BCGS: Carbonatites, Nepheline Syenites & Related Rocks in British Columbia (Chapter 1) (Pell, 1994)

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The British Columbia Geological Survey's 1994 report on Carbonatites, Nepheline Syenites, Kimberlites and Related Rocks in British Columbia by Jennifer Pell.

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BCGS: Carbonatites, Nepheline Syenites & Related Rocks in British Columbia (Chapter 1) (Pell, 1994)

  1. 1. Province of British Columbia MINERAL RESOURCES DIVISIONMinistry of Energy, Mines and Geological Survey BranchPetroleum ResourcesHon. Anne Edwards, Minister CARBONATITES, NEPHELINE SYENITES, KIMBERLITES AND RELATED ROCKS;IN BRITISH COLUMBIA This is a Contribution to the CanaddBritish Columbia Mineral Development Agreement1985-1990 By Jennifer Pel1 BULLETIN 88
  2. 2. ~~~~~~~ ~ Pell, Jennifer,1956- -~ Canadian Catalooulno in Publication Data ~~ Carbonatites, nepheline syenites, kimberlites and related rocks in British Columbia (Bulletin, ISSN 0226-7497;88) Issued byGeologicalSurveyBranch. This is a Contribution to the CanadaIBritisb Columbia Mineral Development Agreement 1985-1990." Indudes bibliographical references:p. ISBN 0-77262170-5 1 Carbonatites -British Columbia. 2. Nepheline . Field Research furthis project was caniec! uut syenite -British Columbia. 3. Kimberlite - British durizgtheperiod 1984 to 1990 Columbia. 4. Geology, Economic - British Columbia. 5. Geochemistry - British Columbia, 6. Petrology - British Columbia. I. British Columbia. Ministry of Energy, Mines and Petroleum Resources. 11. British Columbia. GeologicalSurveyBranch. 111.CanadalBritish Columbia Mineral Development Agreement. IV. Title. V. Series: VICTORIA Bulletin (British Columbia. Ministry of Energy, Mines and BRITISH COLUMBIA Petroleum Resources) ; 8 . 8 CANADA553.609711 TN27.B7P44 C94-960219-1 1994 July 1994
  3. 3. Ministry of Energx Minesand Petroleum Resources TABLE OF CONTENTS ” INTRODUCTION........................................................................ 1 Geochemistry...................................................... Distribution and general characteristics of Geochronology ................................................... 36 carbonatites and nephelinesyenites .................... 2 CARBONATITES AND SYENITE GNEISS COMPLliXES IN Distribution and general characteristics of TO METAMORPHOSED PRECAMBRIAN EARLY kimberlites and alkaline ultrabasic diatreme CAMBRIAN STRATA. OMINECA BELT............................... 31 breccias ................................................................ 3 Manson Creek area (93N/9) ..................................... 37 Geological work .......................................................... 5 Carbonatites ........................................................ 37 Acknowledgments....................................................... 38 5 Silicate phases ..................................................... Fenites ................................................................. 38 CARBONATITE AND SYENITE COMPLEXES IN Geochemistry ...................................................... 39 PALEOZOIC STRATA, ROCKYAND CASSIAR Geochronology ................................................... MOUNTAINS. FORELAND BELT ............................................ 7 - Mount Bisson Mnnroe Creekarea (93N/9; The Aley Carbonatite complex (94B/5)...................... 7 930/5. 12).......................................................... 41 Rauhangite core zone ............................................ 7 41 rocks Alkalic dike ............................................... Sovite zones ........................................................... 7 Pegmatites........................................................... 42margin ‘Amphibolitic’ ........................................... 7 Intrusive breccias ................................................ 42 Alteration halo ....................................................... 8 Fenites ................................................................. 42 Rare-earth-bearing dikes ..................................... 11 Quartz monzonites quartz syenites .............. and 42 Geochemistry ...................................................... 11 Geochemistry...................................................... 42 Geochronology.................................................... 11 Wicheeda Lake complex(Prince and George Geochronology ................................................... 43 Blue River area (83D/3. 6. 7) ................................... 43 claims, 93V5; 93J/8,9) ...................................... 11 Carbonatites ........................................................ 44 Carbonatites and associated syenitic rocks.........13 Nepheline syenites .............................................. 44 Alkaline dikes...................................................... 13 Mafic Silicate Rocks........................................... 44 Geochemistry ...................................................... 13 Fenites ................................................................ 47 Geochronology.................................................... 14 Geochemistry ..................................................... 47 Bearpaw Ridge sodalite syenite (93V4).................... 14 Geochronology .................................................. 53 Sodalite syenite ................................................... 15 Trident Mountain (82M/16) .................................... 53 Nonda formation volcaniclastic rocks ................ 16 Geochemistry..................................................... 54 Orthogneiss.......................................................... 16 Geochronology .................................................. 54 Postorogenic syenite ........................................... 17 Geochemistry ...................................................... 17 CARBONATITES AND SYENITE GNEISSCOMPLEXES Geochronology.................................................... 18 ASSOCIATED WITH CORE GNEISSES IN THE Oh! INECA Ice River complex (82N/1) ....................................... 18 BELT .......................................................................................... 55 Ultramafic series ................................................. 18 Mount Copeland nepheline syenite gneisses Zoned syenite complex ....................................... 19 (82W2) ............................................................. 55 Carbonatites......................................................... 19 Nepheline syenite gneisses ................................ 55 Lamprophyres ..................................................... 20 Alkaline amphibolite .. ......................................... 58 Geochemistry ...................................................... 22 Grey syenruc gneiss ........................................... 58 Geochronology.................................................... 23 Geochemistry ..................................................... 59 Rock Canyon Creek fluorite and rare-earth Geochronology ................................................... 60 element showing (82J13E) ................................. 24 Carbonatites and associated rocks. west flank, . . Minerahzatlon ..................................................... Frenchman Cap 25 (82W2, 7, 10)............... dome 60 Geochemistry ...................................................... 