MINERALS OF THE ZONES OF ENRICHMENT AND OXIDATION

Siderite

Under the zone of secondary enrichment the presence of siderite has already been noted. This mineral occurs sometimes in veinlets replacing either sulphides or chloritic minerals or calcite. At other times siderite is found in a curious box-work honeycomb structure as if it had replaced laminated rock along certain planes and the intervening material had been later dissolved out. The ferrous carbonate has a distinct selective action in replacing a rock, as it usually leaves intact pyrite grains, which then appear contemporaneous with the siderite. Sphalerite and galena are also found more abundantly with the siderite than around in the unaltered formation, and this fact has pointed to the possibility that these two sulphides may have been here deposited by secondary solutions.

Copper minerals in the siderite zone are very faintly coated with chalcocite. Gypsum is also found in vugs, even deeper than siderite, but apparently in late circulation channels.

Siderite probably formed as an advance mineral, as oxidation progressed downward. This may be proven by the closely similar forms taken by limonite in some gossans above enrichment, and the presence of siderite cores in that limonite. The iron carbonate has played an important economic rôle in serving as a precipitant for rich cuprite ores in the Czar mine, where copper-laden solutions have encountered a flat bed in Devonian limestone previously replaced by siderite and kaolin, and have precipitated cuprite and native copper in considerable amount. Throughout the district cuprite crystals can be found on oxidized or partially oxidized siderite.

Chalcocite

In the zone of sulphide enrichment chalcocite was the only secondary mineral of copper formed to any extent. It was usually formed by replacement of the bornite and chalcopyrite alone, leaving the pyrite untouched, or only slightly coated. The broken up, replaced pyrite grains in the primary ore remain in just about the same amount and with the same relations to chalcocite that they had to the primary minerals. When chalcocite and pyrite are the only remaining sulphides, it is difficult. to believe that the iron mineral has not been abundantly attacked by secondary action. Pyrite has been replaced to a considerable extent, however, in the enriched ores of the contact-metamorphic type, by a sooty variety of chalcocite that possibly contains considerable covellite, since the latter has been seen coating pyrite grains in these ores.

In replacing the primary minerals chalcocite behaved in different ways depending on the general feature of the orebody. In the ore of

Sacramento Hill, pyrite is barely coated in the very sericitic portions of the porphyry, and is not affected appreciably in silicified rock. The selective action in replacing bornite is better marked here than anywhere

else as is also the replacement along crystallographic lines of the bornite in the arrangement commonly known as "lattice structure." This form of alteration grades into the usual one of veinlets and irregular remnants, as is shown in Plate 15. The field occurrence of this "lattice

structure" alteration has been quite carefully followed in the district and found everywhere to coincide with slow or incipient enrichment in rich ores, and where the selective enrichment of bornite in place of chalcopyrite is well-marked. This has led to the conclusion that it is a delicate process in which crystallographic weakness can be of some influence. Etching of the chalcocite thus formed shows isometric figures, probably due to included or dissolved cupric sulphide, 10 but there are also stringers in which the chalcocite has orthorhombic cleavage, and these stringers can be seen to form a network of veins. The proximity and association of this orthorhombic chalcocite to major cracks and circulation channels, where even limonite is formed, points to its being due probably to the working over and recrystallization of the copper sulphides first derived from bornite.

Chalcocite of the "sooty" variety is found most commonly in the enriched ores of the zone of contact-metamorphic limestones. Massive chalcocite is here rare, and then only at the top of the zone of enrichment, with considerable native copper present.

In the ores that replace limestone, the formation of chalcocite is accompanied by leaching and formation of abundant cavities in the ore, a process that is reflected in the decrease of values as previously shown. This same thing is true in ores that have a very siliceous gangue, such as those in contact breccia. Pyrite is here oxidized to limonite even before the copper minerals are replaced by chalcocite, as is shown in Plate 16. In the Southwest mine there is an occurrence of chalcocite that is somewhat different from the usual. The Devonian-Escabrosa contact beds are replaced by silica, almost chalcedonic in character, accompanied by abundant fine flaky specularite. This siliceous mass connects up with the main silica breccia replacement in this mine.

The top Devonian beds under those silicified have been hardly altered at all, except for a very slight amount of quartz addition, some recrystallization of calcite in stringers, and the formation of very fine veinlets as well as disseminated grains about a millimeter in diameter of what was apparently bornite and chalcopyrite, now replaced almost entirely by chalcocite. The original copper minerals have been found partially replaced by chalcocite in the center of coarse calcite veins that protected them. Pyrite is very rarely found.

The formation of chalcocite has taken place in this instance under conditions which would hardly allow the circulation of acid waters. A considerable amount of carbonates has formed, but in general the sulphide is relatively abundant. The possibility of chalcocite formation. with the acid generated in the replaced mineral itself is indicated in this orebody, though this is by no means perfectly clear.

10 Posnjak, Allen and Merwin: Economic Geology, vol. 10, No. 6.

Covellite

Covellite is formed in very minor amounts especially in the narrow oxidized zone above the contact breccia ores. It replaces chalcopyrite in preference to bornite, and is found always in microscopic needles. The only other occurrences of this mineral are where the enriched zone touches highly silicified, not very sericitic ores.

Bornite and chalcopyrite have been found in many ores of the district, formed by secondary agencies, as transition products, but they are of no economic importance.

Gangue Minerals

The gangue minerals that accompany the enriched sulphides are halloysite, kaolin, gibbsite, alunite, and serpentine with mixtures of quartz, calcite and the oxides of iron. Of the aluminous minerals, halloysite seems to be the most common, especially in the zone of highly altered limestones. Where the rock is sericitized so that the mica flakes are well formed, sericite persists unaltered through the process of enrichment and oxidation.

Oxide Ores and Native Copper

In the oxide ores just above the secondary sulphides, cuprite, native copper, tennorite, melanochalcite and delafossite are found, but the first two are the only ones of economic importance. The association of cuprite and siderite has already been given. The zone of oxide has in many instances encroached entirely on the sulphides, leaving only a remnant here and there of the latter. Flat orebodies along the limestone bedding are especially susceptible to this process.

The minerals of the residual orebodies are mostly the carbonates malachite and azurite, with some chrysocolla, aurichalcite and brochantite. Besides these, there are the sulphides already mentioned.

Carbonates

The association and the occurrence of the carbonates has been very well described by Dr. Douglas11 and by Ransome12 and no additional features can be given, except to repeat that these minerals are found all through the oxidized portion of the ores that replaced unaltered sediments, and are still a very important part of the ore reserves of the district.

Other Ores

In recent years oxidized lead ores have become more and more important, especially from around highly siliceous replacements in Martin and

11 Transactions, vol. 29, pp. 511-546 (1900).

12 Loc. cit., p. 125.