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Mineralogy, geochemistry and petrology of a pyrochlore-bearing carbonatite at Seabrook Lake, Ontario Osatenko, Myron John 1967

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Mineralogy, Geochemistry and Petrology of a Pyrochlore-bearing Carbonatite at Seabrook Lake, Ontario by Myron John Osateriko B.Sc, University of B r i t i s h Columbia, 1965 A Thesis Submitted i n P a r t i a l Fulfilment ' of the Requirements for the Degree of MASTER OF SCIENCE i n the Department of GEOLOGY We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1967 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I ag ree t h a t t h 3 L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r ag ree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depa r tment The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8, Canada i ABSTRACT The Seabrook Lake carbonatite complex i s one of the smallest of nine known carbonatite complexes i n central Ontario. The complex, which i s one-half square mile i n area and pear-shaped i n plan, consists of f e n i t i z e d granite and breccia, mafic breccia, i j o l i t e and related breccia, and carbonatite. The bulbous northern part of the complex consists of a plug-like core of carbonatite surrounded by mafic breccia and carbonatite dykes. The narrow southern part consists of i j o l i t e and related breccia. Enveloping a l l of these rocks.is a fe n i t i z e d aureole which grades outward to unaltered granite that underlies much of the surrounding area. The carbonatite i s composed of c a l c i t e with the following minor mineral, i n decreasing order of abundance: goethite, microcline, magnesioriebeckite-riebeckite, magnetite-ulvospinel, apatite, hematite, pyr i t e , a l b i t e , b i o t i t e , c h l o r i t e , pyrochlore, brookite, sphene, ferroan dolomite (ankerite?), aegirine, chalcopyrite, wollastonite and quartz. The chemical constituents are as follows: Major CaO + C 0 2 Minor F e ^ , S i 0 2 , MgO, N b ^ , SrO, BaO, Na 20, K 20, MnO, A 1 2 0 3 , P ^ , S and H 20. Trace Cu, Pb, Zn, As, Ce, Y, L i , Cr, Co, N i , V, In, Zr, and T i . The complex i s believed to have formed by d e s i l i c a t i o n and metasomatism of fractured and brecciated granite by a soda-iron-rich carbonatite magma of unknown o r i g i n . i i TABLE OF CONTENTS Page I. Introduction A. Scope of Investigation 1 B. Acknowledgments 2 C. Location arid Topography 2 D. History and Previous Work 3 E. General Geology of the Seabrook Lake Area 6 F. Nomenclature 7 II. Geology of the Carbonatite Complex 11 A. Granite 15 B. Diabase 16 C. Fenitized Granite and Breccia 16 D. Transition Fenite 23 E. Mafic Breccia 23 F. Ijolite 30 G. Hematite-rich rock 36 H. Carbonatites 36 (1) Distribution and Occurrence 39 (2) Structure 40 (3) Chemical and Mineralogical Composition of the Carbonatite a) Chemical composition of the carbonatite b) Detailed mineralogy i . Mineral descriptions i i . Paragenesis i i i . Average mode of the carbonatite 40 40 40 40 73 73 i i i Page c) Trace elements and their d i s t r i b u t i o n i n the carbonatite 75 (4) Comparison of the Seabrook Lake Carbon-a t i t e with Carbonatite from Other Parts of the World 75 a) Description of carbonatites 77 b) Comparison of chemical composition of carbonatites 85 III. Petrogenesis of the Seabrook Lake Complex 91 Selected References 97 Appendix 102 Methods of Quantitative and Qualitative Analysis i . X-ray fluorescence 102 i i . Emission and absorption flame photometry 108 i i i . Colorimetric 113 X-ray d i f f r a c t i o n study of microline composition 113 LIST OF ILLUSTRATIONS Figure Page 1. Location map 5 2. General geology map 8 3. Location of carbonatite complexes related to Seabrook Lake 12 4. Detailed geology map of the Seabrook Lake complex 14 5. Photomicrograph of twinned microline 19 6. Hand specimen photograph of fenitized granite 19 7. Photomicrograph of microdine replaced by carbonate at the f e n i t i z e d granite-mafic breccia contact 21 8. Photomicrograph of quartz nearly completely re-placed by aegirine i n the f e n i t i z e d zone. 21 9. Photomicrograph of melanite surrounding magnetite i n the t r a n s i t i o n fenite 24 10. Photomicrograph of lamellar twinning i n perovskite 24 11. Ground magnetic map of the Seabrook Lake complex 26 12. Hand specimen photograph of hematitic mafic breccia 27 13. Hand specimen photograph of mafic breccia 27 14. Photomicrograph of aligned magnesioriebeckite i n mafic breccia 28 15. Paragenesis of the me t a l l i c mineral sequence i n mafic breccia 29 16. Photomicrograph of oriented inclusions i n nepheline i n the i j o l i t e zone 33 17. Photomicrograph of cancrinite a l t e r a t i o n on nepheline i n the i j o l i t e zone 33 Figure Page 18. Photomicrograph of a reaction rim on aegirine-augite i n the i j o l i t e zone 34 19. Photomicrograph of p o i k i l i t i c pyroxene i n the i j o l i t e zone 34 20. Relationship of TiO,, content to unit c e l l edge i n titanium-bearing and radite 37 21. Carbonatite location map 38 22. Hand specimen photograph of massive carbonatite 41 23. Hand specimen photograph of f o l i a t e d carbonatite 41 24. Photomicrograph of allotriomorphic-granular texture of the carbonatite 44 25. Photomicrograph of aligned c a l c i t e crystals i n the carbonatite 44 26. Photomicrograph of shear zone i n the carbonatite f i l l e d with apatite, hematite and pyrochlore 45 27. Variation of MnO with MgO i n the carbonatite 48 28. Photomicrograph of i n t e r s t i t i a l microcline i n the carbonatite 50 29. Hand specimen photograph of riebeckite clumps i n the carbonatite 50 30. Photomicrograph of riebeckite rimming magnesio-riebeckite i n the carbonatite 52 31. Photomicrograph of riebeckite rimming a large magnesioriebeckite grain i n the carbonatite 52 32. Relationship of density to Fe2°3 + F e 0 content i n the series magnesioriebeckite-riebeckite 56 vi Figure Page 33. Photomicrograph of apatite replaced by magnetite in the carbonatite 63 34. Photomicrograph of a reaction rim on biotite in the carbonatite 63 35. Photomicrograph of brookite with association hematite in the carbonatite 67 36. Photomicrograph of pyrochlore in the carbonatite 67 37. Photomicrograph of zoned pyrochlore in the carbonatite 68 38. Photomicrograph of pyrochlore replaced by magnetite in the carbonatite 68 39. Variation of Nb20,- with iron in the carbonatite 69 40. X-ray powder photograph of pyrochlore 70 41. Paragenesis of the carbonatite minerals 74 42. Triangular composition diagram for the rocks at Alno, Sweden 83 43. Triangular composition diagram of carbonatite compositions 86A 44. Comparison of Nb^ O^  in carbonatites 88 45. Comparison of SrO in carbonatites 89 46. Comparison of P20,- in carbonatites 90 47. Optinum grinding curves for X-ray fluorescent determinations 105 48. Homogeneity check of internal standards used for X-ray fluorescent determinations 107 v i i Figure Page 49. Working curve for niobium determinations 109 50. Working curve for strontium determinations 110 51. Working curve for iron determinations 111 52. Working curve for sodium determinations 114 53. Working curve for potassium determinations 115 54. Working curve for magnesium determinations 116 v i i i LIST OF TABLES Table Page 1. Summary of the major features of four niobium-bearing carbonatite complexes i n central . Ontario 13 2. Gains and losses of minerals and elements i n the f e n i t i z e d zone at Seabrook Lake 22 3. Gain and losses of elements i n fen i t i z e d zones from Norway, Sweden and A f r i c a 22 4. Comparison of X-ray data of schlorlomite 32 5. Summary of q u a l i t a t i v e and quantitative data of the carbonatite 42 6. Comparison of MgO, MnO, SrO and BaO i n limestone and carbonatite 46 7. Spectrographic analysis of microcline 51 8. V a r i a t i o n of magnesioriebeckite, apatite and b i o t i t e , i n the carbonatite, near the carbonatite-f e n i t i z e d granite contact 53 9. X-ray data for magnesioriebeckite 57 10. Spectrographic analysis of magnetite 59 11. Spectrographic analysis of pyrite 62 12. X-ray data for brookite 65A 13. X-ray data for pyrochlore 70 14. Trace element content and d i s t r i b u t i o n i n carbonatites 76 15. Mineralogy of carbonatites 79 16. Comparison of chemical analyses of carbonatites 80 17. Summary of the qu a l i t a t i v e and quantitative X-ray fluorescent data of the carbonatite 112 Emission and absorption flame photometric data of the carbonatite Cu and MnO determinations of the carbonatite Composition of microcline i n granite, f e n i t i z e d granite and carbonatite I Introduction Carbonatite,. at Seabrook Lake, was unknown until 1954 when niobium was discovered. Previous to this, major niobium deposits were discovered on the Manitou Islands of Lake Nippissing, Ontario in 1952; at Oka, Quebec in 1953; and at Lackner Lake, Ontario, in 1954. Niobium is used as an additive to steel to prevent inter-granular corrosion at high temperatures and pressures. A. Scope of Investigation This thesis deals mainly with the mineralogy and geo-chemistry of the carbonatite at Seabrook Lake. To lay the ground work for this study the author spent eight days during July, 1965 at Seabrook Lake mapping, becoming familiar with the general field relationships and collecting