The Clifton-Morenci District, Arizona: The copper deposits of this district are located at Morenci and Metcalf in eastern Graham County. The ores were discovered in 1872, but remained undeveloped for a long time because of the fact that they were of too low grade, and too far from the railroads. At the present time, however, these large bodies of low-grade copper ore are utilized, most of the work being done by three large companies. The Morenci district, in eastern Arizona, ranks high in the production of copper. It is an area of pre-Cambrian granite and quartzitic schist, unconformably above which are Paleozoic sandstones, limestones, and shales that are locally overlain unconformably by Cretaceous shales and sandstones. These rocks are intruded by masses of granitic porphyries, which form stocks, dikes, laccoliths, and sheets. All these rock have been subjected to uplift, doming, and faulting. The domed area of older rocks is framed in by Tertiary lavas. This district from a geological standpoint embraces many conditions closely related to those at Bisbee. Mineralogically, the conditions differ more widely. The geological section embraces igneous and sedimentary rocks ranging in age from pre-Cambrian to Quarternary. Post-Cretaceous granitic and dioritic porphyries cut all the older formations. Subsequent to the solidification of the porphyry, extensive fissuring occurred in both it and the sediments, resulting in the deposition of quartz, pyrite, chalcopyrite, and sphalerite in the fissures and by replacement of the wall rock. These are low in copper, but there is a close relation between the veins and contact deposits because of the similarity of their metallic contents, and of the similar development of tremolite and diopside where limestone forms the wall. The extensive impregnation of the porphyry also occurred at this time. Subsequent exposure of the deposits by erosion permitted the entrance of surface waters which was followed by weathering and secondary enrichment.
The geologic section involves the following
:
Quaternary (Gila) conglomerate; Tertiary flows of basalt, rhyolites, and
some andesites; Cretaceous shales and sandstones. Several hundred feet
thick; Lower Carboniferous heavy-bedded pure limestones, 180 feet; Devonian
(?) shale and argillaceous limestone, 100 feet; Ordovician limestone, 200 to
400 feet; Cambrian (?) quartzitic sandstone, 200 feet; Pre-Cambrian granite
and quartzitic schists. Intrusions of granitic and dioritic porphyries of
post-Cretaceous age pierce all the older rocks, forming stocks, dikes,
laccoliths, and sheets. All of these rocks have been bowed up and
subsequently faulted by late Cretaceous or early Tertiary movements.
The original ores were pyrite and chalcopyrite, of too low grade to be workable, but they have since become so by a process of secondary enrichment. No copper ores were formed before the porphyry intrusion. Where the latter is in contact with the granite and quartzite, but little change is produced, but where the porphyry is found adjoining the limestones or shales, extensive contact metamorphism developed, resulting in the formation of large masses of garnet and epidote, especially in the Lower Carboniferous limestones. Where alteration has not masked the phenomena, magnetite, pyrite, chalcopyrite, and sphalerite accompany the contact minerals.
The ore bodies in the limestone are often irregular, but more frequently roughly tabular, because of the accumulation of the minerals along the stratification planes, or walls of dikes. The copper ore bodies are veins and disseminated deposits in the granitic and quartz monzonite porphyry and contact-metamorphic deposits in the limestone and shale. The veins and disseminated deposits are most productive. The ore deposits are in or near the intruding porphyry and were probably formed by solutions emanating from igneous bodies. The contact-metamorphic deposits have formed in Paleozoic limestone and shale near the porphyry. Pure limestones and calcareous shales were changed to copper ore consisting of pyrite, chalcopyrite, sphalerite, magnetite, garnet, epidote, diopside, tremolite, and quartz. The ores are dependent upon the porphyries for their existence. The ores occur in the porphyry, or close to its contact, or along dikes of the porphry as it cuts the sedimentary rocks. These fissure veins traversing the granite, porphyry, and clastic rocks carry, in their unoxidized portions, chalcopyrite, pyrite and sphalerite, while in the oxidized portion the leaching out of the copper and its down-ward transference has given rise to secondary chalcocite.
In many parts of the district the copper occurs in fissure veins which cut porphyry, granite, and sedimentary rocks, and were probably formed shortly after the consolidation of the porphyry. These in the lower levels carry pyrite, chalcopyrite, and sphalerite, but no magnetite. Surface leaching of these veins has often left limonite-stained, silicified porphyry outcrops. The veins cut granitic porphyry, and sedimentary rocks. They are composed of pyrite, chalcopyrite, sphalerite, molybdenite, sericite, and quartz. The copper bearing veins are allied to contact metamorphic deposits and pegmatites. In regions containing contact-metamorphic copper deposits it is not altogether unusual to find pyritic veins which exert an alteration on adjoining limestone similar to contact metamorphism, indicating that the vein-forming solutions possessed a high temperature. The veins of cupriferous pyrite at Clifton, Arizona, which intersect porphyry and contact-metamorphic limestone, are probably in part of this kind, for it was observed in many places that where they cut across limestone, tremolite and magnetite had developed adjacent to the vein. The primary deposits contain little copper but are enriched by surface waters.
Accompanying these veins, and of more importance commercially, are often extensive impregnations of the country rock. These disseminated deposits in the highly altered porphyry are leached out above, but lower down show a zone of pyrite and chalcocite, which does not usually extend below 400 feet. The disseminated copper ores in porphyry are formed by filling small but closely spaced cracks in the porphyry and replacing the rock nearby. Some of these deposits are large, and they constitute the mainstay of the camp. The veins and disseminated deposits owe much of their workable ore to processes of enrichment. Near the surface they are oxidized and generally leached of copper. Not all are marked by heavy gossans, and in the outcrops of some there is but little iron. Below the leached zone is a zone of chalcocite ore in which the copper sulfide replaces pyrite and sphalerite, below the chalcocite ore the primary sulfides are found including pyrite, chalcopyrite and sphalerite. Most of the copper in the district is obtained from concentrating disseminated copper ores containing chalcocite in altered porphyry. In 1914 the yield of copper from the concentrating ores was 1.65 per cent, while the smelting ores gave an average yield of 4.7 per cent.
The precious metal content of the copper ore is so low that much of the output of this district is not refined electrolytically unless the copper is not pure enough to put on the market. The intrusions of porphyry produced strong contact metamorphism in the shales and limestones of Paleozoic age, resulting in the contemporaneous and metasomatic development of various contact silicates and sulfides, the contact zone thus receiving large additions of iron, silica, sulfur, copper, and zinc, substances unknown in the sedimentary series away from the porphyry.
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