Copper ore occurs in nearly all the different classes of mineral deposits that have been described, but to the prospector the varying conditions of occurrence and the quality of ore required to be payable under different conditions are perhaps more important.
Perhaps the greater quantity of copper in the world is obtained from disseminations of native copper and sulphides through rocks, the yield of which is often low, but the quantity large, and, in many cases, the accessibility, such as to reduce working costs to a minimum. Probably copper is won from such deposits more cheaply than from any other source.
Native copper occurs disseminated in amygdaloids and conglomerates at Lake Superior, the ores being mined yielding from less than 1 to 4 per cent, copper, the total costs varying from 8s. to 12s. per ton.
Disseminated sulphides are worked at various places in the United States and Mexico, and while the rock mined contains from 1*5 to 2 per cent, copper only, the concentrates yield from 15 J to 30 per cent, copper. The primary ores are poor, but there is generally a zone of surface enrichment, and at the Utah Copper Mine this zone is some 310 feet thick, and has a large surface extent. The ore in the enriched zone is chiefly chalcocite in porphyry, and the cost of working is about 10s. per ton.
At Mansfeld, in Germany, the copper slate contains disseminated copper ore, which is mostly argentiferous fahlore. The chief copper-bearing bed is a bituminous shale, seldom over 18 inches thick, which extends for miles. It is generally poor, but is enriched at places by veins which intersect it. The average contents of the ore over a large area is from 2 to 3 per cent, of copper, with a small proportion of silver and gold.
Copper, again, is found in lodes of various classes, mostly as sulphides, associated generally with quartz as a gangue, but occasionally other minerals, such as fluorspar, barytes, and calcite, are also present.
The ores of copper, or indeed of any metal, are nearly always associated with other ores in greater or less quantity, but those veins are most valuable in which one metal alone is found, since they are more easily concentrated, and their metallurgical treatment is more simple.
While the ore worked in these lodes is still chiefly chalcopyrite and iron pyrites, if we except those ores which are due to surface enrichment, their distribution is less regular than that of disseminated copper. At the best the ore occurs in shoots, separated by barren or unpayable ground, and the costs of both development and extraction are high per ton. These facts imply that the yield of the ore must be greater to pay, and while much ore yielding 3 to 4 per cent, copper is mined from such deposits, it is seldom that much profit can be realised from ore of lower grade. This higher grade of ore further increases the cost of smelting per ton, and it is by no means unusual to find the total costs per ton amounting to three or four times those of disseminated ores. It will be evident, therefore, that the yield of lodes, to be payable, must be considerably above those first deposits that have been mentioned.
There is, however, yet another type of copper deposits which plays an -important part in copper mining. In this are included all those massive low grade deposits of cupriferous pyrites, of which Rio Tinto may be taken as a type. Where such ores are worked they are usually found as large masses of homogeneous ore, which can be easily mined, and do not require concentration. The fact that such ores contain a large percentage of sulphur, as well as copper, make them valuable for the manufacture of sulphuric acid, and after the sulphur has been recovered the copper can be extracted from the residues. The costs of mining and treating these ores appear to lie perhaps midway between the other two, but if the mines are so situated that a ready market can be found for sulphur or sulphuric acid, the actual cost of copper, when the ore is worked for the two products, is quite as low as in disseminated ores, and deposits yielding from 1 to 2 per cent, copper will often pay well to work.