specimens. Because the author could spend only a short time at Seabrook Lake i t was decided to restrict the study of the carbonatite with only a brief discussion of the related rocks . Much of the laboratory work consisted of determining min-erals and textures in fifty thin sections and polished sections . X-ray powder photographs were used to identify rare or difficult minerals, but i t was found that the composition of the K-feldspar 2 as well as the ulVospinel phase of the magnetite could be adequately determined only with a X-ray diffractogram. Detailed chemical analy-s i s of the carbonatite was undertaken using the following techniques: X-ray spectroscopy (Nb, Sr, Fe, Ce, Y, Ba) emission and absorption flame photometry (K, Na, Mg) and colorimetry (Cu, Mn). The carbonatites are compared mi n e r a l o g i c a l l y and chemically with those of Sweden, United States, Transvaal, Nyasaland and Kenya. To complete t h i s study genesis of the complex i s considered. B. Acknowledgements The author i s indebted to Dr. F. R. Joubin, of F. R. Joubin and Associates, who k i n d l y permitted examination of t h e i r property. The author wishes to thank G. T h r a l l , of Canadian Nickel Company Limited, f o r providing transportation and supplies f o r a period of eight days. Professors R. M. Thompson, K. C. McTaggart and R. E. Delavault, of the Un i v e r s i t y of B r i t i s h Columbia, offered help during laboratory study and encouragement i n the preparation of the manu-s c r i p t . Special thanks are due t o ^ y Butters f o r kind l y providing i n s t r u c t i o n on the use of the high temperature e l e c t r i c furnace. C. Location and Topography The Seabrook Lake complex i s situated i n the southwest 3 corner of Township 5E, District of Algoma, Ontario (Figure 1). It is accessible from Aubrey Falls by a logging road that winds north-westward for approximately seven miles to Tidy Bay on Seabrook Lake. From this point the complex is most easily reached by boat to the south shore of the lake, 1.25 miles away. The area surrounding Seabrook Lake is one of bold relief in contrast to the flat monotonous terrain that lies to the north and west. In the immediate Seabrook Lake vicinity the maximum relief is approximately 800'. A peninsula in the southwest part of the lake marks the heart of the carbonatite complex. Here the arcuate trend of the shoreline reflects the contact between the country rock granite and the highly fractured and brecciated fenitized rocks which surround the complex. The peninsula resembles an elliptical saucer with a rim that rises 50 - 100 feet above the lake. This feature is the result of differential weathering of the carbonatites, mafic breccias and fenitized rocks (Figure 4) . D. History and Previous Work The area in the immediate vicinity of the Mississagi Road (Hwy. 129) consists chiefly of granitic rocks that were considered relatively unimportant from an economic viewpoint. For this reason as well as inaccessibility, few geologists have visited this region. In 1894, Robert Bell (Harding, 1950) completed a canoe traverse along 4 the Mississagi River from its headwaters to Lake Huron. In 1902, L. C. Graton (op. cit.) examined the geology exposed along the Mississagi and Wenebegon Rivers, and also at Round and Seven Mile Lakes. No economic deposits were reported by either of these ex-plorers (Harding, 1950) . With the opening of the Mississagi Road, during the early part of 1949, prospectors equipped with geiger counters traversed the area along numerous branch roads and trails. In the late summer of that year radioactive showings were discovered at Aubrey Falls and Seabrook Lake. With these discoveries the Ontario Depart-ment of Mines commenced reconnaissance mapping along the Mississagi Road under the direction of W. D. Harding. This survey was completed in the f a l l of 1949. Harding describes the Seabrook Lake find as giving high geiger counter readings from a band of crystalline lime-stone within metamorphosed Huronian sediments. During this early period of exploration W. Bussineau held the mineral rights to the northern part of the complex. His main exploration consisted of stripping of a hematite-rich rock in the northwestern part of the main peninsula (see Figure 4) . The first indication of niobium mineralization was noted in a sample that was taken from a magnetite-rich carbonate rock on the north-east flank of the complex (see location A, Figure 21) . Between 1956 and 1957 Tarbutt Mines Limited completed a ground magnetic and Figure 1. Index map of the Seabrook L a ke C o m p l e x . 6 geological survey and did a l i t t l e drilling. At this time Parsons (1961) of the Ontario Department of Mines first recognized the existence of a carbonatite complex similar to those in Sweden and Africa. E. General Geology of the Seabrook Lake Area The rocks of the Seabrook area are a l l of Precambrian age. They include the following units: Keewatin and Timiskaming, Algoman, Huronian and Keweenawan. The author recognizes that this terminology is in dispute and may well be dropped in the future. It does, however, have some usefulness in suggesting the general nature of the rocks concerned. The following descriptions are taken from Harding (1950) and supplemented with the writer's own observations. Regional geology and township boundaries are showing in Figure 2 (Harding, 1950) . (1) Keewatin and Timiskaming Keewatin and Timiskaming are the oldest rocks of the area and were only identified from a few small outcrops that occur at Tony Lake and in Townships 10D and 3E. Near Tony Lake, in DeGaulle Township, a narrow band of steeply dipping greywackes, which appear to trend east - west, are exposed. Another narrow band consisting of greywacke is exposed in the southwest part of Township 10D, Dis-trict of Sudbury. These rocks are steeply folded and strike north-east. The above rocks occur as inclusions within the granite 7 and granite gneiss. (2) Algoman More than 90 per cent of the area surrounding Seabrook Lake is underlain by g r a n i t i c rocks believed to be of Algoman age. Most of the rocks are f o l i a t e d granite gneisses with scattered areas of massive granite. (3) Huronian Only one small b e l t of Huronian rocks were noted. These rocks, which consist essen t i a l l y of conglomerate, are exposed on the Lafoe Creek Road i n Township IF (Figure 2) . (4) Keweenawan Keweenawan rocks are massive, brick-red, medium-to coarse-grained granite exposed on the shores of Seabrook Lake i n Township 5E. The granite i s fresh and d i f f e r s i n appearance from the somewhat altered f o l i a t e d or massive Algoman granite and granite gneiss. F. Nomencla tur e Because i n the following discussion certain unusual rocks are described i t was thought proper to define them. Carbonatite: A carbonate-rich or silicate-carbonate rock which c r y s t a l l i z e d from a carbonate magma. Fenite: Von Eckermann (1948) defines this term as, " i n s i t u metasomatically altered older contact rocks irrespective of composition, but the term does not apply to mobilized and transported hybridic mixed 8 F I G U R E 2 rocks even i f their origin may be fenitic." Ijolite: A rock consisting of approximately equal proportions of nepheline and aegirine-augite. Urtite: A rock composed mainly of nepheline with less pyroxene than an ijo l i t e . Melteigite: A rock composed mainly of pyroxene with less nepheline than an ijo l i t e . Foyaite (nepheline syenite): A rock composed of orthoclase with nepheline and sodalite. Jacupirangite: A rock composed of titanaugite with minor magnetite, apatite and nepheline. Malignite: A rock composed of approximately 50 per cent aegirine-augite with the remainder pre-dominantly nepheline and orthoclase in about equal amounts. Sovite: A term fi r s t introduced by Brogger (1921) for the calcite-rich member of his carbonatite group. Juvite: A rock composed of orthoclase and nepheline in approximately equal proportions with less aegirine-augite. This term appears to be equiva-lent to nepheline syenite. Theralite: A rock composed of calcic plagioclase, nepheline and titanaugite with accessory olivine 10 and soda pyroxene. Umptekite: A metasomatic rock composed largely of micro-perthite and sodic amphibole. y 11 II Geology of the Carbonatite Complex The Seabrook Lake carbonatite complex is one of the smallest of the nine known carbonatite complexes in central Ontario. This complex underlies an area of approximately one-half square mile. The general shape of the complex is circular in the north with a tapering appendage three quarters of a mile to the south (Figure 4). The complex consists of fenitized granite and breccia, mafic breccia, ijolite with biotite pyroxenite and re-lated breccia, and carbonatite. The core of the complex is thought to be carbonatite (outcrops are scarce) surrounded by a group of heterogeneous mafic breccias which are cut in many places by narrow dykes of carbonatite. Ijolite with biotite pyroxenite and related breccia crop out to the south. Enclosing the whole of the complex is an aureole of fenitized granite and breccia. Vein-lets of sodic pyroxene and amphibole, emanating from the complex cut the surrounding granite outside the aureole. Figure 3 shows the locations of some of the carbonatite complexes in the vicinity of Seabrook Lake while Table 1 summar-izes some of the major features of four of them. The alignment (Figure 3) of carbonatite complexes along a trend, a few degrees east of north, cannot be explained, but i t may be related to a deep crustal weakness. F igure 3. Locat ion of some Nb-bea r ing c o m p l e x e s in cen t ra l O n t a r i o 13 TABLE 1 Summary of the Major Features of Four Niobium-bearing Complexes i n Cen-t r a l Ontario Name Size (Square Miles) Dominant Rock Types Country Rock Seabrook 1/2 I j o l i t e (breccia) Mafic breccia Fenitized granite Carbonatite Granite and diabase dykes Firesand 1 3/4 Carbonatite B i o t i t e +• pyroxene carbonate rock Greenstone Nemegosenda Nepheline syenite Hydrated red fenite Pyroxenite fenite Carbonatite Gneiss Lackner 9 +• Nepheline syenite Foliated i j o l i t i c M a l i g n i t i c and syenitic rocks I j o l i t e breccia Carbonatite Gneiss Data from Parsons, 1961, p. 7. 