25 Perry River intrusive carbonatites (82Wr)........60 Geochronology.................................................... 27 Fenites - Perry River area .................................. 60 Kechika Riverarea (94L/ll, 12, 13) ........................ 27 Ratchford Creek (Ren) intrusive carbona:ite Distribution and field relationships of (82W7) ........................................................ alkaline rocks ............................................... 28 Intrusive syenite - Perry River area Petrography: syenites .......................................... 30 (82W7, 10).................................................. 62 Petrography: trachytes......................................... 31 Mount Graceextrusive carbonatite Petrography: (82W7, 10).................................................. 63 feldspar-quartz-carbonate-sericiterocks .....31 Geochemistry ...................................................... 66 Petrography: carbonatites.................................... 34 Geochronology ................................................... 68 _- Bulletin 88 iii
  4. 4. Three Valley Gap (82L/16)....................................... 68 Carbonatites and syenite gneisses........................... 111 Carbonatites, fenites, syenites ............................. 69 Kimberlites. lamprophyres and other ultrabasic Geochemistry....................................................... 70 diatremes.......................................................... 111 Geochronology .................................................... 70 . . . . Tectonlc lmphcattons.............................................. 113 ULTRABASIC DIATREMESIN NORTHERN BRITISH REFERENCES ........................................................................ 119 COLUMBIA............................................................................... 71 The Kechika River diatreme and related rocks APPENDICES ......................................................................... 125 (94L/12, 13)........................................................ 71 COLOUR PHOTOS ................................................................. 135 Lithology ............................................................. 71 Geochemistry....................................................... 73 FIGURES Geochronology.................................................... 1. Index map, carbonatite and syenite gneiss 74 Ospika Pipe (94B/5) .................................................. 74 complexes..................................................................... 2 Lithology ............................................................. 2.74 Index map, alkaline ultrabasic diatreme swanns ......... 4 Geochemistry....................................................... 3. Geologicalmap, Aleycarbonatite complex................ 75 6 Geochronology.................................................... 76 4.Major element ternary plots of carbonatites and ULTRABASIC DIATREMES IN THE GOLDEN- "amphibolite" margin, Aley complex......................... 10 COLUMBIAICEFIELDSAREA ............................................. 77 . 5 Chondrite-normalizedrare-earth element plots, Bush Riverarea (Larry claims) (83C13) ................... complex 77 Aley ............................................................ 10 Lens Mountainand Mons Creekareas (Jack and 6. Ternary plots for fenites. Aley complex.................... 10 Mike claims) (82N/14. 15)................................. 81 7 .Location map of the intrusive bodies on the Prince Valenciennes Riverpipes and George groups of claims. Wicheeda Lake..........11 (Mark claims) (82N115) ..................................... 83 8. Geological map of the Prince grid, Wicheeda Lakt: 12 .. The HPPipe (82N/10) ............................................... 84 9. Chondrite-normalized rare-earth element plot. Geochemistry o diatremes andf related dikes ........... 87 Wicheeda Lake alkaline complex .............................. 13 Geochronology.......................................................... 87 10. Geologicalmap of Bearpaw Ridge ............................ 14 11. Alkali-silica and agpaitic index plots, ULTRABASIC DIATREMES IN THEBULL RIVER - ELK RIVER AREA. SOUTHERN BRITISH COLUMBIA 17 Bearpaw Ridge ........................................................... (82G. 3) ....................................................................................... 91 12. Major element ternary plots. Bearpaw Ridge ............ 17 Shatch Mountain area (Joff claims) (82Jlll) ............ 91 13. Geology ofthe Ice River complex ............................. 19 The Russell Peak diatremes (82J/6) .......................... 92 14. Alkali-silica and agpaitic index plots. Ice River Blackfoot andQninn diatremes (82G114)................. 94 ultramafic suite ........................................................... 22 Mount Haynes- Swanson Peakarea (Swan 15. Ternary majorelement plots. Ice River ultramafic claims) (82G/14) ................................................ 95 suite............................................................................. 22 The Mary Creek - White River breccia dike 16.Alkali-silica and agpaitic index plots, Ice River (823/3W) ............................................................. 96 . . . syenltlc sulte ............................................................... 23Summer The pipes96 (82G/ll) ..................................... 17. Ternary major element plots, Ice River syenitic Geochemistry of diatremes and dikes ....................... 99 suite ............................................................................. 23 Geochronology........................................................ 101 18. Major element ternary plot. Ice River complex ................................................................. 23 KIMBERLITES IN BRITISH COLUMBIA ........................... 103 I9.carbonatitesthe Rock Canyon Creek Geology of fluorite/ The Cross kimherlite (82J/2) ................................... 103 Geochemistry ..................................................... 105 rare-earth showing...................................................... 25 Geochronology.................................................. 105 20. Chondrite-normalizedrare-earth plot. Rock Canyon Creek fenites ............................................................... 27 ECONOMIC CONSIDERATIONSAND EXPLORATION 21. Generalized geology, Kechika area ........................... 28 POTENTIAL ............................................................................ 107 22 . Geology of the central part of the belt of alkaline Niobium and tantalum ............................................. 107 rocks. Kechikaarea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Rare-earth elements and yttrium ............................. 107 23. Alkali-silica and agpaitic index diagrams. Kechika Zirconium ................................................................ 108 syenites ....................................................................... 32 Phosphates ............................................................... 108 24. Major element ternary plots. Kechika igneous Nepheline and nepheline syenite............................. Vermrcuhte . . .............................................................. 109 109 suite ............................................................................. 32 25. Carbonatite ternary plot, Kechika suite ..................... 34 Molybdenum ........................................................... 109 26. Chondrite-normalizedrare-earth plots, Kechika Wollastonite ............................................................. 109 suite ............................................................................. 34 Titanium................................................................... 109 Diamond .................................................................. 109 27 .Geological map ofthe Lonnie carbonatite complex..37 Gemstones ............................................................... 110 28 .Carbonatite plot. Lonnie complex ............................. 39 29.Major element ternary plots, Manson Creekarea SUMMARY AND CONCLUSIONS ....................................... 11 1 carbonatite complexes................................................ 40 iV Geological Suwey Branch
