Butte

BUTTE (EN: Butte; RU: Бьютт) is the largest deposit of copper of the veinous type within the world. The deposit is situated within the south-western part of the Montana state (USA). The deposit is operated since the 1882. The deposit is confined to the Butte massif of the Upper Cretaceous quartzous monzonites, which are forming the main part of the Boulder batholith. The area of the deposit is more than 20 square kilometres. Within the limits of the deposit, besides the quartzous monzonites, and besides the accompanying them aplites and pegmatites, there are distributed also the stocks and dikes of the quartzous porphyries, and the more young, postmineral rhyolites.

The deposit comprises the great quantity of the veins, which are grouping themselves within the horizontal plane as the shape with the characteristic structure of the "horse's tail" type. There are main the most early large fissure veins of the Anaconda system, which are having the north-eastern strike within the western part of the deposit. Along the strike, the veins of this system have been unsealed for almost 8 kilometres, at the depth of up to 1500 metres, with the average thickness of 6-9 metres (up to 30 metres within the bulges). Another system, which is important in the practical sense, of the so-named "blue" veins, has the north-western strike. The dip of the veins is steep southern, which flattens itself with the depth. The veins of this system traverse everywhere the veins of the Anaconda system, with the left horizontal displacement of up to 60 metres. The length of the industrial places of the most large veins is 300-700 metres along the strike, and 180-540 metres along the dip, with the average thickness of 1.5-6.0 metres, more often the length is 60-90 metres, with the small thickness. The important structural element of the deposit is the faults with the north-eastern strike. The places of the zones within the adjacent veins of the "horse's tail" type, which are suitable for the bulk excavation, commonly have the length of up to 100 metres, and the height of up to 60 metres. Besides this fact, at the south of the central part of the deposit, there is distinguished the mineralized block with the size of 600x150 metres, which has been traced along the vertical to 900 metres.

There have been identified within the ores 66 minerals, the main of them are quartz, pyrite, enargite (it accounts for more than 50% of copper), bornite, chalcocite, and there are less distributed chalcopyrite, sphalerite, rhodochrosite, molybdenite, tennantite, galena, native silver and gold. Enargite and sphalerite concentrate themselves mostly within the Anaconda veins. At the deposit, to the depth of 35-150 metres, there is developed the zone of oxidation, and to the depth of 300 metres, there is developed the zone of the secondary sulfidic beneficiation. The deposit is being developed using the underground method (the depth is to 1500 metres), with the shrinkage, and with the borehole breaking of the ore from the updip of the layers. Since the 1955, at the places of the development of the zones of the adjacent veins of the "horse's tail" type, there is used the opencast method for the development. At one of such places, there acts the "Berkeley" open pit mine, the depth is 330 metres (the stripping ratio is 3.2 tonnes per tonne). More than 70% of the extraction of the copper ores is accounted for the poor ores within the venulous zones (the average content of copper within the ores is 0.67%), which are being developed using the opencast method. During the 1975-77, there were extracted 90-100 thousand tonnes of copper per year. The remaining reserves of the deposit are estimated at 3 million tonnes of copper, with its content within the ore of 0.6-4.5%. The ore is processed at the beneficiation plant, with the productivity of 42 thousand tonnes of the ore per day.

The deposit has been exhausted to the significant measure. Since the start of the operation, there have been extracted at the deposit more than 500 million tonnes of the ore (1980), from which there have been extracted approximately 9 million tonnes of copper, more than 2 million tonnes of zinc, 1.7 million tonnes of manganese, approximately 400 thousand tonnes of lead, 20 thousand tonnes of silver, and 80 tonnes of gold, and also the significant amount of cadmium, bismuth, arsenic, selenium, and tellurium.