14 FIGURE 4 15 A. Granite The Seabrook Lake complex i s completely surrounded by fresh, medium- to course-grained, pink to red granite. Excellent exposures are found on the west side of Southwest Bay, and on the east and south side of Centre Bay. Locally these rocks exhibit coarse pegmatitic as well as gneissic phases. Parsons (1961) has noted that with the appearance of deeper pink to red zones within the granite the rocks become radioactive. Occasionally, narrow fractures and shears carry veinlets of green pyroxene, blue amphi-bole and very minor epidote. These veinlets are presumably formed by emanations from within the complex. Thin section study reveals that the rocks are composed of p e r t h i t i c microcline, a l b i t e and quartz with minor amounts of b i o t i t e , magnetite and hematite. Microcline i s generally medium-to coarse-grained with well developed a l b i t e and p e r i c l i n e twin-ning (Figure 5). Broad exsolution lamellae of a l b i t e i n microdine are common and indicate that the granite has undergone r e l a t i v e l y slow cooling (Smith, 1960) . X-ray studies of the microcline com-ponent of the perthite indicate a composition close to Or^QQ. Again t h i s seems to suggest very slow cooling and a high state of order. The r a t i o of microcline to a l b i t e i s variable but large. Quartz, ranging between 20 - 30 per cent, i s present as anhedral to subhedral grains. 16 B. Diabase The dykes in the Seabrook Lake area, with few exceptions, strike northwest. They cut the country rock granite, but were not found intruding the rocks of the complex. This observation is con-firmed by the termination of distinct linear magnetic anomalies, related to dykes, as the fenitized zone surrounding the complex is approached. These dykes are dark green with a diabasic texture and according to Parsons (1961) resemble those of the type locality in the Matachewan area. C. Fenitized Granite and Breccia Fenitized granite and related breccia form a complete halo around the Seabrook Lake complex. The fenitized granite is medium-to course-grained and consists mainly of subhedral to euhedral K-feldspar. Most of the original quartz of the country rock has been replaced by fine-grained green pyroxene and blue amphibole (Figure 6). This type of replacement has been f a c i l i -tated by the highly fractured nature of the rock. Fenitized gran-ite breccia is present within the fenitized zone. The fragments vary from a fraction of an inch to approximately eight inches in diameter and are angular to subrounded. They are composed of feni-tized granite and may be partly replaced by carbonate. The matrix between the fragments is composed of fine-to medium-grained angular fragments of K-feldspar which are replaced in part by carbonate, green 17 pyroxene and blue amphibole. The fragments show no evidence of rotation or movement. The best exposures of both the fenitized granite and feni-tized granite breccia are on the southwest, northwest and eastern flanks of the main peninsula (Figure 4). This zone varies in width from 150 to 500 feet and is highly fractured and brecciated. A gra-dational contact exists between the granite and the fenitized aureole with the first indications of fenitization being a development of green pyroxene along fractures and shears. Within the fenitized or altered zone green pyroxene and blue amphibole have developed along fractures and quartz-feldspar grain boundaries. This may result in complete replacement of quartz (Figure 6). As the intensity of meta-somatism increases the feldspar becomes more turbid and developes a distinct deep pink-red colour. At this stage of metasomatism the fenitized granite is composed of reddish K-feldspar, green pyroxene, blue amphibole with minor amounts of quartz and carbonate. As the contact between the fenitized zone and the mafic breccia is approached the intensity of metasomatism increases resulting in the introduction of carbonate and the destruction of a l l of the quartz and some of the feldspar. In thin section, the fenitized halo is seen to consist of hematitic microcline, aegirine, magnesioriebeckite with variable minor amounts of carbonate, sodic plagioclase, pyrochlore and quartz. Micro-18 c l i n e occurs as medium- to coarse-grained, subhedral to euhedral grains that appear to be highly hematitic. Gridiron twinning i s present but may be partly destroyed by the f e n i t i z i n g solutions. In the advanced stages of metasomatism the microcline appears as islands within a fine-grained carbonate matrix (Figure 7). Exsolution a l b i t e i s recognizable i n some of the less altered microcline. X-ray d i f -f r a c t i o n study of the microcline component of the perthite indicates a composition of O r ^ suggesting that sodium has replaced some of the potassium. X-ray powder photographs indicate that the K-feldspar i n both the fe n i t i z e d zone and the country rock granite i s microcline. This i s i n contrast to Swift (1952) who reports conversion of micro-c l i n e to orthoclase within a f e n i t i z e d zone at Chishanya, Rhodesia. Aegirine occurs as replacements along fractures and quartz-microcline grain boundaries (Figure 8). In advanced stages of meta-somatism the quartz i s completely replaced leaving only remnants of the replaced mineral. Closely associated with aegirine i s magnesio-riebeckite. Quartz may be present as irregular r e l i c s or as veinlets cutting both microcline and aegirine. The author was able to recognize the following stages i n the development of fen i t i z e d granite and breccia: 19 Figure 5: Twinning i n microcline of the country rock granite. Note oblique exsolution a l b i t e . (Crossed n i c o l s x28). Figure 6: Complete replacement of quartz of granite by soda pyroxene and amphibole i n the f e n i t i z e d zone. Fracture at top of photograph i s f i l l e d with soda pyroxene and amphibole. Grey euhedral to subhedral c r y s t a l s are mic r o c l i n e . Scale i n cm. 20 1. Shattering of the granite. 2. Introduction of aegirine along fractures and shears in granite. 3. Aegirine replacement along fractures and quartz-mi crocline grain boundaries with further fracturing. 4. Replacement of most of the primary quartz by aegirine and magnesioriebeckite. Also destruction of some of the microcline twinning and replacement of the orthoclase molecule by albite (brecciation locally extensive). 5. Replacement of microcline, albite and quartz by carbonate at the inner contact. The fenitized and shattered zone described above resembles the "thermal-shock zone" mentioned by Von Eckermann (1948) from Alno, Sweden. A similar halo is described by Brogger (1921) from the Fen District of Norway. In Africa Dixey, et al. (1937) and Smith (1953) have described similar features. Table 2 summarizes the gains and losses in the fenitized zone. These changes resemble those of other fenitized zones listed in Table 3. Briefly a l l show a general gain in Na and a loss in Si0 2-21 r Figure 7: Advanced stages of carbonate metasomatism at the contact of the f e n i t i z e d zone and mafic breccia. Microcline appear as a large island i n a carbonate matrix. (Crossed nicols x 28). Figure 8: Apparently complete replacement of quartz by aegirine and magnesioriebeckite (prismatic, grey). Light grey mineral i s microcline. (Plain l i g h t x28). 