  5. 5. Ministry of Enemy. Minesand Petmleum Resouxes 30. Ternary fenite plots. Manson Creek area carbonatite . 64 General geology anddiatreme locations in the complexes ................................................................... 40 Golden . Columbia Icefields area .............................. 77 31.Alkali-silica and agpaitic index plots, Lonnie 65. Diatreme breccias and dikes. Bush Riverarea ..........77 complex silicate rocks ................................................ 41 66. Sketch showing distribution of diatreme and related 32. Chondrite-normalizedrare-earth plots, Lonnie rocks on theJack claims. Lens Mountainarea ..........81 and Vergil showings, Manson Creek area .................. 41 67. Diatreme breccias and dikes. Valenciennes River 33. Geology and carbonatitekyenite localities in area ............................................................................. 82 the Blue River ..................................................... 42 area 68. Geology ofthe HP pipe ............................................. 84 34. Geological map the Howard Creek carbonatite of . 69 Major element discriminant diagrams, Golden occurrence................................................................... 44 diatreme swarm .......................................................... 88 35. Geological map and cross-section, Paradise Lake . 70 Major element ternary pIots, Golden diatremes ........88 area .............................................................................. 45 71. Ni-Cr plot, Golden diatremes .................................... 88 . 36 Major elementternary plots, Blue River area . 72 SrRb vs.TiOz, Golden diatremes and related alkaline rocks .............................................................. 48 dikes ........................................................................... 89 37.Carbonatite plot, River Blue area............................... 48 73. General geology and diatreme locations in tht: 38. Ternary fenite plots, Blue River area rocks................ 48 Bull River- White River area .................................... 90 39. Alkali-silica and agpaitic index plots. Blue 74. Geology ofthe Joff pipe, Shatch Mountain .......91 ar.a River area syenites...................................................... 50 75. Geology of the Russell Peak diatreme ...................... 92 40. Chondrite-normalizedrare-earth plots, Blue River area showings ............................................................. 50 76. Geology of the Blackfoot diatreme .......................... 94 77. Stratigraphy, Swanson Peak .............................. area 95 41 . Geology of theTrident Mountain Area, Selkirk Mountains ................................................................... 51 78. Diatreme breccias, Summer andGalbraith 42.Alkali-silica and agpaitic index plots, Trident creeks area ................................................................. 97 Mountain syenites....................................................... 52 79. Major element ternary plots, southern diatremes 99 ...... 43. Major element ternary plots, Trident Mountain 80. Major element discriminant plots, southern syenites ....................................................................... 100 swarm 53 diatreme ........................................................ . 44 Geology of the Frenchman Cap area ......................... 56 Ni . = Cr plot, southern 100 81 diatremes ............................. 45. Geological map, Mount Copeland ..................... 57 area 82. Sketch of the Crossing Creek kimberlite pipe 46. Alkali-silica and agpaitic index plots, Mount facing north .............................................................. 102 Copeland syenites ....................................................... 59 . 83 Major element discriminant plots, Cross 47.Major elementternary plots, Mount Copeland kimberlite ................................................................. 103 syenites ....................................................................... 59 84.Majorelement ternary plots, Cross kimberlite ........103 48. Carbonatite plot, Mount Grace and Perry River 85.Ni = Cr plot, Cross kimberlite ................................. 104 areas ............................................................................ 65 86. Structural position ofdiatremes............................... 112 49. Chondrite-normalizedrare-earth plots. carbonatites, 87. (a) Carbonatites and related rocks in Western west flank, Frenchman Cap dome .............................. 65 North America (b)Diamonds and diatremes i:1 50. Detailed section of the Mount Grace carbonatite, Western North America ............................................ 116 Blais Creek showing ................................................... 66 51. Alkali-silica and agpaitic index plots. Perry River TABLES syenites ....................................................................... 66 1. Chemical analyses. Aley carbonatite complex ............ 9 52. Major element ternary plots, Perry River and 2. Chemical analyses. Bearpaw Ridge.......................... 16 Mount Grace area alkaline rocks................................ 67 3. Chemical compositions Ice River of 53. Fenite plots. Perry Riverarea ..................................... 67 complex rocks ............................................................ 21 54. Carbonatite plot. Three Valley Gap............................ 69 4 . Ice River complex geochronologysummary .......... - 24 55.Major element ternary plots. Three Valley Gap ......... 69 5. Chemical analyses. Rock Canyon Creek................... 26 56. Chondrite-normalizedrare-earth plot, Three 6. Geochemistry ofselected samples, Kechika area .....33 Valley Gap .................................................................. 69 7. Chemical analyses of alkaline rocks, Manson 57. Major elementdiscriminant plots. Kechika diatreme Creek area................................................................... 39 and related dikes and tuffs .......................................... 72 8. Chemical analyses of alkaline rocks, Blue River 58. Major elementternary plots, Kechika diatreme and area ............................................................................. 49 related rocks ................................................................ 72 9. Chemical analyses of Trident Mountain syenites......53 59. Ni-Cr plot, Kechika diatreme, dikes and tuffs ...........72 10. Chemical composition of selected rocks from the 60. Chondrite-normalizedrare-earth plot, Kechika Mount Copelandsyenite complex ............................. 58 diatreme andrelated dikes and tuffs ........................... 73 11. Chemical analyses of alkaline rocks, west fla.~k. 61.Major elementdiscriminant plots. Ospika pipe ......... 74 Frenchman Cap Dome ............................................... 64 62. Major element ternary plots, Ospika pipe .................. 75 12.Chemical analyses, Three Valley Gap,alkalir e 63. Ni-Cr plot, Ospika pipe .............................................. 75 rocks ........................................................................... 68 Bulletin 88 V
