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

R. P. SAGE
p. 116) as a mixture of vermiculite and biotite of hydrothermal origin resulting
from the loss of K, increase in Fe and constant Al. Twyman (1983, p.115) indi
cated that the micas in the olivine-clinohumite sovites and arfvedsonite sovites
define two different chemical trends which join at high Al and Mg contents.
Magnetite is a ubiquitous mineral and occurs as anhedral to subhedral dis
seminated grains. The mineral has ragged to smooth curved grain boundaries. In
one section magnetite occurs as relatively small grains from the breakdown of
olivine. Magnetite rarely is skeletal in form. In one case the skeletal magnetite
consists of a series of parallel crystal plates of magnetite alternating with carbon
ate.
Few bands of magnetite are present at the Argor complex but many are pre
sent at Cargill (Twyman 1983, p. 150). Microprobe studies by Twyman (1983,
p. 150) indicate similar magnetite compositions at the two carbonatite complexes:
the magnetites are low in TiO2 (0.38-6.0196) and MnO (0.0-0.4096). Ilmenite,
which contains 48.9 to 52.696 TiO2 , has more MnO (1.26-2.9596) than magnet
ite.
Twyman (1983, p.97) reported the presence of other opaques. These are
pyrrhotite, pyrite and a cobalt pentlandite which occurs as blebs within the pyr
rhotite. The cobalt pentlandite is present only in his olivine-clinohumite sovite
and ilmenite and pyrite are present only in arfvedsonite sovite (Twyman 1983,
p.97).
Clinohumite occurs as more or less isolated grains and as patchy replace
ments of margins of the olivine grains. The grains vary from anhedral to sub
hedral in form and display a yellow color in thin section. Minor alteration to
serpentine may occur along fractures in the clinohumite. Several thin sections
display clinohumite enclosing rounded blebs of carbonate. Clinohumite may also
enclose acicular, euhedral amphibole crystals. This is an inherited feature re
tained from the olivine from which the clinohumite was derived*. The author
interprets the clinohumite-olivine relationship to be one of reaction of the olivine
with late magmatic fluids.
Kocman and Rucklidge (1973) investigated the crystal structure of
clinohumite from the Cargill complex. They referred to the clinohumite as
titaniferous since microprobe analysis indicated 3.1496 TiO2 . Microprobe analysis
of the clinohumite gave 36.196 SiO2, 3.1496 TiO2, 14.696 FeO, 0.6296 MnO,
44.796 MgO, 1.1596 F, and 1.4696 H2O (calculated) (Kocman and Rucklidge
1973, p.39). The presence of fluorine indicates the carbonatite magma likely
contained appreciable fluorine particularly during the later phases of crystalliza
tion when peritectic reaction of the olivine with the carbonatite magma occurred.
The clinohumite has been interpreted by Twyman (1983, p.96) as the result
of peritectic reaction of olivine with the carbonatite magma, an interpretation the
author would concur with (Sage 1983). Twyman (1983, p.97) reported that
clinohumite is commonly associated with magnetite, is locally replaced by rich
terite and dolomite by peritectic reaction, and is locally altered to serpentine and
magnetite. On the basis of microprobe data the clinohumite displays increasing Fe
and Ti from core to rim (Twyman 1983, p. 135).
Carbonate forms an interlocking mosaic of anhedral grains. In those sections
displaying deformation textures, grains of carbonate may display bent rhombo-
*The identification of clinohumite was confirmed by X-ray diffraction techniques by W. Hicks,
Geoscience Laboratories, Ontario Geological Survey.
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