Barite

Regionally, the area is underlain by sediments which are part of a thick sequence of Precambrian rocks in the upper part of the Purcell Supergroup and lower part of the Windermere Supergroup. The Dutch Creek and Mount Nelson formations of the Helikian Purcell Supergroup form a conformable sequence overlain unconformably by the Toby Formation, at the base of the Windermere.

A basal white quartzite is the most easily identified marker bed in the Mineral King mine area and grades upward into a sequence of dolomites and argillites, of the Hg2 member of the Gateway Formation. The dolomites and argillites form a thick, apparently conformable, succession consisting of very fine grained buff to brown-weathering grey dolomite with argillaceous interbeds. Dolomite beds range from less than 30 centimetres to a couple of metres thick. Interbeds of argillite, a metre to tens of metres thick, are found in the dolomite, and much of the dolomite is argillaceous. The argillite is mainly dark grey but locally is greenish or brownish. The dolomites are separated from the quartzite by a fault of unknown but probably small displacement.

The basal quartzite is underlain by 304 to 914 metres of dark grey to black slate and argillite of the Hg1 member of the Gateway Formation. The contact of the quartzite with the underlying slate is sharp and apparently conformable. The slates are underlain by approximately 30 metres of dolomite which grades downward into a couple of hundred metres of interbedded dolomite and siliceous argillite.

The Toby Formation unconformably overlies the Mount Nelson and Gateway formations. It is comprised of mainly conglomerate and is found only in a zone of complicated structure.

In the Mineral King area the rocks are complexly folded and are transected by many faults. The regional structure consists of relatively open folds plunging gently to the northwest, which together form a broad geanticline extending across most of the Purcell Mountains. Southwesterly dipping thrust faults and north to northwesterly trending normal faults are common.

Many north and northwesterly trending faults (nine have been recognized) are present at the Mineral King mine . They are mainly steeply dipping normal faults downthrown on the west. Two important faults and several smaller faults are westerly dipping thrusts. The pattern of folding and of thrust faulting has resulted from a relative movement of west over east. The rocks have been folded, faulted, crushed and brecciated with subordinate shearing and flowage.

The Mineral King mine can be divided into three parts - a western, central and an eastern part, separated by faults. In the western part, the formations dip steeply to the northeast and are on the northeast limb of an asymmetrical anticline. A broad anticline is outlined in the eastern part of the area and plunges approximately 10 degrees northwest; the axial plane is essentially vertical. This anticline is one of a series of open folds which result in a low cumulative easterly dip extending several kilometres east. Between the eastern and western parts of the area the structure is dominated by a series of folds and related thrust faults in which the western side has tended to move upward and to the east over the eastern side. The folds have the form of dragfolds on the western limb of an open anticline, and because in section (looking northwest) they resemble a letter "N", they are referred to as N-shaped dragfolds. The pattern of folding and faulting is important because mineralization at the mine has been partly controlled by N-shaped dragfolds.

Widely scattered dark green fine-grained dikes, up to 4.5 metres wide, transect the area. They appear to be altered diorites and commonly strike north or northwest, dip steeply and are fairly continuous.

The Mineral King orebodies are replacements of dolomite in the upper Hg2 member of the Lower Gateway Formation, called the "mine dolomite", by sphalerite, galena, pyrite, barite and quartz. The orebodies plunge gently to the northwest, have a relatively low dip, and mainly appear to conform to fold structures within the dolomite. Toward the northwest the plunge steepens, and the orebodies are more or less continuous with other orebodies in the lower part of the mine which follow steeply dipping faults. The faults strike to the north, and sulphides and quartz occur along them as replacements and fillings.

The principal sulphides are sphalerite, galena, pyrite and minor bournonite, in a gangue of dolomite, barite and quartz. In the upper part of the mine, sphalerite and galena commonly occur as irregular masses and lenses or in more or less regular bands in dolomite. In barite their distribution is much more irregular. Pyrite is found closely associated with galena and sphalerite as well as in separate bands and lenses within or on the margins of the orebodies. Bournonite is most conspicuous in barite, where it occurs as intersecting veinlets and less commonly as massive clusters several centimetres across. Meneghinite occurs rarely with siliceous ore.

The "mine dolomite" in general is synclinal with the axis plunging 325 degrees at 30 to 35 degrees and axial plane dipping steeply to the west. The syncline is almost isoclinal. Conglomerate and dark grey argillite form the trough lying above the dolomite. Slate and argillite of the Dutch Creek Formation occur on the east on what is known as the footwall side of the syncline and are in fault contact with the dolomite. Rocks to the west, known as the hangingwall sequence, are calcareous phyllites, greenish sericitic and chloritic phyllites and argillites. At the hangingwall contact of the "mine dolomite" or at some place within the hangingwall sequence is another fault parallel to the formations. Thus the "mine dolomite" and conglomerate form a synclinal wedge between two faults.

The form of the orebodies is extremely complex. The ore above 3 level (elevation 1661 metres) occur in four trough-like structures varying from a tight V-shape on the west to an open syncline on the east. These were called the A, B, C and D zones. Below 3 level the form of the orebodies changes. The A zone loses the V-shape and becomes a northerly trending tabular body with a steep dip, apparently controlled by replacement along a fault or fracture zone. Other orebodies were found down the projected plunge of the upper zones, which were called B, C and D, although they were more or less isolated orebodies with no connection with the upper zones. Fractures, vertical fault zones, and incipient shattering of the dolomite controlled replacement. Orebodies below 3 level in general were higher grade than those above, but some zones in the lowest levels were too low grade to mine.

Ore was mined for almost 1219 metres down the plunge of the structure and through a vertical interval of approximately 457 metres.

Barite is scattered irregularly through most of the ore, and much of it contains sulphides. Zones of fairly pure barite found between the C and D zones have been mined for the barite alone. Masses of barite mined were very irregular, in general having a gentle plunge to the northwest. They are a couple of tens of metres thick in section and a couple of hundred metres long parallel to the plunge. Sulphides are more abundant around the margins than in the central parts of the barite zones.

The production of barite began in 1959 and was mined from the upper levels. Barite production for the period 1959-1967 totalled 22,780 tonnes of crude barite. During the period 1970-1973 inclusive, 38,818 tonnes of barite concentrate was shipped (barite was recovered from the tailings pond of the Mineral King mine). Barite has been recovered on a seasonal basis until 1982.

The Mineral King mine produced continuously from 1954 until 1967 and is developed by several adits and extensive underground workings.