  6. 6. 13. Geochemistry of selected diatreme breccias and 24. Intrusive carbonatite band surrounded by dark related dikes and tuffs, Kechika area .......................... 71 amphibolitefenite and some grey syenitic fenite,14. Chemical analyses, Ospika pipe ................................. 74 Perry River area .......................................................... 6015. Field characteristics of HP pipe breccia phases ......... 84 25. Swirled carbonatite in amphibole fenite, Peny16. Chemical analyses, Golden area diatremes ................ 86 River area.................................................................... 6117. Chemical analyses, Bull River- Elk River 26. Interlayered amphibolitic fenite and syenitic fenite, and diatremes related rocks ........................................ 98 Peny River area .......................................................... 6118. Chemical analyses, CrossingCreek kimberlite........ 104 27. Ratchford Creek carbonatite interlayered with19. Timingof alkaline intrusion and orogenesis ............114 amphibolitic fenite and containing fenitized fragments of country rock .......................................... 62PLATES 28. Part of the thickened section of the Mount Grace 1. Xenoliths of quartzite and syenite in the breccia extrusive carbonatite .................................................. 63 phase of the "amphibolitic" margin, Aley complex..... 8 29. Interbedded sedimentary marble with carbonatite 2. Quartzite xenoliths weathering ontof the agglomerate and tuff layers; Mount Grace "amphibolite"margin, Aley complex ........................... 8 carbonatite near Blais Creek ...................................... 63 30. Large feldspar clots in biotite-rich carbonatite, 3. REE-enriched dikes in carbonate hostrocks, Aley complex......................................................................... 8 Three Valley Gap area ................................................ 68 31. Dolostone clast with reaction rim, Ospikapipe ......... 73 4. Boulder containing syenite dike crosscutting bonded volcaniclastic rocks, Bearpaw Ridge............. 15 32. Rusty weathering,clast-supported megabreccia, Bush River area ......................................................... 78 5. White-weathering syenite with disseminated sodalite, Bearpaw Ridge ............................................. 15 33. Dark greenweathering breccia, Bush Riverarea ... ..78 6. Folded andfoliated dioritic orthogneiss exposed, 34. Limestone-cored annoured xenolith in diatreme Bearpaw ............................................................ 15 breccia, Bush River area ......................................... ..79 35. Alteredmica macrocryst diatreme breccia,in 7. Contact between syenites of the Ice River complex River Bush area ......................................................... 79 and steeply dipping carbonate rocks, Bntress Peak ....18 36. Boulder from dike, Bush Riverarea with breccia a 8. Jacupirangite cut by fine-grained nepheline syenite core and finer grained, macrocryst-rich rim .............79 dikelets, north of Mount Mollison, River Ice complex....................................................................... 20 37. Laminated margin of a fine-grained dike, Bush River area................................................................... 79 9. White-weathering, coarse-grained carbonatite dike, Mount Sharp area, Ice River complex ........................ 20 38. Photomicrograph ofan altered pyroxene crystal in a matrix containing abundant altered mica10. Intrafonnational conglomerate with fluorite matrix, from a dike, Bush River area ...................................... 80 Rock Canyon Creek .................................................... 27 39. Fragments of sedimentary rocks a buff- in11. Potassium feldspar porphyroclasts in a fine-grained weathering, quartz xenocryst-rich breccia, carbonate-sericite-feldspar-quartz Creek matrix from a Mons ................................................................ 80 sheared leucosyenite, Kechika area ............................ 30 40. Photomicrograph ofquartz xenocryst-rich breccia,12. Typicaltrachyte, Kechika area .................................. ,30 similar to that shown in previous plate ...................... 8013. An apatite-rich zone in the quartz-feldspar-sericite- 41. Serpentinized olivine macrocrysts in a fine- carbonate-(apatite) rocks, Kechika area ..................... 31 grained, massive diatreme phase, Valenciennes14. Blue pleochroic amphibole and grained finer River area.................................................................... 82 aegirine in ultrafenite, Lonnie area ............................. 38 42. Boudinaged dikesubparallel to bedding in15. Fz folds in banded nephelinesyenite, Paradise buff-coloured carbonates, Valenciennes River area ...83 Lake............................................................................. 43 43. Alteredphenocrysts in a dike, Valenciennes16. Phlogopite in carbonatite, from Verity ....................... 46 River area.................................................................... 8317. Welllayered carbonatite, Howard Creek ................... 46 44.Sharp contact between breccia phases, HP pipe ........8518. Migmatitic lencosome layered syenites, in 45. Large gabbroicxenolith in a strongly foliated Paradise Lake .............................................................. pipe 47 breccia, HP .......................................................... 8519. Migmatitic segregations of coarse perthite 46. Optically zoned andradite garnets, HP pipe............... 85 crystals in layered nepheline syenite gneiss, 47. Biotite macrocryst coring accretionary lapilli, Paradise .............................................................. pipe HP ....................................................................... 8520. Coarse-grained ilmenite segregation in 48. Graded bedding in extrusive epiclastic layer, leucosyenite, Trident pipe .................................. Mountain 51 Joff ...................................................................... 9121. Typicalbanded nephelinesyenites, Trident 49. Epiclastic crater-infill breccia, Joff pipe, Mountain ..................................................................... 51 immediately overlain by well-bedded, pink and22. Leucosyenite dikes cutting mafic, biotite-amphibole buff-weatheringstrata of the basalDevonian unit .....92 gneiss, Trident Mountain ............................................ 52 50. Well-bedded crater-infill material, Russell Peak23. Xenolith of mafic gneiss in biotite-rich syenite, diatreme .................................................................... ..93 Trident Mountain ........................................................ 52 51. Porphyritic volcanic rock, Russell Peak diatreme 93 ..... -vi Geological Survey Bnznch
