Cargill Township carbonatite complex, District of ... - Geology Ontario

R. P. SAGE
cores to magnesio-arfvedsonite rims. The amphibole composition is richer in Fe
and alkalies and poorer in Ca and Mg as differentiation proceeds (Twyman 1983,
p. 103). Twyman (1983, p. 100) reported little or no Mn, Ti, and Al in the amphi-
boles. At Cargill the K content of the amphiboles was observed to increase with
differentiation (Twyman 1983, p. 146).
Allen (1972, p.105) reported rare zircon, diopside, and ankerite in this rock
unit. Rare zircon and very rare pyrochlore were reported by Twyman (1983,
p.97, 140). Probe analysis of this rare pyrochlore indicates Ta, U, and light rare
earth elements increase and Nb and Ca decrease from core to rim (Twyman
1983, p. 143). Twyman (1983, p. 143) correlated the pyrochlore at Cargill with
the early formed pyrochlore found at the nearby Argor carbonatite.
Sharpe (1987) completed microprobe analyses on both the carbonate and
mafic silicate phases. Microprobe analysis of one sample from International Min
erals and Chemical Corporation Limited diamond drill hole 23, drilled in the
centre of the southern complex indicated the presence of vesuvianite (Sharpe
1987, p.27). This is the first known occurrence of vesuvianite within a car
bonatite of which the author is aware. Sharpe (1987, p.26, 27, 32, 37, 38, 43,
44, 54) presented in tabular form considerable microprobe and chemical data on
the Cargill carbonatite to which the reader may wish to refer.
Silicocarbonatite
With increasing accessory mineral content, sovite grades into Silicocarbonatite. In
thin section the Silicocarbonatite is fine to medium grained, equigranular, hypid
iomorphic to allotriomorphic, with straight to curved grain boundaries.
An estimate of the mode is 5 to 5596 phlogopite, l to 3096 magnetite, O to
1096 clinohumite, 15 to 5096 carbonate, O to 596 apatite, O to 3096 olivine, and 2
to 7096 amphibole.
Phlogopite forms anhedral to subhedral grains, and commonly displays a
darker reddish brown rim similar to that observed in the sovite. Some grains
display kinking or bending of the (001) cleavage.
Magnetite forms anhedral to subhedral grains. The magnetite occurs as dis
seminated grains and also as granular aggregates.
Clinohumite occurs as anhedral grains along fractures within, and along grain
boundaries of olivine. The clinohumite has a distinctive yellow color in thin sec
tion.
Carbonate grains form an anhedral interlocking mosaic. Twin planes on the
carbonate grains may be bent in those samples displaying cataclastic textures.
Apatite occurs as rounded bead-like grains and as rounded elongated grains.
Olivine occurs as anhedral to crudely euhedral grains. Trace to minor serpen
tine may also be present.
Amphibole occurs as acicular crystals with a pale green color. Slight changes
in birefringence between core and rims of some grains suggests that the amphi
bole may, at least locally, be compositionally zoned.
Rauhaugite
Within the central zone of the complex, dolomite becomes the most abundant
carbonate phase. The presence of dolomite was confirmed by X-ray diffraction
by W. Hicks, Geoscience Laboratories, Ontario Geological Survey. The
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