22 TABLE 2 Gains and Losses of Minerals and Elements i n the Fenitized Zone at Seabrook Lake Mineral Gain Element Gain Mineral Lost Element Lost Aegirine Magnesioriebeckite Hematite Na Fe , A l , Mg Quartz Orthoclase molecule from microcline S i K Carbonate Ca, C0 2 Pyrochlore (rare) Nb TABLE 3 Gains and Losses i n Fenitized Zones from Norway, Sweden, and A f r i c a . Area Gains Losses Fen, Norway (Brogger, 1921) Na, Ca, A l S i 0 2 , K Alno, Sweden (Von Eckermann, 1948) K, Ba, Ca, H 20 S i 0 2 , Na Chishanya, Rhodesia (Swift, 1952 cited i n Smith, 1956)' Na, Ca, Mg, Fe, P S i 0 2 Chilwa, Nyasaland (Smith, 1953) K, Na, P S i 0 2 Spitzkop, Transvaal (Strauss and F e + 3 Truter, 1951) Na, A l , S i 0 2 23 D. Transition Fenite The t r a n s i t i o n fenite forms the gradational contact zone between the f e n i t i z e d outer zone of the complex and the inner mafic breccia zone. I t i s exposed only on a small projection on the east side of the main peninsula (Figure 4). In hand specimen this t r a n s i t i o n fenite i s fi n e - to med-ium-grained and i s composed of euhedral b i o t i t e , dark green pyroxene, i n t e r s t i t i a l carbonate with much less K-feldspar than the fenit i z e d zone. In thin-section this rock i s composed of euhedral, red-dish brown b i o t i t e , pale green clinopyroxene, i n t e r s t i t i a l carbonate and subhedral to euhedral s e r i c i t i z e d K-feldspar with minor apatite, melanite, magnetite, c h l o r i t e , sphene and perovskite. Melanite i s yellow brown to deep brown and i s found rimming magnetite or closely associated with i t (Figure 9). Perovskite i s euhedral and shows com-plicated lamellar twinning (Figure 10). E. Mafic Breccia The main part of the peninsula i s underlain by a group of mafic breccias (Figures 12, 13) grading outwards to fe n i t i z e d granite and f e n i t i z e d granite breccia. The fragments range from a fraction of an inch to approximately eight inches i n diameter and are sub-Figure 9: Euhedral magnetite (opaque) surrounded by melanite (dark grey). (Plain l i g h t x28). Figure 10: Complex lamellar twinning of perovskite. (Crossed nicols x28). 25 rounded to rounded. The subrounded fragments are pseudomorphs af-ter f e n i t i z e d granite fragments and are extensively replaced by b i o t i t e , amphibole, magnetite and carbonate. Only minor remnant pink feldspar remains. The rounded fragments are carbonatite. The matrix between the fragments i s fine-grained and i s composed of b i o -t i t e , amphibole, carbonate, magnetite and minor disseminated p y r i t e and galena. Outcrops of mafic b r e c c i a are few, but magnetic data (Figure 11) suggests that these rocks underlie the area designated • i n Figure 4. Outcrops of mafic breccia weather r a p i d l y and b i o t i t e -r i c h s o i l s are common i n areas underlain by these rocks. In t h i n section the mafic breccias are composed of dolo-m i t i c c a l c i t e , b i o t i t e , magnesioriebeckite and magnetite. ; K-feld-spar i s present i n trace amounts as r e l i c i r r e g u l a r grains. C h l o r i t e , hematite, sphene, brookite, p y r i t e , chalcopyrite, sphalerite and galena are minor constituents. The magnesioriebeckite c r y s t a l s are aligned and do not show reaction rims (Figure 14). Paragenesis of the m e t a l l i c mineral sequence i s i l l u s t r a t e d on Figure 15. The mafic b r e c c i a appears to have been derived from the f e n i t i z e d granite and f e n i t i z e d granite b r e c c i a by a d d i t i o n a l or superimposed metasomatism. This i s supported by the gradational contact between the f e n i t i z e d zone and mafic b r e c c i a , and also by the r e l i c pink K-feldspar i n the mafic b r e c c i a . Figure 11: Ground magnetic map of the Seabrook Lake complex. Survey by Tarbutt Mines Ltd. Published by Ontario Dept. of Mines (Parsons, 1961). F i g u r e 12: Hand specimen o f h e m a t i t i c m a f i c b r e c c i a . Note f r a g -m e n t a l c h a r a c t e r . S c a l e i n cm. F i g u r e 13: Hand specimen o f m a f i c b r e c c i a w i t h g r a n i t i c and c a r -b o n a t i t e f r a g m e n t s . Specimen t a k e n n e a r t h e f e n i t i z e d g r a n i t e b r e c c i a - m a f i c b r e c c i a c o n t a c t . S c a l e i n cm. 28 Figure 14: Alignment of magnesioriebeckite (prismatic, grey) i n mafic breccia zone. Magnetite (opaque), carbonate (white) and b i o t i t e (dark grey) are also present. (Crossed n i c o l s x28). magnetite galena chalcopyrite sphalerite pyrine chalcopyrite Figure 15: Paragenesis of metallic sequence in mafic breccia. 30 F. I j o l i t e I j o l i t e forms the southern part of the peninsula and the elongated extension to the south (Figure 4). I t consists of f i n e -to coarse-grained, anhedral to euhedral nepheline and fi n e - to medium-grained, dark green pyroxene and b i o t i t e . In some of the coarser i j o l i t e , nepheline occurs i n rectangular crystals which appear to be pseudomorphous after feldspar. The proportions of nepheline to pyroxene varies markedly i n the space of a single outcrop. Although these rocks are generally medium-grained they vary from f i n e - to extremely coarse-grained assemblages. Brecciated i r j o l i t e and b i o t i t e pyroxenite, as well as biotite-feldspar-pyroxene fragments, have been noted by Parsons (1961) within this general unit. Contacts with the other units of the complex appear to be gradational (Parsons, 1961). In thin section i j o l i t e i s found to be composed essen-t i a l l y of variable amounts of nepheline, aegirine-augite, reddish brown b i o t i t e , black garnet (schorlomite) with minor sphene, hema-t i t e , magnetite, cancrinite, carbonate and apatite. Nepheline i s present as f i n e , anhedral to coarse, euhedral grains. Lath-like inclusions were noted and i n some grains are oriented p a r a l l e l to euhedral c r y s t a l boundaries (Figure 16). Can-c r i n i t e a l t e r a t i o n , which i s p a r a l l e l to the weakly developed cleavage, i s pronounced and variable. Both o p t i c a l tests and X-ray 31 powder photographs confirm the cancrinite identification (Figure 17). Aegirine-augite, which is generally interstitial to nephe-line, occurs as subhedral to euhedral grains. Pleochroism is distinct and varies from emerald green to light brownish green. In some crystals a rim of highly pleochroic aegirine-augite surrounds a core of weakly pleochroic pyroxene (Figure 18) . The cleavage in both of these phases is continuous. Twinning is usually sharp with some twin planes exhibiting a slightly diffuse boundary. Near the contact of the fenitized granite and the ijolite the pyroxene is twinned and weakly zoned but i t does not possess the deep pleochroism that is found near the centre of the ijolite mass. Poikilitic pyroxenes are present near the contact of the ijolite (Figure 19). A black garnet, which has the properties of schorlomite (high titanium-bearing andradite), was identified by both optical and X-ray methods. In thin section i t is anhedral and deep brown to black. Table 4 shows the X-ray data of andradite, schorlomite, and the Seabrook black garnet. Note that in al l cases the d-spacings from Seabrook are higher than andradite and schorlomite. The unit cell edge for the Seabrook schorlomite is 12.15 A which resembles schorlomite from Oberbergen (Kunitz, 1936 cited in Deer, Howie and Zussman, 1962) and Jivaara (Zedlitz, 1935, cited in Deer, Howie and Zussman, 1962) . 32 TABLE 4 Comparison of the X-ray Data of Schorlomite from the Seabrook Lake I j -o l i t e with Andradite (synthetic) and Schorlomite from Magnet Cove, Ark, 1 2 3 I dm I dm I dm 13 4.263 10 4.31 5 4.27 60 3.015 50 3.026 60 3.05 100 2.696 80 2.702 100 2.72 13 2.571 10 2.584 45 2.462 60 2.468 60 2.48 17 2.365 10 2.366 17 2.202 10 2.205 5 2.22 25 1.956 20 1.964 10 1.951 11 1.907 10 1.909 10 1.845 20 1.781 9 1.741 10 1.743 30 1.752 25 1.673 70 1.679 30 1.688 3 1.641 60 1.611 100 1.614 100 1.627 13 1.507 30 1.512 5 1.519 3 1.421 10 1.421 13 1.348 50 1.351 20 1.358 20 1.316 50 1.319 20 1.328 13 1.286 40 1.290 20 1.300 3 1.231 5 1.218 25 1.120 20 1.127 15 1.101 20 1.110 13 1.066 20 1.073 7 1.005 7 0.991 17 0.9781 20 0.9860 1. Andradite from a synthetic mixture, A.S.T.M. 10 - 288. 2. Schorlomite from Magnet Cove, Arkansas, A.S.T.M. 7 - 390. 3. Schorlomite from the Seabrook Lake i j o l i t e . 33 Figure 16: Oriented lath-like inclusions in nepheline (light grey). I n t e r s t i t i a l aegirine-augite (grey) also present. (Plain light x80) . Figure 17: Cancrinite alteration (irregular grey streaks) i n nepheline (light grey). Aegirine-augite also present (grey). (Crossed nicols x80). Figure 18: Highly pleochroic rim around lesser pleochroic aegirine-augite (grey) i n the i j o l i t e zone. White grains are nepheline. (Crossed nicols x28). Figure 19: P o i k i l i t i c pyroxene i n the i j o l i t e zone. Included grains are nepheline. (Crossed nicols x28). 