  7. 7. Minisfry of Energy, Mines and Petroleum52. Vesiculated glass lapilli in diatreme breccia, APPENDICES Quinn Creek................................................................ 94 1. Rare-earth element analysesfrom some carbonatite53. Pillowed flow, Swansou Peak .................................... 96 suites ......................................................................... x2754. Chrome spine1 macrocryst, Summer diatreme 2. (A) U-Pb zircon data, British Columbia breccia ......................................................................... 97 carhnatites and nephelinesyenites .................. 12855. Pyroxenite inclusion forming the core of an (B) Uranium-lead analytical data, accretionary lapillus, central breccia phase, Cross B.C. diatrernes .................................................. 129 kimberlite .................................................................. 102 (C) Summary of RblSr analytical data,56. Alteredolivine macrocrysts and phenocrysts, B.C. diatremes ................................................... :I30 phlogopite phenocrysts and opaque oxidesa in (D) Summary of K-Ar analytical data, magmatic matrix, Cross kimberlite .......................... 102 B.C. diatremes .................................................. ~131Bulletin 88 vii
  8. 8. ~.British Columbia -viii Geological Survey B n
  9. 9. Ministry of Energy, Mines and Petroleum Resources " INTRODUCTION A previously poorly documented alkaline igneous may contain diamond, but only as a rare constihtent. h eprovince is present in the Canadian Cordillera. It comprises term kimberlite was introduced into geological literature thecarbonatites, nepheline and sodalite syenites, some ijolite- in 1887to describe the hostrocksof diamonds at Kimberley,series rocks, one kimberlite locality and numerous ul- South Africa. Since that time many rocks carrying oli-tramafic and lamprophyric diatreme breccias, all of whichintruded the Cordilleran miogeoclinal succession prior to vine+phlogopite+carbonat&clinopyroxene+feldspathoid+the deformation and metamorphism associated with the spinel have erroneously been referred to as kinberlites,Jura-Cretaceous Columbian orogeny. rocks which should be placed in the 1amprophSre group Carbonatites are ultrabasic igneous rocks composed of (Clement etal., 1984). Inaccurate orincorrectclasrificationmore than 50%carbonate minerals. They may contain sig- only complicates understandingof petrogenetic: eco- the andnificant amounts of olivine, magnetite, pyroxene, sodic am- nomic implications.phibole, biotite, vermiculite, apatite, columbite, zircon, Kimberlite has traditionally heeu considered the onlyrare-earth minerals and pyrochlore. Carbonatites occur most important primary source of diamond. Recent studiescommonly as intrusive bodies, generally associated with (Scott-Smith and Skinner, 1984a, 1984b; Jacques et al.,other alkalineigneous rocks (Pecora, 1956;Heinrich, 1966) 1986; Scott-Smith et al., 1986) have shown that diamondssuch as nepheline syenites, ijolites, urtites, melteigites may also be present in economic concentrations in lamproi-(nepheline+maficsilicateskfeldspathoidsinvariouspropor-tions) and jacupirangites (alkaline pyroxenites). Metaso- tes. Lamproites are ultrapotassic rocks that are chemicallymatic rocks (fenites), which are generally enriched in andmineralogically distinct fromkimberlites. characterizedsodium andferric iron and depleted in silica, are also com- by thepresence of phenocrystic and/or groundmass leucite,monly associated with carbonatites, often marginal to the titanium-rich phlogopite, clinopyroxene, amphihole (!.ita-intrusive complexes. Extrusive carbonatites are less com- nium and potassium-rich richterite), olivine and !;anidinertmon, but havebeen described westemUganda (von Knor- in glass (Scott-Smith and Skinner, 1984b).Diamonds;have oc-ring and du Bois, 1961). northern Tanzania (Dawson,1962, casionally been reported from carbonatites and pt:ridotites, 1964, Hay, 1983). Kenya (Le Bas and Dixon, 1965; Le Bas, 1977; Deans and Roberts, 1984) and Germany (Keller, but, to date, the only known economic primary sources re-1981). main kimberlites and lamproites. Diatreme breccia pipes in Many cabonatite bodies are valuable sources a num- of British Columbia have been targets diamond exploration forber of commodities. Niobium has producedat Oka and been since the mid-1970s (Grieve, 1981; Dummett et tzl., 1985)St. Honor&, Quebec and at Araxa, Brazil; the Mountain Pass even though mostare nottrue kimberlites; microdiamondscarbonatite in California is the largest producer of rare-earth have been discovered in heavy mineralseparates from twoelements in the westem world; and copper and byproduct of these pipes (Dummett et al., 1985). Diamondr are alsoapatite, magnetite, vermiculite and zirconium oxide are pro- known to occur in kimberlites from the Colorado-Wyomingduced at Palabora, South Africa (Heinrich, 1966; Currie, State-Line district (McCallum and Marbarak,1976), from1976a). Nepheline syenite is an important raw material used the Mountain diatreme in Yukon (Godwin andPrice, 1987)in the glass and ceramics industries. Small amounts have and from placer deposits in Alaska (Forbes et d, 1987)also been usedin paints and as fillers in plastics. The BlueMountain region Ontario is the largest western world pro- of where stones over 1 carat in size have been recovered.ducer of nepheline syenite (Cume, 1976a). Since the 1950s The parental magmas of alkaline igneous rocks area number of carbonatite complexes in British Columbia meltswhichformdeepinthemant1e.Thesemeltsc~ostcom-have been prospected for various commodities at different monly intrude cratonic or shield areas with a longhistorytimes; their vermiculite, niobium, zirconium and rare-earth of tectonic stability (Heinrich, 1966; Dawson, 1980) andpotential has been explored. None haveany history ofpro- their emplacementis often indirectly associated with normalduction. faults, grabens or failed rifts. In British Columbia, the car- Kimberlites arevolatile-rich, potassic, ultrabasic igne- bonatites and related rocks wereintruded into the sedinlen-ous rocks which occur as small volcanic pipes, dikes andsills. They have adistinctly inequigranular texture resulting tary prism deposited along the rifted continental margin,from the presenceof macrocrysts (olivinefphlogopite, pi- making this a somewhat anomalous alkaline province in acroilmenite, chrome spinel, magnesian garnet, clinopy- structural setting which differsfrommostothers worldwide.roxene and orthopyroxene) in a fine-grained matrix. The set Consequently, thissuite has important implications becausematrix contains phenocrystic and/or groundmass it documents the characteristics of carhonatites and relatedolivinefphlogopite, carbonate, serpentine, clinopyroxene rocks emplaced in a continental margin environ~nent, andandmany otherminerals (Clementetal., 1984). Kimberlites details the subsequent effects of orogenesis. "Bullefin 88 I
