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

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CARBONATITE - ALKALIC ROCK COMPLEXES: CARGILL TOWNSHIP pyroxenites to have resulted solely from the metasomatic effects of the car- bonatitic fluids. Amphibole replacement of pyroxene, in part at least, was likely the result of interstitial deuteric fluids and thus an autometamorphic effect related to the magmatic event that formed the complex. Some of the amphibole may be related to reaction with metasomatic fluids derived from the carbonatite. GEOCHRONOLOGY Gittins e t al. (1967) obtained the first isotopic age on the Cargill Township Carbonatite Complex. K-Ar isotopic ages obtained from biotite were 1790, 1820, and 1860 Ma. In 1986, Kwon obtained a U-Pb isotopic age of 1906 30 Ma on zircon. He noted that the U-Th-Pb did not behave as a closed system. Kwon (1986) plotted 2oepb7204pb - 207pb7204pb isotope ratios and obtained an errorchron age of 1890 100 Ma. He also obtained an Sm-Nd errorchron age of 1903 100 Ma (Kwon 1986). He interpreted his isotopic data as suggesting that a thermal distur bance had occurred at approximately 500 Ma. The lead isotopic data indicates that the carbonatite magma has not been contaminated by Kapuskasing gneissic rocks. Kwon (1986) compared the lead isotopes at Cargill Township with those at the nearby Borden Township carbonatite which may be 30 Ma younger and con cluded that the differences can be explained only by a heterogeneous mantle source. Kwon (personal communication, 1987) revised the Cargill zircon U-Pb isotopic age to 1907 4 Ma. Sharpe (1987) reported an Rb-Sr whole rock-mineral isochron age of 1891 15 Ma with an initial 87Sr7^Sr ratio of 0.70185 0.00004. Sharpe (1987) indicated that variation in the 87Sr786Sr ratio exceeds analytical uncer tainty which she suggested was a reflection of a heterogeneous mantle source. The ratio is consistent with a mantle source uncontaminated by crustal rocks (Sharpe 1987). STRUCTURAL GEOLOGY REGIONAL SETTING The Cargill Township complex lies within the Kapuskasing Subprovince (Struc tural Zone) of the Superior Province. This subprovince is characterized geophy- sically as a northeast-trending zone of gravity highs and pronounced linear aeromagnetic trends (Innes 1960; ODM-GSC 1970). This anomalous gravity zone has been interpreted as reflecting an upwarp in the Conrad discontinuity caused by major regional faulting and the formation of a complex horst structure (Wilson and Brisbin 1965; Bennett et al. 1967). This prominent regional struc ture extends southward from the south end of Hudson Bay, becoming broader and more ill-defined as it approaches the Lake Superior basin. The Cargill Town ship Carbonatite Complex has been emplaced into this structure, which also con tains many other carbonatite-alkalic rock complexes north and south of Cargill Township. Recent studies of the Kapuskasing Structural Zone indicate it is an oblique section through about 20 km of Archean crust uplifted along a northwest-dipping thrust fault (Percival and Card 1983). To the east, high-grade gneissic rocks are in sharp fault contact with low-grade rocks and to the west high-grade rocks grade westward to low-grade rocks over a distance exceeding 100 km (Percival and Card 1983). The author concurs with Percival and Card (1983) in that the Kapuskasing Structural Zone is likely a tilted upthrust segment of Archean crust. 24
INTERNAL STRUCTURES R. P. SAGE The dumbbell shape of the complex likely resulted from right lateral displacement along a northeast-trending fault that cuts the complex; strike-slip movement is approximately 4.0 km (Sandvik and Erdosh 1977, p.94). The fault may have been a pre-intrusive feature which controlled the emplacement of the carbonate and later became re-activated (Sandvik and Erdosh 1977, p.94). Most of the isotopically dated alkalic rock-carbonatite complexes within the Kapuskasing Subprovince are Late Precambrian (~ 1.0 Ga) in age and the faulting of this Middle Precambrian intrusion may be a function of reactivation of earlier struc ture concomittant with this younger event. Some evidence of post-emplacement adjustment along the fault is suggested by the presence of unfenitized quartz dio rite gneiss in contact with the southern subcomplex along the strike of the inter preted fault zone, and the presence of visible fenitization peripheral to the west subcomplex away from the fault (Sandvik and Erdosh 1977). This distribution of fenitized and unfenitized rock suggests that the quartz diorite gneiss marginal to the south subcomplex was not originally in direct contact with the intrusion at the time of its emplacement. Sandvik and Erdosh (1977, p.45) have also postulated shearing peripheral to the complex. Sandvik and Erdosh (1977, p.93) reported that the contact zone between carbonatite and pyroxenite consists of interbanded carbonatite and pyroxenite varying from a few centimetres to several metres thick. The banding is steep to vertical in dip. In examination of a limited amount of diamond drill core of Inter national Minerals and Chemical Corporation (Canada) Limited, the author noted that mineralogic banding in the carbonate phases was parallel to, or closely paral lel to the axis of the core. This suggests a vertical to subvertical dip to the banding since the core was from vertical holes. Allen (1972, p.57, 58) reported layering on the order of 2 to 10 cm in the pyroxene-rich section of diamond drill core of Kennco Exploration (Canada) Limited. The banding is at approximately 45 0 to the core axis. The drill hole from which the core came was not specified by Allen (1972, p.54). Since the drilling of Kennco Exploration (Canada) Limited was at various angles the signifi cance of the orientation of the banding is uncertain. Banding as displayed in the core of International Minerals and Chemical Cor poration (Canada) Limited and reported by Allen (1972) from core of Kennco Exploration (Canada) Limited is defined by variable concentrations of different mineral species. Banding analysis suggests that internal structures within the Car gill Township complex are vertical to subvertical in attitude. RECOMMENDATIONS FOR FUTURE STUDY The study of Allen (1972) was concerned mainly with the pyroxenide border of the complex. Locations of the samples studied are not presented within his thesis. Petrographic examination of samples collected by the author has been very brief and additional study is is warranted. The work of Twyman (1983) and Sharpe (1987) provided a large volume of chemical and microprobe data. Additional chemical and microprobe studies would be best directed to similar but more poorly known complexes so that petrogenetic modeling can be expanded and modified. 25
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 and 22: R. P. SAGE more euhedral when in co
Page 23 and 24: Other Mafic to Ultramafic Rocks R.
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: R. P. SAGE containing approximately
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 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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Cargill Township carbonatite complex, District of ... - Geology Ontario

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