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

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CARBONATITE - ALKALIC ROCK COMPLEXES: CARGILL TOWNSHIP vine. Magnetite has been observed to completely enclose the pyroxene and one magnetite grain was noted to contain a small grain of olivine. One thin section displays subhedral to euhedral magnetite. This section also contains amphibole and pyroxene displaying optically continuous structures which are complexly in tertwined suggesting either simultaneous crystallization or a secondary origin for the amphibole. The euhedral magnetite grains are commonly poikilitically en closed in the amphibole but euhedral grains straddling the pyroxene-amphibole boundary are also present. Magnetite is rarely poikilitically enclosed in pyroxene. In the amphibole-rich rocks or soda pyroxene-rich rocks it may display a de cided skeletal texture consisting of alternating plates of carbonate and magnetite. Allen (1972, p.47) identified the oxide phases as titanomagnetite, magnetite with ilmenite lamellae, magnetite with exsolved ulvospinel, ilmenite, ilmenite plus magnetite, and magnetite with exsolved ulvospinel and hercynitic spinel. The sul phide phase which consists of pyrrhotite with exsolved chalcopyrite, probably formed as an immiscible sulphide liquid (Allen 1972, p.47). The total sulphide content approaches 1096 and rarely 1596. Pyrrhotite is by far the dominant phase. The author purposely avoided samples of high sulphide content for thin section study and complete rock analysis. The sulphides occur interstitially to the oxide and silicate phases and could be the result of either liquid immiscibility or late- stage magmatic crystallization. Allen (1972, p.47-48) interpreted the oxides in rocks containing low concen trations of oxide minerals as being intercumulus in nature, but those rocks with a high oxide content as being likely cumulates. Phlogopite is rarely present in the clinopyroxene-rich rocks examined by the author. Where present it occurs as small grains along cracks in the pyroxene and along grain margins. One phlogopite grain was noted to be poikilitically enclosed in pyroxene. Allen (1972, p.86) reported phlogophite as anhedral spongy net works poikilitically containing unaltered relicts of unreplaced amphibole and/or pyroxene. The author did not observe this last textural relation but agrees with Allen (1972, p.96) that the phlogopite is likely non-igneous and is a late replace ment phenomenon. This replacement may be the result of late-stage deuteric fluids reacting with the earlier crystallized phases. However the replacement is more likely the result of alkali metasomatism by fluids derived from the younger carbonatite magma which intrudes the pyroxenite. The phlogopite reaches its greatest development in the area of contact between carbonatite and pyroxenite. The phlogopite formation is a form of fenitization and the process has been dis cussed by Gittins et di. (1975). The general lack or very low content of phlogopite observed by the author in the pyroxenite rocks is likely due to the fact that the author avoided sampling rocks that appeared to be altered or of ques tionable genesis and these generally contain greater quantities of mica. Trace to minor amounts of sphene and carbonate are present in one or more of the thin sections. Neglecting the minor components, the clinopyroxene-rich rocks are visually estimated to contain 55 to 8096 clinopyroxene, O to 2096 olivine, 5 to 1596 amphi bole, and 5 to 3596 opaques. Twyman (1983, p.98) reported banding in the mafic silicate rocks based on mineralogy and grain size and reported that brecciated zones parallel this band ing. The mafic silicate rocks were interpreted by Twyman (1983, p. 100) to be cumulates. Twyman (1983, p.49) completed a limited number of whole rock chemical analyses of the mafic silicate rocks. 12
Other Mafic to Ultramafic Rocks R. P. SAGE The mafic altered rocks appear to be a transitional phase between the carbonatite and clinopyroxenites and may or may not be hybrid. One thin section of an al tered rock contains a visually estimated 109& biotite, 459fc green amphibole, 159& magnetite, and 309& carbonate. The amphibole is pleochroic, green to dark green and distinctly different from both the brown to greenish brown amphiboles in the clinopyroxene-rich rocks and the very pale green acicular amphiboles of the ca rbonatite. The amphibole is likely a soda-iron-rich variety. The magnetite is an hedral, and commonly skeletal, consisting of platey grains that alternate with car bonate. The biotite forms books of anhedral to subhedral outline and rarely dis plays dark brown cores which imply that the grains are compositionally zoned. Allen (1972, p.76-77) described