  10. 10. DISTRIBUTION AND GENERAL Rocky Mountain Trench; the eastern edge of the OminecaCHARACTERISTICS OF Belt; and in the vicinity of Frenchman Cap dome, a core gneiss complex, also within the Omineca Belt. The eaiiternCARBONATITESAND NEPHELINE or Foreland Belt (Figure 1) hosts carbonatites and d a t e dSYENITES rocks within Paleozoic strata, predominantly in the Main and Western rangesof the Rocky Mountains. This con- belt In British Columbia, carbonatites, nepheline and so- tains the Aley carbonatite complex (Mader, 1986, 1!)87),dalite syenite gneisses and related alkaline rocks are found Wicheeda Lake showing (Prince and George claims, Bet-in a broad zone which parallel to, and on either side of the isRocky Mountain Trench. Carbonatites relatedrocks are and manis, 1987; Maderand Greenwood, 1988),BearpawRidge sodalite syenite, the Ice River syenite and carbonatite corn-also reported from anumber of areas in the western UnitedStates, for example, the McClure Mountain, Iron Hill, Gem plex (Cnrrie,1975,1976a) and Rock Canyon Creek fluo- the rite and rare-earth showing, a carbonatite-related depositPark and Wet Mountain areas of Colorado (Lmen, 1942;Olson and Wallace, 1956; Parker and Sharp, 1970; Nash, (Hora and Kwong, 1986). The Aley, Ice River and Bearpaw1972; Hildebrand and Conklin, 1974; Armbrustmacher, Ridge intrusions are subcircular to elliptical in plan, gcner- ally have extensive metasomaticalteration or contact meta-1979, 1984; Armbrustmacher et al., 1979), the Lemitar morphic halos and are hosted by Middle Cambrir.n toMountains, central New Mexico (McLemore, 1987) and theMountain Pass area, California-Nevada State-Line (Olson Middle Devonian miogeoclinal rocks. Alkalic rocks i:1theetal., 1954; Jaffe, 1955; Warhol, 1980; Woyski, 1980). Wicheeda Lake area define a linear zone, and consist of small plugs, dikes and sills. The Rock Canyon Creek show- Three discrete areas hosting carbonatites can be defined ing is an elongate zone of fluorite and rare-earth metaso-within British Columbia: the Foreland Belt, east of the matic alteration in Devonian carbonate rocks, possibly Figure 1. Index map, carbonatite and nepheline syenite gneiss complexes.2 Geological Survey Bxanck
  11. 11. ~~ ____ ____~ ~~~ ~~ ~related to a buriedcarbonatite. Carbonatites in this belt ex- syenite gneiss (Fyles, 1970;Cnme, 1976b) occurs along thehibit varied mineralogy andare enriched in niobium, fluo- southern margin of the gneiss dome. The extrusive Mountrine and rare-earth elements relative to other British Grace carbonatite tuff, intrusive carbonatites with thickColumbia occurrences. During the Colnmbian orogeny the fenitited margins and syenite gneisses occur along theintrusions were subjected to sub-greenschist to greenschist northern and western flanks of the dome. Bothof .he inm-facies metamorphism. The obviouseffects of deformation sive andextrusive carbonatites are moderately enriched inare minor, the intrusions appear to have behaved as rigid rare-earth elements, but no significant niobium mineraliza-bodies during orogenesis and were simply rotated, tilted tion has beenreported.and/or transported eastwards in thrust slices. Locally, small Several tens of kilometres to the south of tht: French-faults cut the alkalic rocks. man Cap Dome, near Three Valley Gap, another carbonatite The Kechika River complex is located a few kilometres is hosted bymigmatitic gneisses of uncertain affinity. It ex-west of the Rocky Mountain Trench, in the Cassiar Moun- hibits many similarities in fieldrelationships and geochemi-tains (Fox, 1987; Pell et al., 1989). It is morphologically cal signatures to the intrusions of the Blue River andsimilar to the Wicheeda Lake showing,consisting of dikes Manson Creek areas along the eastern margin of theand plugs and probable pyroclastic layers distributed in a Omineca Belt.linear belt. Although not the Rocky Mountains,it exhibits inmany similarities to alkalic rocks in the Foreland Belt and DISTRIBUTION AND GENERALfor the purposes of this discussion, will be consideredwith CHARACTERISTICS OF KIMBERLITESthem. AND ALKALINE ULTRABASIC Carbonatites and syenites are found along the eastern DIATREME BRECCIASmargin of the Omineca Belt, extending westward from theRocky Mountain Trench for 50 kilometres or more. All the Alkaline ultrabasic diatremes anddikes have been dis-intrusions within this belt are hosted by late Precambrian covered in the Western and Main ranges of the Rocky Moun-(Upper Proterozoic) to Early Cambrian metasedimentary tains and in the Cassiar Mountains of British Columbiarocks. They generally form foliated, sill-like bodies that (Figure 2). With the exception of the Cross diatrenle, all arehave been multiply deformed and metamorphosed to am- hosted byCambrianto Silurian miogeoclinalrocks (Robertspbibolite facies during the middle Mesozoic orogeny; some et al., 1980; Grieve, 1981; Pell, 1986~. 1987b). The Crosssmall plugs and discordant dikes are also present. The car- diatreme, which is located in a moreeasterly structural po-bonatites have thin sodic pyroxene and amphibole-rich feni- sition, is hosted by carbonate rocks the Pennsylvanianto oftic margins. The belt comprisescarbonatites associated with Permian Rocky Mountain Group (Hovdebo,195:‘; Grieve,monzonites and some syenites in the Manson Creek area at 1985). A11 thediatremesintNdedthemiogeoclina1 sequencethe Lonnie and Vergil showings (Rowe, 1958; Currie, of platformal carbonate and clastic rocks prior to the Colum- 1976a). syenites and monzonites in the Mount Bisson - biau orogeny. The effects of deformation and metamor-Munroe Creekarea (Halleran, 1988; Halleran and Russell, phism are manifest in a weak to strongly dweloped 1990), carbonatites with nepheline and sodalite syenites and foliation, some flattening and the development 01: chlorite.some nrtites in the Blue River area, including the Verity, The diatremes weretransported eastwards in thnlst sheetsParadise and Howard Creek localities (Rowe, 1958; Cnme, during orogenesis therefore, have presumably and, been cut1976a; Pell, 1987) and nepheline and sodalite syenites at off from their roots. Diatreme breccias are also fomd in theTrident Mountain and Kinbasket Lake (Cnrrie, 1976a; MackenzieMountains, Yukon (Mountaindiatremc:, GodwinPerkins, 1983). No carbonatites in this zone are known to and Price 1987; Coates Lake diatreme, C. Jeffeison, per-contain potentially economic concentrations of niobiumor sonalcommunication, 1987);kimberlites andrelatedalkalicrare-earth elements; however, pegmatites dramatically en- ultramafic diatremeshave been found in north-central Mon-riched in light rare-earth element havebeen reported from tana (Hearn, 1968; Hearn and McGee, 1983); kimberlitesthe Mount Bissonarea (Halleran and Russell, 1990). also occur in the Colorado-Wyoming State-Lin: District The most westerly contains intrusive and extrusive area (e.g. Sloanpipe,HauseletaL, 1979,1981;McCal:umeraZ.,carhonatites and syenite gneiss bodies in a mixed paragneiss 1975; McCallum and Marbarak, 1976).succession along the margins of the Frenchman Cap gneiss Ultrabasic diatremes are present in five geographic: re-domenorth ofRevelstoke (Wheeler, 1965;McNlillan, 1970; gions of British Columbia. Within each area the ,diatremesMcMillan and Moore, 1974; Hoy and Kwong, 1986; Hoy are, for the most part, petrologically similar. The first suiteand Pell, 1986) in the core of the Omineca Bele (Figure l). is found in the Cranbrook- Bull River (Figure 2) where areaThe Frenchman Cap gneiss dome is one of several late do- examples of crater facies and extrusive rocks have been rec-mal structures located near the eastern margin of the ognized. The upper parts of the diatremes are chwacterizedShuswap Complex (Wheeler, 1965; Read and Brown, by bedded epiclastic and/or pyroclastic material overlying1981). The core of the dome comprisesmixed gneisses of a chaotic fragmental breccia containing abundant vesicu-probable Aphebian age that are nnconformably overlain by lated glass lapilli. In one pipe, small mafic flows dikes and‘mantlinggneiss’, an autochthonous cover sequence which are exposed near top of the crater zone. These rocks the arehosts the carbonatites and syenites. The intrusive and extru- porphyritic and consist of abundant clinopyroxene less andsive alkaline rocks in this area are conformable bodies that abundant olivine phenocrysts, clinopyroxene, oxide and po-were deformed and metamorphosed to upper amphibolite tassium feldspar microphenocrysts in a fine-grainedfacies during the Columbian orogeny. The Mount Copeland groundmass. Deeper levels within the craters are charac-Bulletin 88 3
