Abstract

The Montgary Pegmatite in southeastern Manitoba is a large, complexly zoned, spodumene-bearing body containing the world's largest known reserves of pollucite.

The pegmatite is near the margin of a granitic intrusive rock and forms an elongate, disc-shaped body lying more or less perpendicular to the near-vertical schistosity of the enclosing Precambrian plagioclase amphibolite. The amphibolite has no compositional or textural trends beyond a narrow alteration zone that can be correlated with distance from the pegmatite. The alteration zone, 4–6 feet in average width, contains small amounts of glaucophane, biotite, apatite, and tourmaline in addition to the normal mineral assemblage of the amphibolite.

The pegmatite is subdivided into six zones, two replacement bodies, and a unit that may be a replacement body. The six zones are: (1) a quartz–albite border zone; (2) a perthite–quartz–plagioclase–muscovite wall zone; (3) a spodumene–perthite–plagioclase–quartz intermediate zone; (4) a spodumene–quartz intermediate zone with minor perthite; (5) a microcline-quartz intermediate zone, and (6) a quartz core. Neglecting the quartz core and considering the two spodumene–bearing zones as one, the internal structure is one of concentric shells. Aplitic albite replaces lower portions of the spodumene-bearing zones and upper portions of the footwall portion of the wall zone. Fine-grained lepidolite replaces parts of the microcline-quartz intermediate zone. Pollucite forms very fine-grained, monomineralic bodies cut by veins of quartz; these bodies are tentatively regarded as replacement bodies.

Lithia has been concentrated toward the hanging wall and up the dip of the pegmatite. The attitude of the pegmatite and the shape of the hanging wall amphibolite–pegmatite contact exerted an important control over lithia distribution during crystallization.

An igneous hypothesis of origin is the most acceptable. Pegmatitic fluids, probably related to the nearby granitic intrusive rock, were permissively introduced into a dilatant zone along a sinuous shear. The internal structure is consistent with fractional crystallization accompanied by resurgent boiling of the residual liquid in a restricted system. Deposition of crystals took place from the walls inward. The zonal structure has been influenced, but not caused, by the attitude of the pegmatite. Upward lithia concentration is consistent with the development of a second fluid phase by resurgent boiling following the deposition of wall-zone material. Alkali-replacement bodies were formed late in the history of the pegmatite. The fluids responsible for their formation were probably either vapors formed by resurgent boiling or residual fluids left over after the main pegmatite crystallization.

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