocks laid down in the marine environment. These 

 deposits are generally (1) stratiform or lenticular, 

 and concordant with the bedding of the surround- 

 ing rocks, (2) of small lateral extent as compared 

 to thickness, and (3) composed largely of sulfide 

 minerals with small proportions of silicate gangue. 

 Mineralogically the deposits consist mainly of 

 pyrite and (or) pyrrhotite and varying amounts of 

 chalcopyrite, sphalerite, and galena. The terms 

 "massive sulfide," "cupriferous pyrite," or "massive 

 pyrite" deposits have been used to identify this 

 class. 



The last decade has been extremely productive in 

 theories of origin of massive sulfide deposits. Recog- 



nition of metamorphic features in the North Ameri- 

 can deposits by Kinkel (1962) and Kalliokoski 

 (1965) permitted the interpretation that their em- 

 placement was earlier than the events that deformed 

 them. Regional geologic studies such as those of 

 Goodwin (1965) in the Noranda area, Quebec, made 

 possible the views that the deposits were a part of 

 the volcanic rock complexes that enclosed them and 

 not a later superimposed feature. Detailed studies 

 of Kuroko-type ores in Miocene tuffs of Japan (for 

 example, Horikoshi, 1969) showed that these ores 

 were formed on the sea fioor during the last stages 

 of an eruptive cycle. The equivalence of these de- 

 posits to the metamorphosed massive sulfides of 

 older terranes had by this time become clear to 

 many workers. 



Anderson (1969) presented a thorough review of 

 the genesis of these deposits, and in 1970 the meet- 

 ing of the International Mineralogical Association- 

 International Association on the Genesis of Ore 

 Deposits in Japan brought together an outstanding 

 group of papers on the subject. The description 

 which follows is largely taken from papers by Tat- 

 sumi and Watanabe (1971), Hutchinson and Searle 

 (1971), and L. A. Clark (1971) in the proceedings 

 of that meeting. 



Massive sulfide deposits that are undeformed by 

 later orogenic activity exhibit a wide variety of 

 textures and forms. Ore may be found in dissemina- 

 tions and stockworks or swarms of veins in highly 

 propylitized lava. Above these may be found sulfide 

 ore in cavities between pillows as at Cyprus. Mas- 

 sive sulfide bodies above the stockworks and vein 

 swarms may show banding and colloform texture 

 indicating deposition in open cavities or on the open 

 sea floor. The ore may be intensely brecciated with 

 interfragmental spaces filled with fine-grained 

 sandy pyrite, and evidence exists for gravitational 

 transport of blocks of ore, lava, and soft bottom 

 sediment downslope on the sea floor from the source 

 of the deposit. Overlying the massive ore are bedded 

 deposits rich in iron oxides, silica, and disseminated 

 pyrite. These bedded deposits are thickest over the 

 massive ore, and they thin abruptly laterally. 



The preceding features have given rise to the 

 volcanic exhalative theory of origin. Metal-bearing 

 solutions or vapors are thought to ascend rapidly 

 through fractures in a submarine volcanic pile and 

 to precipitate, partly in colloidal form, in near- 

 surface cavities and on the sea floor. Deposition of 

 sulfides may temporarily seal the vent area, and 

 the resulting increase of vapor pressure may cause 

 the brecciation of previously deposited ore. Mixing 

 of iron-rich solutions with oxygen-rich circulating 



