Cargill Township carbonatite complex, District of ... - Geology Ontario
Cargill Township carbonatite complex, District of ... - Geology Ontario
Cargill Township carbonatite complex, District of ... - Geology Ontario
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CARBONATITE - ALKALIC ROCK COMPLEXES: CARGILL TOWNSHIP<br />
vine. Magnetite has been observed to completely enclose the pyroxene and one<br />
magnetite grain was noted to contain a small grain <strong>of</strong> olivine. One thin section<br />
displays subhedral to euhedral magnetite. This section also contains amphibole<br />
and pyroxene displaying optically continuous structures which are <strong>complex</strong>ly in<br />
tertwined suggesting either simultaneous crystallization or a secondary origin for<br />
the amphibole. The euhedral magnetite grains are commonly poikilitically en<br />
closed in the amphibole but euhedral grains straddling the pyroxene-amphibole<br />
boundary are also present. Magnetite is rarely poikilitically enclosed in pyroxene.<br />
In the amphibole-rich rocks or soda pyroxene-rich rocks it may display a de<br />
cided skeletal texture consisting <strong>of</strong> alternating plates <strong>of</strong> carbonate and magnetite.<br />
Allen (1972, p.47) identified the oxide phases as titanomagnetite, magnetite<br />
with ilmenite lamellae, magnetite with exsolved ulvospinel, ilmenite, ilmenite plus<br />
magnetite, and magnetite with exsolved ulvospinel and hercynitic spinel. The sul<br />
phide phase which consists <strong>of</strong> pyrrhotite with exsolved chalcopyrite, probably<br />
formed as an immiscible sulphide liquid (Allen 1972, p.47). The total sulphide<br />
content approaches 1096 and rarely 1596. Pyrrhotite is by far the dominant phase.<br />
The author purposely avoided samples <strong>of</strong> high sulphide content for thin section<br />
study and complete rock analysis. The sulphides occur interstitially to the oxide<br />
and silicate phases and could be the result <strong>of</strong> either liquid immiscibility or late-<br />
stage magmatic crystallization.<br />
Allen (1972, p.47-48) interpreted the oxides in rocks containing low concen<br />
trations <strong>of</strong> oxide minerals as being intercumulus in nature, but those rocks with a<br />
high oxide content as being likely cumulates.<br />
Phlogopite is rarely present in the clinopyroxene-rich rocks examined by the<br />
author. Where present it occurs as small grains along cracks in the pyroxene and<br />
along grain margins. One phlogopite grain was noted to be poikilitically enclosed<br />
in pyroxene. Allen (1972, p.86) reported phlogophite as anhedral spongy net<br />
works poikilitically containing unaltered relicts <strong>of</strong> unreplaced amphibole and/or<br />
pyroxene. The author did not observe this last textural relation but agrees with<br />
Allen (1972, p.96) that the phlogopite is likely non-igneous and is a late replace<br />
ment phenomenon. This replacement may be the result <strong>of</strong> late-stage deuteric<br />
fluids reacting with the earlier crystallized phases. However the replacement is<br />
more likely the result <strong>of</strong> alkali metasomatism by fluids derived from the younger<br />
<strong>carbonatite</strong> magma which intrudes the pyroxenite. The phlogopite reaches its<br />
greatest development in the area <strong>of</strong> contact between <strong>carbonatite</strong> and pyroxenite.<br />
The phlogopite formation is a form <strong>of</strong> fenitization and the process has been dis<br />
cussed by Gittins et di. (1975). The general lack or very low content <strong>of</strong><br />
phlogopite observed by the author in the pyroxenite rocks is likely due to the fact<br />
that the author avoided sampling rocks that appeared to be altered or <strong>of</strong> ques<br />
tionable genesis and these generally contain greater quantities <strong>of</strong> mica.<br />
Trace to minor amounts <strong>of</strong> sphene and carbonate are present in one or more<br />
<strong>of</strong> the thin sections.<br />
Neglecting the minor components, the clinopyroxene-rich rocks are visually<br />
estimated to contain 55 to 8096 clinopyroxene, O to 2096 olivine, 5 to 1596 amphi<br />
bole, and 5 to 3596 opaques.<br />
Twyman (1983, p.98) reported banding in the mafic silicate rocks based on<br />
mineralogy and grain size and reported that brecciated zones parallel this band<br />
ing. The mafic silicate rocks were interpreted by Twyman (1983, p. 100) to be<br />
cumulates. Twyman (1983, p.49) completed a limited number <strong>of</strong> whole rock<br />
chemical analyses <strong>of</strong> the mafic silicate rocks.<br />
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