  12. 12. terized by juvenile lapilli-rich breccias with rare macro- (1986). These diatremes and dikes are associated withcrysts of chrome spinel, altered pyroxenes andaltered oli- quartzxenocryst rich breccias,containingsedimentaryarylockvines sporadically distributed throughout. Micas are not fragments and little recognizable igneous material. Mi-present in these rocks. Sedimentaryrock fragments, granitic crodiamonds have reportedly been recovered from hc:avyclasts and a variety ofpyroxenite andperiodotite xenoliths mineral separates taken from two pipes of this type (Dum-have been recovered from these pipes. Tentatively, these mett etal., 1985).rocks are interpreted to have an alkaline lamprophyre affin- In a third area, near Williston Lake and the Os:?ikaity. River, northern Rocky Mountains, a single pipe has k e n The secondsuite, found north of Golden (Figure 2). is discovered near the Aley carbonatite complex. It exhibitscharacterized by macrocryst-rich breccias and dikes. The many similarities to the pipes inthe Golden area; it is a ~nnl-macrocryst population consists of titaniferous augite or tiphase diatreme characterized by macrocrystic greensalite, phlogopite, green chrome diopside, spinel and rare chrome pyroxene,augite, pholgopite and spinel. Basei onolivine, with either clinopyroxeneor phlogopite most abnn- mineralogy it can be classified as an aillikite, a type of ul-dant (Ijewliw, 1987;Pell 1987a). Sedimentary fragments are trabasic lamprophyre.predominant in the breccias; gabbroic and granitic xeno- A diatreme breccia and related dikes are also known inliths, as well as cognate material, spherical structnres and the Kechika area of the Cassiar Mountains northern :Brit- innucleated autoliths are also present locally. These pipes are ish Columbia. These breccias are richin juvenilelapilli, con-multiphase intrusions, with massive and multiple breccia tain abundant sedimentary rock fragments, rare chromephases cut by related dikes. Petrologically, the diatremes spinels and are devoid of xenocrystic micas (Pell ei al.,appear to bear someaffinity to alkaline and ultrabasic mica 1989), similar to breccias in the Cranbrook - Bull Piverlamprophyres (alnoites and aillikites) as defined by Rock area.Figure 2. Index map, alkaline ultrabasic diafxeme swarms (from Pelf, 1987). For details on the Ospika Pipe see Figure 3 or Miider ( 1987).4 Geofogical S u n q Eranch
  13. 13. Ministry of Energy, Mines andPetroleum l E c 3 The last geographically and petrologically distinct rock All the carbonatite-syenite localities (with the exception oftypeis represented by one example, Cross diatreme, lo- the theWicheedaLakeandMountBissonshowings,di:rcove:redcated at Crossing Creek, north of the town of Elkford. To late in the project) and alarge number ofthe diatreme brec-date, this breccia pipe is the only true kimberlite recognized cias were mapped and sampled. The purpose tllis study ofin the southern Canadian Cordillera (Grieve, 1981, 1982; is to document alkaline rock occurrences in British Colm-Hall e#al., 1986; Ijewliw, 1986,1987). It is a multiple intru- bia; to describe their petrography, geochemistry, economicsion with massive and breccia phases containing xenoliths geology and field relationships; and to determine the timingof garnet and spinel lherzolite, serpentinized peridotite, and tectonic controls of emplacement, providing basis for agimmerite and sedimentary material as well as pelletal future, detailed studies. Most new work concenmted onlapilli and xenocrystsof olivine, pyrope garnet, spinel and previously undocumented occurrences; previous1:r studiedphlogopite. Massive phases have a magmatic matrix of ser- suites were examinedfor comparison purposes.pentine, carbonate, microphenocrystic olivine and spinels.No diamonds havebeen reported from this pipe. The ratioof ultramafic to sedimentaryxenoliths is greater in the Cross ACKNOWLEDGMENTSdiatreme than in any the other breccia pipes andit is char- ofacteristic of the diatreme facies of kimberlites (Hawthorne, The Canadaritish Columbia Mineral Development1978; Clement and Reid, 1986). Agreement 1985-1990 has provided the financial support which made this project possible; the Natural Sciences andGEOLOGICALWORK Engineering Research Council of Canada supplied addi- Prior to this study, documentation of carbonatites, tional funding. R.L. Armstrong, The University of Britishsyenite gneisses and alkaline ultramafic diatreme breccias Columbia, provided the rubidium-strontium dating;was limited to studies of a few complexes and brief descrip- J. Harakal and Scott, The University K. ofBritish Columbia,tions of some of the others. In the Foreland Belt, the Ice the potassium-argondating; and R. Parrish, GeologicalSur-River complex had been subject of a number ofstudies, the vey of Canada, the uranium-lead zircon dating. Neutron ac-dating back to the turn of the century (Dawson, 1885; Bar- tivation analyses were provided by Bondar-#:lege &low, 1902; Allan, 1914; Jones, 1955; Rapson, 1963; 1964). Company L dt.The most comprehensive studyof the complex was com- I would like to thank Z.D. Hora for proposin]: the pro-pleted by Currie (1975). Other carbonatite complexes, ject in 1984 and for his continued help and ;pidancesyenites, kimherlites and diatremes the Rocky Mountains in throughout; thanks also go to T.HBy, D.J. Schulze, D.C.had only received brief mention in the literature. Carbona- Hall, J.A.Mott, C. Graf,B.H. Scott-Smith, C.E.Fipke,K.R.tites hosted by metamorphosedstrata along the eastern mar- Pride, U.K. Mader, H.H. Helmstaedt, D.A. Grieve, G.P.E.gin of the Omineca Belt had also received only brief White, M.Fox, R.R Culbert, A. Betmanis, B. French, !LA.mention in overview publications (Rowe, 1958; Currie, Almond, D.L. Pighin and J.K. Russell for helpfill disr:us-1976a). Carbonatites and syenite gneisses associated with sions both in and out of the field; thanks are also extendedcore complexes in the Omineca Belt were discovered and to W.J. McMillan, J.M. Newel1 and B. Grant for review ofstudied in the course of regional mapping (Fyles, 1970; this and related manuscripts. A special thanks goesto OlgaMcMillan, 1970, 1973; McMillan Moore, 1974). Sub- and Ijewliw for two seasonsof capable field assistance and. forsequent studies (Currie, 1976b;HBy and Kwong, 1986; Hay, of agreeing to help unravel some the story hiddell in these1988) provide detail on these suites. rocks, through her Master’s thesis research. Gwen3a Loren- Work by the author was begun in 1984 and included zetti also provided capable and cheerful assistance duringfield mapping during the summers of1984,1985 and 1986. one field season.Bulletin 88 5
