Alaskan Copper Districts

The Alaskan Districts. In Alaska there are four distinct copper fields or districts. The Prince of Wales Island; Prince William Sound; Copper River district, and the Kotsina district. They are described as follows:

(A) On Prince of Wales Island, In southeastern Alaska several contact metamorphic copper ore deposits occur in irregular masses along the contacts of the intrusives with limestones. The most important copper mineral is chalcopyrite. It is associated with pyrrhotite, pyrite and magnetite with a gangue of amphibole and feldspar. Fissure veins and disseminated copper ores are also encountered. Those of Copper Mountain, Prince of Wales Island, present an unusually excellent illustration of deposits occurring at intervals along the contacts of an isolated granite intrusion. The most important ore bodies are contact-metamorphic ones occurring in irregular masses from 10 to 250 feet in dimensions, along the contacts of the intrusive rocks, usually with limestones, the ore composed mainly of chalcopyrite, magnetite, pyrrhotite, and pyrite in a gangue of amphibole, orthoclase, epidote, garnet, and calcite. In addition to these there are lode deposits in shear zones, vein deposits in fissures, and disseminated ores. The ores mined are somewhat low in grade, with a little gold and silver, but high in iron and lime, and form a desirable flux for smelters of Tacoma and British Columbia. At Copper Mountain in the Hetta Inlet district the ores are (1) contact deposits occurring between granite and lime stone or schist, and (2) vein or shear zone deposits, occurring along the bedding planes of the greenstone schist and quartzites. The contact zone is of variable width and is broadest in the limestone.

 

 

(B) On Prince William Sound:  In this district the ore is chalcopyrite disseminated through the metamorphic schists. The copper ore is mainly associated with pyrite and pyrrhotite as a cavity filling. Also as replacement deposits of impregnations in a shear zone in interbedded slates and graywackes. The most important mine is on Latouche Island, and here the ore, is composed of a mixture of chalcopyrite, pyrrhotite, and pyrite.  On the Kasaan Peninsula are several magnetite-chalcopyrite deposits, also containing pyrrhotite and pyrite, in a gangue of amphibole, epidote, orthoclase, garnet, and calcite. Wright believes that the ores were formed after the consolidation of the last intrusions of syenite. At both places shear zones and vein deposits containing copper accompany the contact deposits.

(C) Copper River District: The Copper River district is near Mount Wrangell. This region, which is situated some distance from the coast, and hence difficult of access, has been but little developed, although transportation facilities have now been provided. The primary copper ores are chiefly are chalcocite and bornite associated with pyrrhotite and magnetite as dissemination deposits in masses of greenstone. Some bornite replaces Triassic limestone near a greenstone. The ore is chiefly chalcocite, but other sulfides as well as oxidized ores occur. In the Bonanza mine on the Chitina River, the ore consists of a solid mass of chalcocite in limestone, averaging about 60 per cent copper, with about 22 ounces of silver to the ton. The Chitina copper belt, Alaska,  is an area of greenstone, mainly diabase, which is overlain by Triassic limestones. Above the limestones are later sedimentary rocks; all these rocks are cut by quartz diorite porphyry. The country is rugged, and the region of the deposits has been deeply eroded. The principal deposits are rudely tabular masses of nearly pure chalcocite rich in silver, which occur in fractured or fissured zones in the limestone above the contact with greenstone.

 

 

The chalcocite ore extends to the very surface and accumulates in talus from the cliff . Owing to the rapid mechanical disintegration and the cold climate little or no gossan is developed. Open cavities in the fractured limestone have been filled with ice, and both the country rock and the talus are frozen all summer except for a few feet at the surface. Tolman has shown that in the Bonanza mine chalcocite replaces bornite. The principal deposits are those of the Bonanza and Jumbo mines, owned by the Kennecott Copper Co. The veins in a general way may be considered as extremely thin wedges which gradually taper upward toward their apex and whose bases rest on an inclined fault or bedding plane. The length of the inclined wedges along their base is many times greater than their height from base to top. The height of most of the veins has been determined, but lengthwise along their base they are still being followed by inclined shafts. The average height of the main Bonanza vein from base to top, measured normal to the incline, is about 210 feet in the upper levels and 150 feet on the lower levels. The vein has been followed for a distance of about 1,900 feet, measured along its base, and the width varies from 2 to 50 feet.

The chalcocite is oxidized and changed to carbonates near the surface, but no gossan is  developed. It crops out on the surface and has accumulated as talus and in a glacier below the outcrop. The deposits are remarkable for the richness and unique character of the ore. There is no evidence of primary ore adequate to supply the material from which the chalcocite might have been derived. Some investigators have regarded the ore of primary, others of secondary origin. Bateman and McLaughlin are inclined to the hypothesis that the ore is of deep-seated origin. This hypothesis appears probable for many reasons. One of the most convincing is that some of the deposits are blind or terminate upward against barren rock

 (D) The Kotsina district is situated some little distance inland from the coast. The chief copper mineral is chalcocite. The ore is found as replacement deposits in the Triassic limestones associated with the earlier greenstones.

 

 Return To: Copper Ore Deposits of the USA

 

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