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

R. P. SAGE
mined thickness. Sandvik and Erdosh reported that these thick residual accumu
lations overlie sideritic carbonatite and that the topographic low in the southwest
corner has been interpreted by them to be a sinkhole. An unleached calcitic
carbonatite of the core of the complex is traversed by a third northeast-trending
residuum filled trough which is also underlain by siderite.
Sandvik and Erdosh (1977) reported that the troughs are commonly steep-
sided to nearly vertical, and that depth may exceed width. Sinkholes may also
occur along a ridge axis. Sandvik and Erdosh suggested that peripheral shearing,
later modified by northeast-trending strike-slip faults, and lithologic distribution,
have all exerted control on the distribution of residuum accumulations. Erdosh
(1979) later suggested that jointing and faulting controlled solution of the carbon
ate minerals and that joint-fault intersections are particularly favorable sites for
solution and development of karst-like features. The present author observed
cataclastic textures in the dolomitic protore to the residuum deposits lending sup
port to the above interpretation.
Kelley (1984a) subdivided the phosphate residuum into 3 types: grey, black
and red residuum. The grey residuum is apatite mixed with minor clay (32 to 409&
P2O5); the black residuum is a mixture of organic material, pyrite and apatite (28
to 339k P2O5) and the red residuum is a mixture of iron oxides plus apatite (trace
to 35*7o P2O5). The red residuum contains from 2 to 3096 Fe2O3 (Kelley 1984a).
Kelley (1984a) reported that grey and black residuums form interfingering
lenses adjacent and beneath the trough of preglacial sediments in the centre of
the carbonatite while the red residuum occurs at the northern and western parts
and outer margins of the southern part of the carbonatite complex. A zone of
uranium, niobium and rare earth element enrichment occurs at the contact of the
residuum with overlying preglacial sediments (Kelley 1984a).
Kelley (personal communication) reported that a discrete layer of variable
thickness occurs at the base of the residuum which is composed of magnetite,
apatite, and chert. This layer has been interpreted by Kelley to be the result of
weathering of the carbonatite and deposition by percolating groundwater.
The deep weathering of the Cargill carbonatite, within a granitic terrain that is
not readily amendable to solution or groundwater movement, poses an interesting
problem. Groundwater movement to promote such deep weathering and solution
of the carbonate rock may have been controlled by the Cargill fault which cuts
and offsets the carbonate complex. Perhaps faulting is essential in such a terrain
to provide groundwater access to the carbonatite so that karst structures can form
and a residuum develop.
PREGLACIAL AND GLACIAL DEPOSITS
The residuum is overlain by a blanket of overburden which thickens to the south-
west where it exceeds 165 m (Sandvik and Erdosh 1977, p.95). The overburden
is generally thickest over the residuum troughs and thinnest over the unleached
rock (Sandvik and Erdosh 1977, p.95). This unit is composed of quartz sand that
commonly contains wood chips, organic clay, and kaolinite and minor inter
bedded gravel and silt (Sandvik and Erdosh 1977, p.92). The quartz sand is fine
to coarse grained, white to grey in color, and individual grains are irregular in
shape and appear unweathered (Sandvik and Erdosh 1977, p.92).
Sandvik and Erdosh (1977, p.92) reported that dark brown to black, soft
organic clay interbedded with the quartz sand occurs in thicknesses of up to a few
metres. Thin interbeds of sand and gravel are ubiquitous (Sandvik and Erdosh
1977, p.92).
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