  14. 14. British Columbia - LEGEND 1 Fenitized Group alkali-syenitt River Rood (dolostone) Rood River Group (sandstone, shole) a amphibolite Carbonatite >Devoniar Rare-earth Group Road River (shale) carbonatitedikes SkokiFm. (dolostone, volcanics) KechikoFm. (limestone, marl, siltstone) Figure 3. Geological mapof the Aley carbonatite complex (fromMader), 1987.6 Geological Survey Bnznch
  15. 15. Ministry of Energy, Mines and Petroleum . e s o u x e s " CARBONATITE AND SYENITE COMPLEXES IN PALEOZOIC STRATA, ROCKY AND CASSIAR MOUNTAIINS, FORELAND BEXI "THE ALEY CARBONATITE COMPLEX [(Ca,Ce,Na)(Nb,Ta,Ti)2(0,OH,F)61 forms fibrous to fine- grained aggregatesreplacing pyrochlore; primary fersnute(94BI9 is rare. Columbite [(Fe,Mn)(Nb,Ta)z06] is present as a re- The Aleycarbonatite complex was discovered 1980 in placement of fersmite.and staked by Cominco Ltd. in 1982 (Pride, 1983) for its Mineral banding or layering is common, particularlyniobium potential. It is located approximately 140 kilome- near the margins of the complex, and is charactmized bytres north-northwest of Mackenzie, on the east side of Wil- aligned flattened grains and aggregates apatite. In miner- ofliston Lake between the Peace Reach and the Ospika River alized zones, magnetite, pyrochlore, fersmite and biotiteat latitude 56"27 north, longitude 12345 west. The area is also exhibit some alignment and compositional zoning.generally above treeline (1450 - 2200 melevation) and has Field studies indicate that the mineral layering i:r steeplyexcellent exposure. It is fairly remote; access is by helicop- dipping and strikes approximatelyparallel to the margins ofter from Mackenzie. the complex. It has been interpreted as vertical flow band- The Aley Creek area is underlain by Cambrian to Silu- ing, a primary igneous texture (Mader, 1986) as is observedrian carbonate andclastic rocks of the Kechika, Skoki and in many other carbonatite complexes (e.g., Oka).Road River groups (Thompson, 1978; Pride, 1983). Thismiogeoclinal succession, deposited near the outer edge of SOVITE ZONESthe continental shelf, was intruded by the Aley carbonatite Sovite zones (dikes and sweats) occur locally near thecomplex prior to the main Late Jurassic to Early Cretaceous margin of the rauhaugite core zone andin the sunaundingorogenic event. The youngest unit affected by the intrusion amphibolite zone. The sovites exhibit a more variable min-is the mid-Ordovician(?) Skoki volcanic sequence. Much of eralogy than the rauhaugites. Calcite with or without dalo-the following description of the carbonatite complexis sum- mite dominates (40.95%) and there are accessory to majormarized fromthe work of Mader (1986,1987). amounts of apatite (2-lo%), biotite (0-5%),magnetite (0 to The complexis oval in outline with a diameter of 3 to 40%), richterite, a sodic amphibole (0-5%),pyrochlore (O-3.5 kilometres, occupying an of approximately 7 square 2%). fersmite and pyrite (Pride et al.,1986). Zircon rare area andkilometres. It is cylindrical, with a near-vertical axis and baddeleyite associated with zirkelite have also been re-consists of a rauhaugite (dolomitic carbonatite) core zone ported; mineral banding well developed. issurrounded by an older, outer ring of amphibolite. Somesovite (calcitic carbonatite) and rare-earth carbonate AMPHIBOLITICMARGINsweats occur in the rauhaugite core. A contact aureole of An amphibolitic margin, approximately 1 kilometrerecrystallized carbonate rocks surrounds the amphibolite in width, encircles and complexly interfngers with themargin. Rare-earth-enriched carbonatite dikes intrude the rauhaugite core. The marginal zone includes massive andcontact aureole (Figure 3). Ultrabasic lamprophyre dikes breccia phases. No distinct pattern to the spatial dis ributionand a diatreme breccia pipe (Ospika pipe) intrude altered of the two phases is evident. Carbonatite dikes cut bothand fresh carbonates outside the complex. These will be dis- members, indicating the amphibolite margin pred %ted em-cussed later in thisreport. placement of the carbonatite core zone. The massive phasea medium to coarse-grained, dark isRAUHAUGITE CORE ZONE green rock consisting primarily of sodic amphibole (mag- The coreof the Aley complex is approximately 2 kilo- nesio-arfvedsonite), quartz, albite and aegirine. It is moremetres in diameter. It comprises morethan 50% of the ex- extensively developed than the breccia phase and resemblesposed complex and consists of dolomite (80.99%) and fenites associated with some of the other carhonatite com-apatite (1-10%) witb minor amounts of phlogopite, pyrite, plexes in British Columbia. Mader (1986,1987) recog- h2.smagnetite, monazite, strontianite and zircon. It is generally nized microsyenite textures in the massive amphibolite. anda massive and homogeneous unit, weathering a buff to suggests that it is a primary igneous phase with a metaso-brownish colour. Pyrochlore [(Na,Ca,Ce)2 (Nb,Ta,Ti)z Os matic (fenitic) overprint, as opposed to fenitized country(OH,F)] may b e p r e s e n t i n t h i s z o n e . F e r s m i t e rock. This appearsto be the most reasonableinterpretationBulletin 88 7
  16. 16. -~ ~British Columbia - for the origin of this rock; however, the true nature of the primary igneous phaseis so obscured that classificatim is difficult. The breccia phase contains subroundedclasts of domi- nantly orthoquartzite, with some siltstone, albitite and mi- crosyenite fragments in a matrix that is similar to the massive phase and locally grades into it. The clast-to-natrix ratio is highly variable and clast-supported breccias ate de- veloped locally; on average, clasts comprise l to 30%of the rock volume. Their subrounded nature gives this unit the appearanceof a conglomerate (Plates 1 and 2). The quartzite and siltstone xenoliths range from a milliietres l o ap- few proximately 30 centimetres in diameter. Reaction rims con- sisting predominantly of fine-grained aegirine commonly envelop the xenoliths. The quartzite xenoliths are probably derived from Lower Cambrian quartzite formations which the complex must have sampled during its ascent. Niicro- syenite and albitite clasts may represent deeper level mtru- sives or fenites associated with the original magma chanber. The extreme roundnessof clasts is similar to many of the diatremebreccias and may be a result of gas-streamingabra- sion. ALTERATION HALO Sedimentary rocksadjacent to the Aley complex have been altered for a distance of approximately500 metn:s be-Plate 1. Rounded xenolithsof quartzite and syenitein the breccia yond the amphibolite margin. This alteration halo is char-phase of the "amphibolitic" margin, Aley complex. acterized by a colour change from greyish to a distinct buff hue. The altered rocks may superficially resemble mzlterialPlate 2. Roundedquartzitexenolithsweatheringout of the Plate 3. Chocolatebrown-weathering,REE-enricheddikes in"amphibolite" margin, Aley complex. carbonate hostrocks, Aley complex, (colourphoto, page,3) 5.8 Geological Survey Branch

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