35 Figure 20 illustrates the relationship between titanium content and unit cell edge in the series low to high titanium-bearing andradite, e.g. andradite, melanite and schorlomite. The data were collected from Donnay and Nowacki (1954) and Deer, Howie and Zussman (1962) . The plot suggests a high titanium content for the andradite from the ijolite at Seabrook Lake. Ijolite may form by metasomatic or by magmatic processes. A metasomatic origin was proposed by Strauss and Truter (1951) at Spitzkop and by Von Eckermann (1948) at Alno, Sweden for ijolite that grades into fenite and pyroxenite. At Nemegosenda, Ontario (Figure 3, Table 1) ijolite flanks magnetite veins and grades out into nepheline syenite. Hodder (1961) concluded that these rocks were produced by metasomatism along fractures that now contain magnetite. According to Hodder the metasomatism was accomplished by aqueous solutions rich in iron, calcium, fluorine and phosphate. A magmatic origin was postulated (Larsen, 1942) for the ijolite at Iron H i l l , Colorado. This ijolite, which exhibits a pandio-morphic-granular igneous texture, is composed of variable amounts of nepheline, pyroxene and black garnet. These bodies form dykes and large intrusive masses. King (1949) at Napak, Uganda and Kranck (1928) at Kola reached similar conclusions when they found a con-tinuous variation between ijolite and other rocks. Parsons (1961) concluded that the ijolite at Seabrook Lake was formed by additional metasomatism of fractured and brec-36 ciated f e n i t i z e d granite. This conclusion was based on the gradational contact between the i j o l i t e and f e n i t i z e d granite, the absence of i j o l i t e dykes cutting other rock types, the marked v a r i a t i o n of nepheline and pyroxene i n s i n g l e outcrops, and the presence of r e l i c biotite-feldspar-pyroxene fragments. A meta-somatic o r i g i n i s also supported, i n t h i n section, by the apparent pseudomorphism of nepheline a f t e r feldspar and by the unusual, non-e u t e c t i c mineral paragenesis of f i r s t nepheline and then aegirine-augite. The present author agrees with Parsons that the i j o l i t e at Seabrook Lake i s of metasomatic o r i g i n . G. Hematite-rich rock Hematite-rich rocks crop out on the northwest part of the main peninsula (Figure 4). Their d i s t r i b u t i o n i s apparently e l l i p t i c a l and i s defined by deep reddish brown s o i l s that o v e r l i e this u n i t . This rock i s composed of mafic b r e c c i a and carbonatite that has been extensively replaced by hematite. H. Carbonatites Carbonatites are carbonate-rich rocks, generally dyke-l i k e , that are believed to be magmatic forming by the c r y s t a l l i z a t i o n of a carbonate-rich magma. At Seabrook Lake Parsons (1961) and the author follow this usage and use the term for c a l c i t e - r i c h dykes and avoid i t s use f o r areas of carbonate-bearing rock formed by 12.25 r 12.20 12.1$ 12.10 12.05 12.00 11.95 11.90 0.1 O b 0 3 J L I I I I I I I 1.0 Percent. TiO, 10.0 Figure 20. Relationship of TiO^ content to unit c e l l edge in the series low to high titanium-bearing andradite.The plotted values are as follows: 1. Andradite(Deer,Howie and Zussman,1962) 2. Andradite(Deer,Howie and Zussman,1962) 3. Andradite(Donnay and Nowacki,195U) 14. Andradite (Donna y and Nowacki,1954) 5.Schorlomite(Kunitz,1936 cited in Deer,Howie and Zussman,1962) 6.Melanite(Donnay and Howacki,195U) 7.Schorloraite(Zedlitz,1935 cited in Deer.Howie and Zussman,1962) Figure 21. Map of the main peninsula showing carbonatite l o ca t i ons and s t ruc tu re . 39 metasomatism. 1. Distribution and Occurrence Carbonatites do not crop out as strongly as the other rocks of the complex and only eight separate bodies (Figures 4, 21) were examined by the writer. Some are dyke-like, some irregular, and the large mass in the middle of the peninsula resembles a plug. Carbonatite dykes, which are found only in the northern part of the complex, range from a few inches to approximately thirty-five feet in width. Both massive and foliated types of carbonatites were noted. Each may be present in the same dyke with the foliated occupying the margin of the dyke and the massive the central part. The massive type (Figure 22) is generally medium-grained and buff in colour, and may have minor interstitial pink microcline. The foliated carbonatite (Figure 23) is generally fine-to medium-grained and heterogeneous. Its foliation and layering are outlined by hematite, magnetite, biotite, apatite and magnesioriebeckite. The following minerals can be identified in hand specimen from one or both types: calcite, magnetite, pyrite, limonite, hematite, biotite, amphibole (magnesioriebeckite) and K-feldspar (microcline) . 40 2. Structure The scant distribution of outcrops and the attitudes of foliation suggests that an outer cone sheet system encloses a central structureless core (Figures 4, 21). 3 i Chemical and Mineralogical Composition of the Carbonatite This section will present partial chemical analyses of carbonatites as well as descriptions of their mineralogy. (a) Chemical composition of the carbonatite The carbonatite is essentially composed of CaO and C O 2 (calcite). Table 5 presents the minor constituents. A detailed discussion of each method of analysis is given in the appendix. (b) Detailed mineralogy This section will present mineralogy, paragenesis and mode of the carbonatite. (i) Mineral descriptions The following minerals were identified in the carbon-atite at Seabrook Lake and are listed in approximate order of abundance: calcite, goethite, microcline, magnesioriebeckite-rie-beckite, magnetite-ulvospinel, apatite, pyrite, hematite, biot-ite (chlorite), pyrochlore, brookite, sphene, albite, ferroan dolomite, 41 Figure 23: Hand specimen photograph of f o l i a t e d carbonatite. Scale in cm. TABLE 5 Summary of the Qualitative and Quantitative Data of the Seabrook Lake Carbonatite Sample Nb„Ci SrO BaO Fe Na 20 K 20 MgO MnO Cu Ce Y Number % % (SrO) % % % % p .p .m. M-19 0.01 0.17 d 1.8 - - - - - n.d. d M-43 0.17 0.18 d 3.4 - - - - - d d M-44 0.17 0.20 d 2.7 0.58 0.38 2.12 0.32 2.5 P.tr d M-46 0.05 0.18 d 1.9 - - - - - n.d. d M-50 0.11 0.27 d 2.7 - - - - - d d M-51 0.16 0.30 d 3.1 - - - - - d d M-54 0.06 0.30 d 2.0 0.19 0.10 1.63 0.57 5 p. t r . d M-55 0.14 0.18 d 2.6 - - - - - d d M-60 n.d; 0.25 d 1.0 - - - - - p.tr. d M-66 0.15 0.18 d 3.0 - - - 0.32 5 n.d. d M-68 0.06 0.27 d 1.8 - - - - - d d M-82 0.01 0.30 d 2.3 0.41 0.21 1.96 0.48 5 n.d. d M-86 0.42 0.25 d 2.6 0.44 0.55 2.32 0.19 9 n.d. d M-94 0.27 0.67 d 4.8 0.15 0.27 2.72 0.19 13 d d M-102 0.06 0.24 d 1.4 - - - - - d d M-104 0.06 0.27 d 1.8 0.25 0.03 1.74 0.32 15 n.d. d Average 0.12 0.26 - 2.4 0.34 0.26 2.08 0.34 8 - d d - detected n.d. - not detected p.tr. - possible traces Nb, Sr, Ba, Fe, Ce, Y by X-ray spectroscopy Na, K by emission photometry Mg by absorption photometry Cu, Mn by colorimetry 43 aegirine, chalcopyrite, wollastonite and quartz. Methods of determination include X-ray powder photographs, X-ray diffracto-grams and thin and polished sections. Mineral separations were made with a Franz Isodynamic Separator and with heavy liquids. Calcite Calcite occurs in fine-to medium grains which commonly exhibit an allotriomorphic-granular texture (Figure 24). These grains, in some instances, are elongated and aligned (Figure 25). Micro-shears, which parallel the foliation, are f i l l e d with apatite, hematite, magnesioriebeckite and pyrochlore (Figure 26). Shearing is also indicated by the strained biaxial character of the calcite. Varia-tion in 2V of 0 - 15 degrees is within the range described by Deer, Howie and Zussman (1962) . . The amount of C O 2 and CaO in the carbonatite at location B was estimated by dissolving a known weight of powdered carbonatite in excess dilute HCl. For C O 2 the value is 48 per cent while the CaO value is approximately 41 per cent. CaO was estimated after C O 2 , Sr, Mg and Mn were determined. Most of MgO, MnO, SrO and BaO in the carbonatite is held in solid solution in the calcite. This is suggested 44 Figure 25: Aligned c a l c i t e crystals i n carbonatite. (Crossed nicols x80) . 45 Figure 26: Shear zone f i l l e d with apatite (elongated gray crystals) and hematite (opaque). The euhedral dark crystals are pyrochlore. (Crossed nicols x28). 46 TABLE 6 Comparison of MgO, MhO, SrO, and BaO Content i n Limestone and Carbona-t i t e . A l l values i n p.p.m. Limestone Carbonatite Author SrO BaO MgO MnO SrO BaO MgO MnO Sahama 1,000 400 78,000 500 1949 Higazy 180 6 9,200 2,180 1954 Russell 3,000 100-485 32,000-1954 6,000 80,000 Pecora 6,000- 5,500- 1,000-1956 24,700 110,000 188,000 Smith 3,900-1953 9,600 Present 1,700- SrO 16,300- 2,000-Study 6,850 27,200 6,000 47 by the consistent A I of SrK^ and MhK^ , despite grain s i z e . Another l i n e of evidence i s that separate strontium, barium and manganese minerals cannot be i d e n t i f i e d . Higazy (1954) suggests that at high temperatures larger amounts of Sr and Ba can be accommodated into the c a l c i t e structure than at low temperatures. I t therefore may be possible to relate the Sr and Ba content to the approximate temperature of formation, or at least to establish c r i t e r i a for the formation of carbonate rocks at high temperatures. Analysis of the carbonatite reveals the presence of 1.63 - 2.72 per cent MgO with an average value of 2.08 per cent, about 0.26 per cent SrO with BaO -=c SrO (Table 5 ) . The la s t two components (SrO and BaO) are present i n amounts much i n excess of that normally found i n limestone (Table 6 ) . Figure 27 shows the relationship between MnO and MgO. This line a r relations i s probably related to fixed degrees of s o l i d solution with s p e c i f i c temperature-pressure condi-tions as well as a v a i l a b i l i t y of both constituents during c r y s t a l l i z a t i o n . Microcline Microcline was noted i n minor amounts (1 - 47o) at four of the seven carbonatite l o c a l i t i e s studied. In thin 48 _ J _ i I I I I 1.0 1.$ 2.0 2.$ 3.0 3.5 Percent. MgO Figure 27. Variation of MnO with MgO in the carbonatites at Seabrook Lake, Ontario. 