Other Arizona and New Mexico Locations:
The Jerome District: The Jerome
district is in Yavapai County, east central Arizona. The largest mine of the
district is the United Verde, which was once worked for
native gold but since 1888 has been a steady producer of
copper, giving the district the sixth rank in the United States.The rocks of
this district are pre- Cambrian metamorphics. The copper ores are of the
same age and replace a schist that has been formed by the intense shearing
of the basic porphyry. The ores are
bornite and chalcopyrite with a little
sphalerite. The percentage of
chalcopyrite is higher in the alternating bands of sulfide
copper ores and schist than it is in the more massive material The terranes
are pre-Cambrian and the ore is
chalcocite widely disseminated through the schistose rocks. The
ores occur in a pre-Cambrian schist, and consist of
pyrite, chalcopyrite, some sphalerite, and varying amounts of
quartz, replacing the schist. The ore body is really composed
of a series of irregular lenses. Unlike most of the other Arizona copper
deposits, this ore carries rather high gold and
silver values.
The United Verde mine is in an area of pre-Cambrain schists, faulted upward and overlain unconformably by Paleozoic beds. The ore shoot is oval in plan, about 1,300 feet long horizontally and 700 feet wide. It trends north-northwest and pitches in that direction 45. It has been worked to a vertical depth of 1,200 feet. The great shoot is in reality a complex of smaller but nevertheless large irregular or lenticular copper ore bodies, showing a tendency toward parallelism with the schistosity. The ore was deposited in pre-Cambrian time. The chief mineral is chalcopyrite, associated with which are pyrite specular hematite and sphalerite. The sulfides occur partly in small irregular fractures and along planes of schistosity, but to a large extent they have replaced the schist.
Oxidized ore containing malachite, azurite, and cuprite extended to a depth of about 400 feet and in its upper part was comparatively rich in gold. Below the level of complete oxidation there was chalcocite ore with a relatively high proportion of silver. Recently large bodies of rich chalcocite ore enclosed in schist have been found in the United Verde Extension mine. These are capped by gossan, above which are flat-lying Cambrian sedimentary rocks. The chalcocite copper ore and the leached oxidized material above it have evidently resulted from weathering in pre-Cambrian time.
Ajo, Arizona. The Ajo district, in south-central Arizona, contains disseminated copper ores in porphyry, in which oxidation appears to have been attended by little leaching and chalcocitization. The most notable feature of this region is an intrusive mass of monzonite porphyry, which has domed up the older rhyolite beds. Some rich copper veins occur in the porphyry and in the rhyolite, but the most valuable deposits developed are in a mass of shattered porphyry that occupies about 55 acres and has a maximum depth of 600 feet, carrying about 12,000,000 tons of carbonate copper ore, below which lies about 28,000,000 tons of sulfide ore. Unlike the disseminated deposits at Bingham and Ely, in which the copper ore is largely chalcocite, the disseminated ores in the Ajo district are mainly chalcopyrite and bornite. The outcrop and oxidized zone exhibit copper minerals conspicuously. They consist of silicified monzonite porphyry, with seams and stains of malachite, limonite, specular hematite, and a little chrysocolla. The oxidized ore extends downward to an almost horizontal plane about 150 feet below the highest hills. This plane is approximately the present ground-water level, and the transition from carbonate to sulfide ore is very abrupt.
Silver Bell, Arizona. The Silver Bell district, located southwest of Tucson, extensive primary chalcopyrite deposits have been worked. The mines are near the summit of one of the numerous desert ranges of that region; the copper ores were smelted, without concentration, at the Sasco plant. Several small masses of Paleozoic limestone are engulfed in a large mass of granite porphyry and along their contacts metamorphism is irregularly developed in part by marmorization, in part by garnetization. The ore consists of chalcopyrite and light-brown garnet, said to be andradite garnet, with a little magnetite, sphalerite, galena, and molybdenite. Much of the ore averaged 7 per cent, copper, but that smelted would average about 4 per cent. About 800 tons were mined per day in 1909. A trace of gold and 1 to 2 ounces of silver per ton are present. The oxidation is shallow, wholly fresh rock being encountered at the 200-foot level. The porphyry is locally silicified, but otherwise not greatly altered, except for some disseminated pyrite and chalcopyrite. No extensive chalcocite zone has been found in the porphyry.
Santa Rita, New Mexico.
The Santa Rita district, in Grant County, New Mexico, is an area of
limestone, sandstone, and shale of Paleozoic and Cretaceous age, intruded by
dioritic and
quartz monzonite porphyry. The
deposits are disseminated in porphyry but differ from the disseminated
deposits of Bingham and Ely, for much of the copper occurs as native metal
and cuprite. Chalcocite evidently has replaced pyrite to form the rich
secondary copper ore, but the chalcocite itself has been converted by
oxidation processes to
cuprite and native copper. In the disseminated ores on the
Chino ground 90,000,000 tons of ore averaging 1.75 per cent, of copper have
been developed. Most of it is near the surface. The average thickness of the
capping is 82 feet, and the average thickness of the ore below the capping
is 107 feet. Other Deposits. Santa Rita, is becoming important chiefly on
account of its great disseminated deposits in highly altered and shattered
sedimentary and intrusive rocks, in which the copper occurs largely native
or as the oxide, although chalcocite is by no means uncommon.
Another interesting district of the disseminated type is that of the Burro Mountains in New Mexico. This region is of interest especially because of the relationships of the primary sulfides found within the copper ore body. The rocks are greenstone and sericitic schists, intruded in places by granite and gabbro. The schists have been derived from a series of andesites and quartz porphyries with a preponderating amount of tuffs. The fissure veins, which occur in the chloritized and epidotized andesite, contain primary bornite and chalcocite, in a gangue of quartz, and subordinate calcite or epidote. The ore-bearing solutions are thought to have come from the granite, whose intrusion postdates the development of schistosity in the volcanics.
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