Pyromorphite will be readily distinguished from copper ores by its high specific gravity ; besides which it is not always green, but often yellowish-green, yellow, or brown. Copper uranite, which crystallises in laminae, exhibits on the larger faces a pearly lustre, and fuses before the blowpipe to a blackish mass. The arseniates and phosphates of copper are soluble in ammonia, and the arseniates give before the blowpipe the characteristic smell of garlic. Atacamite gives the blue flame characteristic of chloride of copper when brought near the flame of a candle, it not being necessary to previously moisten the mineral with acid.
It seems probable that the only primary ore of copper is chalcopyrite, which in its pure form contains 34 per cent, of copper. In the permanent ores of mines it frequently happens that iron pyrites contains a few units, and pyritic ores may exist with any percentage of copper less than that of pure chalcopyrite.
Beyond the fact that the richer ores are of a deeper yellow, and the mineral is more sectile, there is nothing in their appearance to distinguish the richer from the poorer ores, and assays are always necessary to determine their value. It is frequently the case that a picked specimen of apparently pure chalcopyrite mixed with a small quantity of quartz will yield only 25 per cent, of copper, or even less, instead of over 33 per cent., which it should do theoretically. This low return is not due to the presence of quartz alone, but to an admixture of iron pyrites, and it is seldom the case that concentrated pyritic ores yield more than 15 per cent, copper on an average in a large consignment.
When a pyritic ore carrying a small percentage of copper is exposed to surface oxidation, sulphates of iron and copper are first formed. The sulphate of iron further changes to oxide, which constitutes the gossan, while the sulphate of copper is dissolved by percolating surface waters, and, part of it at any rate, carried down in the lode until it meets unaltered pyrites, which once more extracts the copper-forming chalcopyrite.
This operation goes on again and again, until a mass of gossan has been formed, and the copper carried down has enriched the ore below in copper. As long as the enrichment only produces chalcopyrite, it is difficult, without microscopic examination, to determine except by analogy, whether the ore when first struck below the gossan is a zone of enrichment or the permanent ore of the mine.
Chalcopyrite, however, acts on copper sulphate solutions in the same way, producing sulphides of copper containing a higher percentage, such as bornite, covellite, and copper glance, the presence of which show unmistakably the existence of a zone of enrichment.
Any further oxidation of these richer sulphides must necessarily result partly in the formation of oxides of copper e.g., cuprite and melaconite and native copper would also seem to be a product of the decomposition of sulphides, whether found as plates on the sides of cavities or fissures, as ramified crystals in soft clays, or as grains or masses in amygdaloids or conglomerates, as at Lake Superior.
The sulphate of copper solutions, however, lose their copper in other ways as well. When supersaturated they deposit chalcanthite by crystallisation ; when they meet with the necessary precipitants, carbonates, phosphates, arseniates, silicates, and oxychloride of copper are formed in the zone ot oxidation ; and, in the zone of surface enrichment, sulphides of antimony and arsenic appear to have the same power as sulphides ot copper to form fresh minerals, such as tetrahedrite, tenantite, enargite, and bournonite, all of which may be looked on as minerals of the surface enrichment zone.
Copper ores, especially chalcopyrite grey copper and melaconite, occasionally contain silver and gold, and at Lake Superior native copper often contains nuclei of pure silver enclosed in the mass of copper without being alloyed with it. Even low grade cupriferous pyrites, such as those of Huelva, in Spain, often contain a small quantity of silver and gold, but still sufficient to give a reasonable profit in working.
The gossan forming the cap of a lode is simply that portion from which the soluble products of decomposition have been leached, and what remains is in the form of a more or less spongy and honey-combed mass. It is consequently easy to anticipate from the nature of a gossan if the ore lying 'below is likely to be a rich compact copper ore, or whether it is mostly composed of iron pyrites carrying little or no copper. In the first instance the large percentage of copper which has been removed must have left the iron in a very porous condition ; while in the latter the gossan will generally be more compact.
Copper pyrites is not generally found pure immediately below the gossan, but a richer ore, commonly called " black ore," which has no special mineralogical name, is first met with. It is black and earthy, like manganese or black copper ore, but, if broken, nests of copper pyrites will generally be found in the centre, and the ore passes from black to yellow through intermediate shades of bronze. What is called " peacock ore " is only copper pyrites, the surface exhibiting iridescent colours. By leaving a piece of clean yellow copper pyrites in water for some time it will become coated in this way.
The easy decomposition of copper ore under the influences of the atmosphere explains why the waters at some copper mines are quite unfit to drink. It is well known that tools abandoned for a time in old workings become covered with a coating of metallic copper, as if they had been left in a bath of sulphate of copper.