The Globe District: Globe, Miami, and Ray
are in Gila County, central western Arizona. Globe is one of the oldest copper-bearing districts in Arizona, having produced almost steadily since 1881. This district became well known through the Old Dominion mine at Globe, long before the now more important copper ores at Miami were developed. Here the ore bodies occur as lenticular replacements in limestones and fault lodes, or fissure zones in diabase. The ores in the upper levels are of the oxidized type. At the lower levels they are enriched. The original primary copper ore was a sulfide of iron and copper, which in its upper part has undergone leaching and oxidation, accompanied by secondary enrichment of the ore below. Some bodies of primary ore with commercial significance have been developed. The formations are in an area that includes a pre-Cambrian crystalline rock complex, the Pinal schist cut by various granitic intrusives. These are overlain unconformably is a thick series of Paleozoic sediments including conglomerates, quartzites, shales and limestones. In Mesozoic times probably there followed an intrusion of diabase and granitic rocks, and then after an erosion interval Tertiary dacite volcanics and sediments, the Gila conglomerate being prominent among the latter. Faulting of both pre- and post-Tertiary age is known.
The principal deposits of Globe are lodes and replacement deposits in limestone and diabase, of which the largest are in the Old Dominion mine. The primary ores consisted largely of pyrite and chalcopyrite, with bornite and specular hematite. These ores are oxidized to a depth of 800 feet and at places deeper and they contain copper as carbonates and cuprite. Enriched chalcocite ore is found in and below the oxidized ore. Much of the ore contains about 6 per cent, of copper. Miami is about 6 miles west, and Ray about 20 miles southwest of Globe. They are the only districts known in the United States that contain large deposits of chalcocite copper ore disseminated in schist. At Miami the Pinal schist is intruded by granite and granite porphyry. Disseminated deposits form a chain 2 miles long and a quarter of a mile in maximum width, from the Miami mine on the east through the Inspiration, Keystone, and Live Oak mines toward the west. The surface is stained in places with iron oxide and copper carbonate. Around Miami the great disseminations of chalcocite, in the Pinal schist near the Schultze (Mesozoic) granite are of importance. The original ore was a sulfide of iron and copper, which in its upper part has undergone leaching and oxidation, accompanied by secondary enrichment of the ore below. The section therefore shows a leached capping, followed by an irregular zone of oxidized ore, and this in turn by a secondary enrichment zone, showing grains and stringers of pyrite and chalcopyrite replaced by chalcocite. The ore-bearing solutions are believed to have come from the Schultze granite, and resulted not only in deposition of ore, but also a more or less complete silicification of the schist.
These disseminated copper ores represent such an important type in the West to-day that a few figures showing their low grade, extent, etc., as explanatory of their working at a profit, may be given. Leached material with little copper extends downward from 50 to 600 feet below the surface. Below that is a blanket of secondary chalcocite ore which has a maximum thickness of about 300 feet. Estimates give a total of 80,000,000 to 90,000,000 tons of ore averaging between 2 and 2.5 per cent, of copper. The ore of workable grade will probably amount to 150,000,000 tons. Although much of this copper ore lies so deep that it can not be mined profitably with steam shovels, it can be worked because it is of somewhat higher grade than the ore of porphyry mines that use steam shovels, and because the secondary ore carries very little pyrite. The mill concentrates are rich. They consist mainly of chalcocite, and only a small percentage of the ore milled is put through the smelter. The copper per cent in ore milled averaged 2.28 per cent. The mill extraction was 69.93 per cent, and the concentrates contained 39.31 per cent copper. The gold and silver content runs fairly low, but they are saved in the refining process.
Ray Copper District:
The geology of the Ray district, also known as Mineral Creek,
shows that the deposits are found in sedimentary rocks associated with
faults and fissures, or as disseminations in the Pinal schist and granite,
this second type being the more important. In 1914 the ore concentrated
averaged 1.76 per cent copper. At the beginning of 1915 the ore reserves
were estimated at 74,765,789 tons, averaging 2.214 per cent copper. The
deposits at Ray are similar to those of Miami. The Pinal schist is intruded
by granite porphyry. The ore bodies aremainly in the schist, although masses
of granite porphyry within the generally metallized area also have been
converted to ore. The deposits are of the disseminated type; the siliceous,
sericitized schist is sheeted, fractured, and rilled with innumerable
closely spaced veinlets of copper sulfides, which occur also in the schist
between the veinlets. The section shows a leached capping, followed by an
irregular zone of oxidized copper ore, and this in turn by a secondary
enrichment zone, showing grains and stringers of pyrite and
chalcopyrite replaced by
chalcocite. The primary ore is probably connected genetically
with the granite porphyry. The ore bodies underlie a group of hills stained
here and there with copper minerals. Within this area there is a continuous
ore body about 8,000 feet long and 2,500 feet in greatest width. As at
Miami, the layer of ore has many irregular undulations that apparently have
no relation to the present topography. The average thickness of the ore body
is 101 feet; of the overburden 250 feet. The depth to ore ranges from 10 to
300 feet, and the thickness of the ore from a thin film to 400 feet.
Return To: Copper Ore Deposits of the USA