a hornblende-oligoclase clinopyroxenite rock. Allen (1972, p.76) found only two specimens and its presence was not observed by the author. This rock is similar to the other clinopyroxene-rich rocks but contains 5 to 896 calcic oligoclase. The oligoclase is described as inter cumulus, and occurs as ragged anhedral grains surrounded by kaersutite amphi bole (Allen 1972, p.78). The plagioclase is poorly twinned, slightly sericitized, and contains abundant rod-like apatite needles. The author observed the late stage veins and segregations described by Allen (1972, p.86-94) in the diamond drill core from the pyroxenite but did not analyse them. The simple veins have a maximum thickness of l to 1.5 cm and consist solely of hornblende plus minor sphene, calcite and apatite (Allen 1972, p.88). The complex veins and segregations have thicknesses up to and exceeding 14 cm, and have a mineralogy consisting of hornblende, biotite, sphene, soda augite, aegirine augite, apatite, and calcite all of which occur in highly variable amounts (Allen 1972, p.88). The grain size of the complex veins approaches pegmatitic in places (Allen 1972, p.88). Allen (1972, p.86) considered the min erals of the veins to have formed by crystallization of late residual liquids of the magma. Phlogopite-clinopyroxenite rocks were not examined by the author. Allen (1972, p.94-99) considered the phlogopite to be the product of metasomatic reaction. Allen (1972, p.97) noted that the phlogopite replaces both pyroxene and amphibole with the amphibole being most susceptible to replacement. The preferred habit of phlogopite is interstitial. Allen (1972, p.97) also reported nar row phlogopite-filled fractures where the pyroxene borders appear partially resorbed in contact with phlogopite. In areas of concentrated mica, the clinopyroxene rims at the contact with the phlogopite may be weakly sodic. Re placement of serpentine by phlogopite is also reported by Allen (1972 p.97). While examining the Cargill complex, blocks of coarse-grained phlogopite pyroxenite were noted to have been dug up along skid trails. The phlogopite sheets are up to 8 cm in diameter and associated with slightly smaller pyroxenes. Some magnetite is present. Due to the deeply weathered and coarse grained na ture of the rock they were not sampled for thin sectioning or chemical analysis. At the Argor carbonatite complex Twyman (1983, p. 109) interpreted the hybrid rocks to have resulted from assimilation of the wall rocks by carbonatite magma. Sharpe (1987) estimated the hybrid zone at Cargill to be 20 to 40 m wide. 13
Page 1 and 2: THESE TERMS GOVERN YOUR USE OF THIS
Page 3 and 4: Ontario Geology of Ministry of Mine
Page 5: Foreword The Cargill Township Carbo
Page 8 and 9: TABLES 1. Table Of Lithologic Units
Page 11 and 12: Geology of Carbonatite - Alkalic Ro
Page 13 and 14: Introduction The Cargill Township C
Page 15 and 16: R. P. SAGE Township and extends int
Page 17 and 18: General Geology The Cargill Townshi
Page 19 and 20: R. P. SAGE A crandallite-rich blank
Page 21: R. P. SAGE more euhedral when in co
Page 25 and 26: R. P. SAGE p. 116) as a mixture of
Page 27 and 28: R. P. SAGE cores to magnesio-arfved
Page 29 and 30: R. P. SAGE mined thickness. Sandvik
Page 31 and 32: TABLE 2. STRATIGRAPHY OF OVERBURDEN
Page 33 and 34: R. P. SAGE containing approximately
Page 35 and 36: INTERNAL STRUCTURES R. P. SAGE The
Page 37 and 38: Economic Geology The Cargill Townsh
Page 39 and 40: R. P. SAGE Thoburn (1984) prepared
Page 41 and 42: Appendix A Petrographic Description
Page 43 and 44: R. P. SAGE Clinohumite is anhedral
Page 45 and 46: R. P. SAGE with brown cores and red
Page 47 and 48: R. P. SAGE hedral grains disseminat
Page 49 and 50: R. P. SAGE pyroxene and occurs alon
Page 51 and 52: R. P. SAGE grains disseminated thro
Page 53 and 54: TABLE A-2. MAJOR ELEMENT ANALYSES (
Page 55 and 56: TABLE A-2. CONTINUED. Ref. No. SiO2
Page 57 and 58: TABLE A-2. CONTINUED. Ref. No. SiO2
Page 59 and 60: R. P. SAGE TABLE A-3. TRACE ELEMENT
Page 61 and 62: TABLE A-3. CONTINUED. Ref. No. Ag A
Page 71 and 72: TABLE A-4. CONTINUED. Ref. No. QZ C
Page 73 and 74:
TABLE A-4. CONTINUED. Ref. No. QZ C
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
R.P. SAGE TABLE A-5. AVERAGE CHEMIC
Page 83 and 84:
Appendix B — Electron Microprobe
Page 85 and 86:
R.P. SAGE TABLE B-2. MICROPROBE MIN
Page 87 and 88:
R. P. SAGE TABLE B-3. MICROPROBE MI
Page 89 and 90:
TABLE B-5. MICROPROBE MINERAL ANALY
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
References Allen, C. 1972: The Petr
Page 99 and 100:
Index Aeromagnetic data, 7, 21, 24
Page 101 and 102:
Kinking Biotite, Rauhaugite, 18 Phl
Page 103:
Chart A — Cargill Carbonatite Com
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