Cross Lake Project (1983-86)

Geologist P.G. Lenton Year 83 STATION 5207
EASTING 575399 NORTHING 6059941 ZONE 14 DATUM NAD83
Date of mapping 08/18/1983

STRUCTURAL DATA

PETROLOGIC DATA

Rock Number: 1

Rock Type: pegmatite
Field Description:

SAMPLING
-complete cross sections of minerals and saccharoidal albite and samples of saccharoidal albite dike
DESCRIPTION FROM NW to SE
CONTACT: shows 5 to 8 cm exomorphic zone of moderate tourmalinization. Tourmaline growth consumes mafics and the greywacke goes from uniform grey rock to thin rind of white quartz-feldspar rock with 1 to 3 mm tourmalines
PEGMATITE CONTACT: thin silicified rind 2 or 3 mm thick with abundant fine grained tourmaline. Passes into quartz - albite - tourmaline - blue apatite zone with muscovite fans opening away from contact. This averages 10 cm and may be a wall zone. Red garnet, commonly intergrown with tourmaline, is a minor phase here.
-there is a sharp transition into platy pinkish grey and subhedral greyish green albite up to 7 cm with coarser muscovite, larger tourmaline (1x5 cm) 1-1.5 cm apatite and anhedral masses of chocolate brown microcline up to 15 x 30 cm (average 5x10 cm).
-Albite decreases in abundance about the middle of the intermediate zone and dike becomes dominantly quartz and muscovite
-Microcline increases in size and abundance in the inner portion of the intermediate zone.
-transition to the quartz core is marked by accumulations of blue apatite as aggregates 5 to 15 cm across. Muscovite increases rapidly in size and abundance forming huge fans of books up to 25 cm large radiating into the core zone. The muscovite fans contain radially orientated tourmaline crystals up to 1 x 10 cm.
-plates of columbite up to 5 cm across occur in a narrow zone along the intermedite zone - quartz core boundary.
CORE ZONE
-composed dominantly of quartz with webwork fracture system veined with cleavlandite
-both core margins are associated with large accumularions of alkali Fe-Mn aluminophosphate intergrown with blue apatite, blue black tourmaline and large fans of ultracoarse muscovite.
- 30-50 cm brown microcline crystals are scattered through core margins into central core.
-central core is occupied by giant masses of brown MC up to several metres in length
- 1 to 5 cm brassy weathering alkali Fe-Mn aluminophosphate are scattered randomly through core right up to and in the central core microcline (margins only)
-saccharoidal albite veins cross the quartz core and core microcline.
-clevelandite veining in quartz core is quite rich in small columbite crystals
-Southwest of core microcline mirrors the northeast upto the transition into the intermediate zone.
-from the SW alkali Fe-Mn aluminophosphate masses outward there is a sudden drastic decrease in quartz into a 50-80 cm zone of massive clevelandite+muscovite+tourmaline+ minor microcline
-transition into the next unit is again rapid, consisting of the vertical disappearance of albite leaving a coarse rock comprising 50% quartz and 50% mix of muscovite, apatite, tourmaline, minor microcline and traces of albite. This extends 15 to 45 cm.
-Beryl makes its 1st appearance here in the form of an 18.3 cm wide pale apple green crystal with a quartz-albite core (scabbard beryl)
-The final unit upto the SW contact is massive white quartz containing disseminated 1-3 cm apple green or lemon yellow beryl.
-veins of clevelandite+muscovite+tourmaline and veins of brown microcline.
Either quartz or massive clevelandite occupied the country rock contact
LARGE SAMPLES
#1. Saccharoidal albite from narrow dike, 25.4-2.4
#2. Saccharoidal albite from narrow dike, 4.6-0.7
#3. Quartz core material, 7.0+0.0
#4. Margin of narrow dike - tourmalinization, 25.4-2.4
#5. Margin of main dike
#6. Country rock pebbly wacke

PHOTOS

Geological map portraying pegmatite 37-84-5207. The dike is intruded into the bedding / foliation plane of the greywacke - conglomerate sequence. Prominant folding follows an upright northeast plunging attitude.

18.3 cm pale green beryl -- upper edge of core zone (15.8/+3.6). Note the "scabbard" structure with the core of the beryl crystal filled with quartz and albite with minor muscovite. Beryl crystal in surrouinded by quartz, beige platey albite and muscovite books.

Same view of large beryl crystal with exposure wetted to enhance view of crystal form.

Close-up of wet surface of beryl. Note the pale translucent green colour of beryl.

Large fans of massive muscovite with clots of Mn-Fe alkali phosphates around the fans. Mineralogy probably triphylite, sicklerite, bobfergusonite ? and other phospahtes. Location is upper core contact (14.7/+3.6).

Large masses of choclate brown microcline in the quartz core zone at 2.7/+7.4. Note the networks of quartz-albite veins crisscrossing the microcline.

Cluster of pale green beryls (2.3/+1.9 ). Location is at the edge of the quartz core (to right containing choclate brown microcline) and the upper intermediate zone where folding has attenuated the zone thickness.

Smaller dike of largely unzoned granitic muscovite rich pegmatite in which the central portion is largely replaced by saccharoidal albite. Location (19.0/-3.5).
Note the exomorphic features of bleaching of the mafic wacke and development of zone of tourmalinization along the contact.

Additional view of the albitized dike at (19.0/-3.5)

Huge chocolate microcline in the core zone at (18.5/+1.2)

Brown microcline showing webwork of albitization.

Margin of large mass of brown microcline agains quiartz of core zone.

Folding in saccharoidal albite dike showing development of fabric and folding of fabric about a SE axis.

Detail of deformation in dike from previous picture with foliation and folding highlited.

General view of dike looking up base line at 060 degrees. Note the steep SE dip on the contact with the country rock

Radiating fans of muscovite at base of core (looking up section) at 14.0/-2.0

General views of hanging wall contact and quartz rich upper intermediate zone. (centred around 13.5/+4.0)
The hanging wall was not exposed for Anderson who interpreted the exposure as only the lower half of a thicker dike. This part of the exposure was interpreted as the quartz core instead of an assymetric upper intermediate zone.

General views of hanging wall contact and quartz rich upper intermediate zone. (centred around 13.5/+4.0)
The hanging wall contact with the mafic wacke country rock is at the bottom of the picture.

Manitoba Geological Survey, 2017