49 section i t i s i n t e r s t i t i a l to c a l c i t e and closely assoc-iated with minor a l b i t e (Figure 2 8 ) . Positive i d e n t i -f i c a t i o n was made with an X-ray powder photograph while an X-ray diffractogram ( A 2 6 201 ) determined a compo-s i t i o n of approximately C r ^ . This i s s l i g h t l y more sodic than the apparent source country rock microcline. A spec-trographs qua l i t a t i v e analysis of microcline shows the presence of barium (Table 7) and the author found that the index of refraction of this microcline i s somewhat higher than that normally accepted. Roy (1965) states that Ba w i l l greatly increase the index of refraction of K-feld-spar. Magnesioriebeckite-riebeckite Magnesioriebeckite i s widespread but only at location B (see Figure 21) i s i t easily seen i n hand specimen. Here i t occurs i n coarse, irregular clumps with a radiating acicular habit (Figure 2 9 ). I t may also occur p a r a l l e l to and outlining the carbonatite f o l i a t i o n . Magnetite and coarse b i o t i t e can be seen i n hand specimen to be d i r e c t l y associated with magnesioriebeckite. Along the carbonatite-fenitized granite contact, at location B, a noticeable increase i n the percentage of magnesioriebeckite, b i o t i t e and apatite was noted. Table 8 presents the per-Figure 28: I n t e r s t i t i a l microcline within a c a l c i t e matrix. (Crossed nicols x 28). Figure 29: Magnesioriebeckite-riebeckite clumps within carbonatite. Scale i n cm. TABLE 7 Spectrographic Analysis of Microcline from the Seabrook Lake Carbonatites K - S Mn • - W Na - S P - W S i - S Pb • - V.W. A l - S Zn • - p.tr. Ba - W-M Be • - n.d. Sr - W B - n.d. Ca - W Mg • n.d. S - Strong M - Moderate W - Weak V.W. - Very Weak p.tr. - possible traces n.d. - not detected 52 Figure 30: Irregular zonal cappings of riebeckite (black) on magnesioriebeckite (grey) i n a c a l c i t e matrix. Apatite (Ap) and magnetite (opaque) are present. (Plain l i g h t x80). Figure 31: Irregular riebeckite (black) rim on a large magnesio-riebeckite grain. (Plain l i g h t x28). 53 centages of each mineral at distances of 2% and 7 feet from the contact, i l l u s t r a t i n g the general nature of the d i s t r i b u t i o n of these minerals. TABLE 8 Mineral Variation, at Location B, i n the Carbonatite Inward from the Carbonatite-Fenitized granite Contact. Mineral Distance away from contact (feet) 2k 7 Magnesioriebeckite Apatite B i o t i t e 2 2 In thi n section magnesioriebeckite i s b i a x i a l (-) with a moderate to high 2V and strong dispersion. Pleochroic colours vary from l i g h t v i o l e t to deep prussian blue. Com-plete peripheral rimming of riebeckite on magnesioriebeckite is rare, but irregular cappings are exceedingly common, (Figures 30 and 31). These cappings are invariably deep blue with l i t t l e pleochroic v a r i a t i o n . This peculiar rim-ming may be due i n part to disruption of larger crystals after the formation of the reaction rims; hence, only ter-minal zonation is v i s i b l e , or to differences i n the rate of 54 c r y s t a l growth along different crystallographic directions. Deer, Howie and Zussman (1962) attribute the rimming to 4*2 -E-3 -f-2 -t~3 progressive replacement of Mg and A l by Fe and Fe They also found that with this replacement the depth of pleochroism increased and the extinction angle decreased. These variations i n optic properties are found also i n the magnesioriebeckite-riebeckite from the Seabrook Lake car-bonatite. The determination of the inner and the outer riebeckite zones was established by thin section, density and X-ray determinations. Using diiodomethane the density of the 3 inner zone was estimated to be 3.18 + 0.02 gm./cm. . A plot of the va r i a t i o n of FeO 4- Fe2°3 w i t h density (Deer, Howie and Zussman, 1962) suggests that the Seabrook Lake riebeckite i s a FeO + Fe^O^ poor or magnesium-rich riebec-k i t e (Figure 32). X-ray powder photograph of a mixture of the core and the outer rim agrees closely with c r o c i d o l i t e (fibrous riebeckite) and gives two d-spacings (3.30 and 2.29) that can be indexed only to magnesioriebeckite. Table 9 compares the X-ray data of c r o c i d o l i t e and magnesiorie-beckite. I t i s concluded that the inner riebeckite zone i s a magnesium-rich riebeckite (magnesioriebeckite) with riebeckite forming irregular outer rims. Synthesis of magnesioriebeckite and riebeckite has 55 been accomplished by Ernst (1960). He found that at low vapor pressure and temperatures above 800°C magnesiorie-beckite breaks down to a high temperature association con-s i s t i n g of hematite, magnesioferrite, o l i v i n e , aegirine, Mg20.5 M^g, Fe) 0 . 12 SiO^ and vapor. At vapor pressures of approximately 300 bars and above 910°C magnesioriebeckite melts incongruently to hematite, magnesioferrite, o l i v i n e , orthopyroxene, l i q u i d and vapor. Riebeckite breaks down at a temperature approximately 150°C below that of magnesio-riebeckite. The results of Ernst's studies show that magnes-ioeriebeckite i s stable at magmatic temperatures. I t also suggests that at the time of formation the temperature of o the carbonatite must have been less than 900 C. This i s i n keeping with the experimental work by Wyllie and Tuttle (1959, 1960) who suggest that liquids i n the systemsCaO-CX^-^O, Ca0-Mg0-C02-H20, and Ca 3(P0 4) 2-CaC0 3-Ca(OH) 2 can exist at temperatures between 600 - 700°C. Magnetite Magnetitie i s distributed rather i r r e g u l a r l y along the contact of the carbonatite and the country rock. This i s especially noticeable at location A (northeast part of the complex) where the carbonatite, near the contact, contains a narrow zone of 8 - 10 per cent medium- to coarse-grained, 3.U0 3.35 3.30 3.25 3.20 3.15 3.10 3.05 -10 >riebeckite riebeckite Seabrook o ^ o magnesioriebeckite magnesioriebeckite O 20 Percent. Fe^+FeO 30 ho 50 Figure 32. Relationship of density to Fe20j+Fe0 content in the series magnesi or iebeckite-riebeckite. Note that riebeckite from Seabrook falls In the range of Mg-riebeckite. Data taken from Deer,Howie and Zussman,1962. 57 TABLE 9 Comparison of Seabrook Lake Magnesioriebeckite with Riebeckite, Croci-d o l i t e and Magnesioriebeckite 1 2 3 4 I dm I dm I dm I dm 30 9.30 30 9.30 40 8.39 100 8.42 100 8.42 100 8.42 30 4.90 40 4.51 25 4.51 50 4.52 25 4.506 30 3.88 25 3.86 20 3.67 50 3.42 60 3.42 65 3.42 9 3.34 80 3.30 15 3.27 60 3.280 100 3.25 30 3.24 60 3.13 90 3.135 10 3.06 80 3.09 10 2.99 30 2.97 40 . 2.987 14 2.926 50 2.83 11 2.81 20 2.79 30 2.807 90 2.62 25 2.72 100 2.719 80 2.724 20 2.699 10 2.59 7 2.60 40 2.622 30 2.596 10 2.54 7 2.54 60 2.530 40 2.513 3 2.38 20 2.33 5 2.33 40 2.327 10 2.317 20 2.28 40 2.261 18 2.293 50 2.17 9 2.18 50 2.173 20 2.178 10 2.132 10 201 5 2.03 20 2.012 10 1.951 10 1.990 15 1.910 10 1.892 15 1.792 7 1.809 20 1.799 3 1.684 10 1.683 60 1.662 7 1.661 50 1.655 15 1.639 3 1.639 10 1.637 30 1.609 7 1.619 20 1.612 15 1.511 30 1.518 15 1.497 7 1.504 10 1.493 50 1.446 50 1.424 30 1.377 15 1.317 15 1.295 30 1.550 1. Magnesioriebeckite from Seabrook Lake. 2. Riebeckite (A.S.T.M. 9 - 436) 3. Cro c i d o l i t e (A.S.T.M. 14 - 230) 4. Magnesioriebeckite (A.S.T.M. 13 - 499) 58 euhedral magnetite. I t s occurrence i s also noteworthy at location B (western part of the complex) where i t i s associated with magnesioriebeckite. Zoning, within the magnetite, occurs at location A and i s seen as minute i n -clusions oriented p a r a l l e l to the c r y s t a l boundaries. Thin section studies shows that at some l o c a l i t i e s pseudomorphs of hematite after magnetite are common. Table 10 gives a qu a l i t a t i v e spectrographs analysis of the magnetite from the location A carbonatite and com-pares i t to chemical analyses of magnetite from Magnet Cove, Arkansas and Loolekop, Transvaal. The results from Seabrook are presented with a designation that only im-plies very approximate r e l a t i v e amounts. The unit c e l l edge, as determined by X-ray, for magne-t i t e from the Seabrook carbonatite i s 8.40 A which i s close to the c e l l edge for pure magnetite, 8.391 A (Clark, et a l . , 1931) . This value i s i d e n t i c a l to magnetite from Oka, Quebec (Davidson, 1963) and only s l i g h t l y larger than magne-t i t e from Loolekop, Transvaal. This larger unit c e l l i s probably related to the TiO^ content which at Seabrook ranges from 1 - 1 0 per cent. Polished section examination under high power (700x) o i l immersion reveals a fine network of oriented lamellae. 