Cross Lake Project (1983-86)

Geologist P.G. Lenton Year 83 STATION 5211
EASTING 572983 NORTHING 6049117 ZONE 14 DATUM NAD83
Date of mapping 08/18/1983

STRUCTURAL DATA

PETROLOGIC DATA

Rock Number: 1

Rock Type: pegmatite
Field Description:

DIKE A-9
Strike 070/ variable, near vertical. (See caption of first photo that updates this based on greater exposure from 1988)
-Mineralogy: spodumene, toumaline, albite, quartz, beryl, columbite, muscovite
-Dike is of variable width averaging 1.5 m. It is crudely zoned with a quartz rich core zone with extremely dominant very large spodumene and tourmaline and rare blocky microclines. Flanked by intermediate/wall zones of platy grey albite, tourmaline, minor apatite. Columbite is most common in the grey albite zone or included within blocky spodumenes. Beryl is concentrated along the boundary between a quartz core and both flanking albitic zones
-spodumenes range in size from 1 cm to 34 cm. Tourmalines can reach 63 cm. The dike is moderately sheared parallel to the contacts with the fabric particularly evident in the massive quartz which is microfractured and annealed into a beige weathering sacchoroidal textured core zone. Garnet is pale orange, probable spessartine, concentrated near the country rock contact. Secondary muscovite of a bright green and flackey nature is developed along central core fractures.
-In the continuation of the dike at the back of the bay to the SE, there is much more sachroidal albite developed as core zone veins. Beryl is particularly abundant and once again concentrated along the inner edge of the platy grey albite zone. Columbite is very abundant in the saccharoidal albite.

PHOTOS

Photo taken in 1988 during extreme low water levels.
The view is east along the shore toward the exposure of station 5211 (Anderson A-9). At the time of mapping in 1983 the only exposed pegmatite was the knob of pegmatite at the tree line at the farthest back and a small portion of the pegmatite that is lighter weathered colour on the right side of photo.
The original impression that the dike was near vertical is obviously wrong - the dike strikes parallel to the shore and dips north into the lake. The impression of steep dip comes from the near vertical shear fabric visible in the country rock and upright isoclinal folds in the pegmatite that causes the scalloped look of the contact along the tree line. The pegmatite comes out of the bush and comes along the shore passing to the left and behind the point of view of the camera.

Massive blocky subhedral to euhedral tourmaline. View is of the quartz rich core zone with fractured spodumene crystals and large euhedral spodumene growing mainly perpendicular to the core on both sides.

Contact between core zone and the flanking albite/tourmaline zone

White beryl in the albitic zone.

Radial growth of columbite plates within a blocky spodumene.

General view of core zone showing the fabric imposed by shearing. The columnar tourmaline and tourmaline crystals are aligned in the shear plane, broken into blocks and entended along C in the shear plane again suggesting that crstallization was syndeformational.

Blocky tourmalines in albitic zone near contact. The shear plane parallels the contact with the country rock. Note the large tourmaline shows pronounce dextral rotation with quartz concentrating in the pressure shadow.

Concentration of small white beryls along the inner margin of the wall zone.

Same as previous photo but shows increase in beryl size approaching a spodumene cluster.

General view looking east down the dike.

Detail of platey grey albite zone. The albite zone is relatively quartz poor but quartz increaases in the bleached looking contact wall zone.

Remarkably persistent zoned distribution of tourmaline, beryl and spodumene. Note to the left of the scale card how the white beryl crystals are larger in the vicinity of spodumene crytals and will actually mantle the spodumene.

Remanent shattered blocky microcline in the microfractured, annealed quartz core. It appears much of the shearing in the pegmatite was taken up in the quartz core and along the margins of the core.

Manitoba Geological Survey, 2017