59 TABLE 10 Comparison of the Major Constituents i n Magnetite from a Carbonatite at Seabrook Lake with Magnetite from Magnet Cove, Arkansas and Loolekop Transvaal. Seabrook Arkansas (%) Transvaal (%) Fe 20 3 57.11 73.40 Fe S FeO 21.83 23.49 Ti S T i 0 2 6.98 0.54 A l M A 12°3 3.62 0.75 Cr ¥ C r 2 0 3 0.01 -V W-M V2°5 0.10 0.55 Mn M MnO 11.82 -Mg M MgO 7.18 2.53 Zn W ZnO n.d. -Ni n.d. NiO n.d. 0.01 a = o 8.40 A° a = 8.39 A° o a = 8.387-f-O.i o — S - Strong M - Moderate W - Weak n.d. - not detected 60 These lamellae are uniform i n size and are approximately 5 microns long and one micron wide. They are darker than the magnetite and exsolve along three directions (100) which i s indicative of exsolution of one spinel from another. Ilmenite commonly exsolves from magnetite but would show or-iented lamellae along four directions (111) . The i d e n t i f i c a t i o n of these lamellar intergrowths i s d i f f i c u l t due to the minute siz e . Using the orientation of these lamellae, as well as Parsons' titanium content, the exsolution phase probably represents a titanium-rich sp i n e l . X-ray powder photographs f a i l e d to detect a broadening or doubling of the d-spacings, or the presence of hercynite or o ilmenite. A weak r e f l e c t i o n was detected at 1.30 A. This r e f l e c t i o n i s the only l i n e of lilvospinel that does not interfere with those of magnetite and i s of s u f f i c i e n t i n -tensity to be i d e n t i f i e d with an X-ray powder photograph. The magnetite was scanned with an X-ray diffractometer to 0 i d e n t i f y the strong magnetite ref l e c t i o n s of 2.530 A and 2.966 A (A.S.T.M. 11 - 614). The diffractogram revealed two d i s t i n c t l y s p l i t peaks; one set corresponding to magne-t i t e and the other (reflections 2.585 A and 2.984 A) to the reflec t i o n s of a r t i f i c i a l iilvospinel that are given by Pouillard (1949). From these supposedly lilvospinel r e f l e c -a tions a c e l l edge of 8.50 A can be calculated, which agrees 61 with the unit c e l l of 8.49 A for iilvospinel given by Vincent, et a l . (1957). Applying Vincent's exsolution graph to the Seabrook Lake carbonatite i t is concluded that at the time of formation of the exsolution texture the temperature of o the carbonatite was approximately 600 C or s l i g h t l y l e s s . Apatite Apatite c r y s t a l s , within the carbonatite, occur as fine-grained disseminations and i n irregular stringers. They also occur as replacements along shear zones. The variety of apatite that occurs as disseminations and stringers i s closely associated with magnetite, magnesior-iebeckite and b i o t i t e . The replacement type consists of t i g h t l y packed, anhedral masses which replace mylonitic zones. Thin section and spectrographic evidence suggests that apatite i s only present i n minor amounts; however, l o c a l l y , such as location B (western part of the complex) the -?2^ 5 c o n t e n t m a y reach approximately 3 per cent. F i g -ure 33 shows magnetite replacing apatite. Pyrite Pyrite i s present as fine-grained disseminations that are readily observable i n hand specimen. Typically the pyrite percentage i s less than 1. In the northern part of 62 location E (eastern part of the main peninsula) i t may range up to 15. The c e l l edge of the Seabrook Lake p y r i t e , as deter-0 mined by X-ray powder photograph, i s 5.43 A. This i s s l i g h t l y higher than the 5.419 A value given by Berry and Thompson (1962). According to Neuhaus (1942) the unit c e l l can be increased by the addition of As. His results show that o p t i c a l l y continuous pyrite containing 5 per 0 cent As increases the c e l l edge to 5.442 A. At Seabrook spectrographic evidence indicates a moderate amount of arsenic i n the pyrite (Table 11). TABLE 11 Trace Constituents i n Py r i t e from a Carbonatite at Seabrook Lake Fe V.S. Zn V.W. Mn W-M Pb V.W. As M B i n.d. Mo W Au n.d. Co W Ag n.d. Cr W V.S. - Very Strong V.W. - Very weak M - Moderate n.d. - Not detected W - Weak Figure 34: Reaction rim around b i o t i t e . Matrix i s c a l c i t e . (Plain l i g h t x80). 64 B i o t i t e B i o t i t e occurs e r r a t i c a l l y throughout the contact margins of the ca-r-bonatites, but i t does not occur i n amounts greater than 10 per cent. In thin section i t may occur as coarse, euhedral crystals which show deep red-brown pleochroism, or as fine-grained disseminations which show a l i g h t yellow-brown to pale brown pleochroism. The deeply pleochroic variety i s only found at location B (western part of the complex) and occurs i n two grain sizes that r e f l e c t s at leasttwo stages of paragenesis or possibly c r y s t a l l i z a t i o n over an extended range. The f i r s t formed deeply pleochroic variety occurs as coarse, euhedral grains which possess ragged c r y s t a l boundaries and i s replaced by c a l c i t e . This early formed b i o t i t e i s usually extensively altered to c h l o r i t e and replaced by magneaoriebeckite, magnetite and hematite. The second deeply pleochroic b i o t i t e replaces c a l c i t e and i s fine-grained. This type shows irregular reaction, rims around an older bio-t i t e , which exhibits s l i g h t l y different o p t i c a l properties (Figure 34). These rims are presumably formed by reaction with the carbonate f l u i d that i s s l i g h t l y enriched i n i r o n . The deep red brown pleochroism is attributed to high t i t a n -ium or iron content (H a l l , 1941 and Parsons, 1961). 65 Brookite Brookite occurs as fine-grained disseminations that can only be i d e n t i f i e d i n th i n section. The most dis-t i n c t i v e o p t i c a l property i s i t s l i g h t grey to blue grey pleochroism. Brookite occurs as disseminations and com-monly is associated with sphene and hematite (Figure 35). Positive i d e n t i f i c a t i o n was made with an X-ray powder photograph. Table 12 gives the X-ray data for i t and for brookite from Magnet Cove, Arkansas (Berry and Thompson, 1962). Pyrochlore Pyrochlore i s present i n the carbonatites as minute, honey yellow octahedra which vary i n size from 70 to 500 microns (Figure 36). These octahedra are only v i s i b l e i n thi n section and occur as disseminations and as concen-trations along hematite-bearing fractures. An average of sixteen Nb 20^ determinations indicate that pyrochlore i s present i n amounts of approximately 0.2 per cent. Zoning is rare but may be present i n some of the large disseminated grains (Figure 37). Parsons (1961) reports a spectrographic analysis of clean magnetite from location A (northeast part of the complex) as containing "titanium 1 - 10 per cent and niobium 0.5 - 5.0 per cent i n addition to iron but no clue 65A TABLE 12 Comparison of the X-ray Powder Data for Brookite from a Carbonatite at Seabrook Lake with Brookite from Magnet Cove, Arkansas Seabrook Magnet Cove I d I d m m 10 3.51 10 3.49 2 2.90 7 2.88 ih 2.42 2 2.47 1 2.38 1 2.39 1% 2.21 1 2.23 1 2.13 2 1.957 4 1.894 2 1.888'* 3 1.845 2 1.746 3 1.671 % 1.681 4 1.638 3 1.653 4 1.605 2 1.537 2 1.482 h 1.488 1 1.450 1 1.459 3 1.435 3 1.415 1 1.363 % 1.365 1 1.341 1.334 ) 1% 1.265 1.238 )A.S.T.M. 4 1.044 1.037) 3-0380 66 as to the form i n which the niobium occurs.". Thin section studies c l e a r l y show that magnetite i s paragenetically l a t e r than the pyrochlore and i n many sections was seen to i n -clude this mineral (Figure 38). The association i n a few thin sections between magne-t i t e , hematite and pyrochlore suggested that a relationship might exist between the t o t a l iron and Nb^ O,.. Figure 39 i s a plot showing the results of f i f t e e n Wo^O^ and iron analyses. The plotted trend c l e a r l y shows a relationship between the two components with high M^O v a-- u e s corres-ponding to high iron percentages. The X-ray powder photograph of pyrochlore i s sharp indicating a non-metamict state that i s confirmed by nega-ti v e alfa-track tests. The unit c e l l of the pyrochlore at location A i s 10.38 A which compares to the 10.35 A value given by Dana (1963) and 10.37 A by A.S.T.M. 3-110. Figure 40 i s an X-ray powder photograph of pyrochlore. Table 13 presents the X-ray powder data of pyrochlore. 67 Figure 36: Large pyrochlore grain associated with c a l c i t e (white) and magnesioriebeckite (grey). ) P l a i n l i g h t x80). 68 Figure 37: Zoned pyrochlore i n c a l c i t e matrix. (Plain l i g h t x80). Figure 38: Magnetite enclosing pyrochlore, c a l c i t e and magnesio-riebeckite. (Plain l i g h t x80). 70 Figure 40: X-ray powder photograph of pyrochlore (FeK^ with MnO f i l t e r ) Most pyrochlore i s metamict and X-ray powder photographs showing a clear pattern are rare. TABLE 13 X-ray Powder Data of Pyrochlore from a Carbonatite at Seabrook Lake I d m hkl 3 6.07 111 1% 3.14 311 10 3.01 222 3 2.60 400 1% 2.02 511 333 7 1.845 440 5 1.573 662 1 1.502 444 % 1.459 711 551 h 1.359 731 553 h 1.305 800 2 1.196 662 2 1.168 840 2 1.064 2 1.004 0 Unit c e l l edge 10.38 A 71 A l b i t e A l b i t e i s a very minor constituent which commonly i s associated with microcline. A composition of An,, was deter-mined by the "perpendicular to a" method. Sphene Sphene i s present i n very minor amounts and i s gener-a l l y associated with pleochroic b i o t i t e , apatite and brookite. Ferroan dolomite (Ankerite) Ferroan dolomite i s present as irregular aureoles around some of the magnetite grains. The grains are yellow brown 3 and have a s p e c i f i c gravity of greater than 2.85 gms/cm . X-ray powder photograph suggests dolomite; however, the lines could also be indexed to ankerite. Aegirine Aegirine was noted i n only one carbonatite as irregular c r y s t a l fragments which appear to be xenocrysts from the fe n i t i z e d zone picked up during i n j e c t i o n of the carbonatite. Chalcopyrite Chalcopyrite i s present i n minor amounts i n the pyr i t e -r i c h carbonatite and i s d i r e c t l y associated with pyrite and 72 magnetite. Wollastonite Wollastonite was noted from only two carbonatite bodies on the east flank of the main peninsula. Quartz Quartz was noted i n one grain at location C (northern flank of the main peninsula). Secondary Minerals  Goethite Goethite i s found only as a secondary mineral i n the surface oxidized zone of the carbonatite dykes. I t occurs as fine-grained replacements along fractures and c a l c i t e grain boundaries. Occasionally this mineral exhibits a fibrous, radiating habit. Goethite was confirmed by X-ray powder photograph. Hematite Hematite occurs as fine-grained disseminations and as ve i n - l i k e replacements which cross cut most of the minerals present i n the carbonatites. 73 ( i i ) Paragenesis A complete account of paragenesis of the minerals w i t h i n the carbonatite i s extremely d i f f i c u l t . D e t a i l e d work i n -dicates complex overlapping periods of c r y s t a l l i z a t i o n . Figure 4 1 i l l u s t r a t e s the paragenesis and divides the c r y s t a l l i z a t i o n into four somewhat a r b i t r a r y stages. The f i r s t i s represented by Stage I i n which pyrochlore, brookite, sphene and c a l c i t e c r y s t a l l i z e . A p a t i t e and s i l i c a t e s c r y s t a l l i z e i n Stage II. Magnetite forms i n Stage III followed by the secondary minerals i n the l a s t stage. ( i i i ) Average mode of the carbonatite An average mode for the Seabrook Lake carbonatite i s d i f f i c u l t to determine as not a l l the minerals mentioned i n the previous mineralogy s e c t i o n are present w i t h i n a s i n g l e carbonatite dyke. The following mode gives only the approximate percentage of each mineral: C a l c i t e 8 5 - 907o M i c r o c l i n e 1 - 2% Magnesioriebeckite-riebeckite 2 - 37= Magnetite 17. Apatite 1% P y r i t e 1% Hematite 1 - . .27 . Goethite 4 - 57o B i o t i t e 1% C h l o r i t e 17. Pyrochlore 1 / 5 % Brookite 0 17. A l b i t e 17. Sphene 17. Ferroan dolomite 17. Aegirine t r . Chalcopyrite t r . Wollastonite t r . Quartz t r . goethite Stage k ~ hematite Stage 3 magnetite feldspar magnesioriebeckite riebeckite - Stage 2 . biotite apatite calcite br ookite,sphene pyrochlore Figure 41. Paragenesis of the minerals in the carbonatites at Seabrook Lake, Ontario. Time ~*4 75 (c) Trace elements and their d i s t r i b u t i o n i n the carbon-a t i t e . Trace elements i n the carbonatite were detected by means of X-ray spectrography, quartz-prism spectrography and wet c o l o r i -metric techniques. The d i s t r i b u t i o n of trace constituents i n magnetite, p y r i t e and microcline has been previously discussed i n the mineralogy section. The purpose of this section i s to summarize the d i s t r i -bution of trace elements i n the Seabrook Lake carbonatite and to compare them to trace constituents i n carbonatites from A f r i c a (Table 14). 4. Comparison of the Seabrook Lake Carbonatite with Carbonatite from Other parts of the World This section w i l l describe the general features of carbon-ati t e s from various places with special emphasis on the mineralogy and chemical composition. After a b r i e f discussion of each l o c a l i t y a comparison of the chemical composition of carbonatites w i l l be presented. The l o c a l i t i e s chosen are: Magnet Cove, Arkansas; Spitzkop, Transvaal; Alno, Sweden; Chilwa, Nyasaland; and Mrima H i l l , Kenya. In Table 15 the mineralogy of these f i v e representative carbonatite complexes i s subdivided into those minerals invariably present, commonly present and infrequently present. The minerals i n the sec-ond category are generally represented by f i v e or s i x members. Minerals present at Seabrook Lake are starred. Table 16 gives the 7 6 TABLE 1 4 Trace Element Content and Di s t r i b u t i o n i n the Carbonatites at Dorowa and Shawa, Loolekop and Seabrook with Comparison to Higazy's Averages A B C-Distribution i n Carbonatite D - Di s t r i b u t i o n Rb n.d. 1 0 Present i n phlogopite (tr) n.d. L i 5 3 Present i n phlogopite (tr) t r b i o t i t e Ba 1 9 8 5 2 5 0 Pres ent i n carbonate & apatite Sr carbonate, microcline Sr 7 7 5 0 1 0 0 0 Present i n carbonate & apatite 2 2 0 0 carbonate, microcline Cr 1 1 1 Present i n magnetite (tr) t r magnetite, pyrite Co 1 4 1 7 Present i n magnetite (tr) t r p y r ite Ni 5 0 1 0 Pr es ent i n magnetite ( t r ) , V a l l e r i i t e (tr) and Pentlandite t r magnetite, pyrite (poss .) Zr 6 0 4 0 Present as baddeleyit e t r pyrochlore Ce tr pyrochlore Cu 1 5 2 Copper sulphides 8 chalcopyrite V 1 8 n.g. Pr es ent i n magnetite ( 0 . 3 7 7 o ) t r magnetite Ga 1 1 n.g. n.d. T l n.d. n.g. n.g. n.d. Sn n.d. n.g. n.g. n.d. Pb 1 0 1 0 n.g. t r p y r ite Sc n.d. 1 0 n.g. n.d. Mo 4 n.g. n.g. t r p y r ite Ge n.d. n.g. n.g. n.d. Be n.d. 1 n.g. n.d. Ag 6 n.g. n.g. n.d. In n.d. n.g. n.g. t r Na n.g. 3 0 0 n.g. 2 6 0 0 Na amphibole K n.g. 2 0 0 n.g. 1 9 0 0 b i o t i t e , microcline Nb n.g. 3 0 n.g. 9 0 0 pyrochlore T i n.g. 3 5 0 n.g. 5 0 - 1 0 0 magnetite Ta n.d. l i m i t of detection U n.g. n.d. 3 0 0 ] A - Higazy's Averages of African carbonatites (Higazy, 1 9 5 4 ) B - Dorowa and Shawa (Johnson, 1 9 6 1 ) C - Loolekop, (Russell, et a l . , 1 9 5 4 ) D - Seabrook Lake, Ontario. t r n.d. n.g. A l l results i n p.p.m. 77 chemical analyses of the carbonatites that are discussed i n this sec-t i o n . (a) Description of carbonatites Magnet Cove, Arkansas. (Erickson and Blade, 1963) The Magnet Cove a l k a l i c complex, which covers an area of 4.6 square miles, i s composed of a series of ring dykes of post-Mississippian age that intrude faulted and folded Paleozoic sedi-ments. The r i n g dyke complex has a core of i j o l i t e and carbonatite, an intermediate r i n g of trachyte and phonolite, an outer zone of nepheline syenite, and two i r r e g u l a r l y distributed zones of jacupir-angite. Mineralogically, the rocks can be divided into those con-taining feldspar and those free of feldspar. The carbonatites occur i n irregular bodies i n the central part of the complex. Locally, apatite, magnetite, p y r i t e , m o n t i c e l l i t e , perovskite and kimzeyite (zirconium garnet) are found i n the c a l c i t e groundmass. Minerals less abundant are r u t i l e , anatase, brookite, wavellite, phlogopite (?), limonite, b i o t i t e , pyroxene, and amphibole. The carbonatites contain about 937- CaCO^. Phosphate, s i l i c a and mag-nesia are the only oxides present i n amounts greater than 1 per cent. Trace elements that exceed 0.01 per cent are: Sr, Mn, Ba, T i , and V. Niobium i s present i n amounts less than 0.01 per cent and i s mainly present i n perovskite. 78 Erickson and Blade conclude that a l l the igneous rocks at Magnet Cove were derived by d i f f e r e n t i a t i o n and f r a c t i o n a l c r y s t a l -l i z a t i o n of a phonolitic magma r i c h i n a l k a l i , lime and v o l a t i l e s . Carbonatites are also included i n this igneous o r i g i n . Table 16, Column 1 presents an average chemical analysis of the carbonatite. Spitskop, Transvaal. (Strauss and Truter, 1951) The complex consists of an irregular outer zone of a l k a l i granite, quartz syenite and red and white umptekite (syenite) a l l of which were derived by metasomatism of the country rock granite. The umptekites are followed inwards by f a y a l i t e d i o r i t e and t h e r a l i t e . Both these rocks are considered metasomatic. The central mass of the complex consists of red, coarse-grained i j o l i t e ; black, fin e -grained i j o l i t e ; dark melteigite and jacupirangite. The l a s t two rock types are regarded as f e n i t i z e d f a y a l i t e d i o r i t e and t h e r a l i t e . The red i j o l i t e i s considered to be of magmatic o r i g i n . Inside the complex are two large ring dykes of foyaite - '. Carbonatite occurs i n the south-east part of the complex as a large mass which appears to intrude a l l the a l k a l i c rocks. This rock i s composed essentially of c a l c i t e and dolomite with small, irregular masses of apatite and magnetite. Also present are disseminations of pyr i t e , limonite, riebeckite (?) and serpentinous material. Strauss and Truter consider the carbon-a t i t e s to be magmatic. Chemical analysis of this carbonatite i s shown on Table 16, Column 7. 79 TABLE 15 L i s t of a l l Minerals of Carbonatites from Magnet Cove, Arkansas; Spitzkop, Transvaal; Alno Island,. Sweden; Mrima H i l l , Kenya; and Chilwa, Nyasaland. Note that the mineralogy of carbonatites can be divided into minerals invariably present, commonly present and infrequently present. Minerals present i n the Seabrook Lake carbonatite are starred. 1 2 3 c a l c i t e b a r i t e columbite magnetite f l u o r i t e thorianite * apatite zircon c a s s i t e r i t e * pyrite * K-feldspar baddeleyite olivine-serpentine pyrrhotite monazite * b i o t i t e * amphibole sphalerite Niobium mineral: aegirine-augite v a l l e r i i t e pyrochlore * T i 0 2 pentlandite niobian perovskite dolomite * chalcopyrite ilmenite chalcocite * hematite bornite * goethite fluorcenite sphene synchysite galena Jl* quartz perovskite * a l b i t e vermiculite phlogopite aegirine epidote chondrodite * wollastonite scapolite s e l l a i t e melanite bastnaesite wavellite kimzeyite 1. Minerals invariably present 2. Minerals commonly present 3. Minerals infrequently present 80 TABLE 16 Comparison of Chemical Analyses of Carbonatites .1 .2 .3 .4 .5 .6 .7 Si0 2 1.90 4.29 3.36 2.40 3.40 3.15 1.17 A 12°3 0.33 1.32 1.69 1.02 1.60 0.45 n.d. Fe 0 0.42 9.28 6.13 0.34 5.60 ) ) )2.9 )0.43 FeO 0.32 n.d. 2.99 0.29 n.d. ) ) MnO 0.26 0.96 0.31 0.17 1.50 0.48 0.33 MgO 1.05 0.25 3.10 1.45 tr 1.96 1.82 CaO 53.37 45.62 44.35 52.78 47.80 ) 53.12 )89.0 co 2 39.41 34.88 32.80 39.35 36.44 )) 40.98 K 20 0.16 0.22 0.50 0.28 n.r. 0.21 n.d. Na20 0.00 0.06 0.04 0.13 n.r. 0.41 n.d. SrO n.r. 0.11 n.d. n.d. n.r. 0.30 n.r. BaO 0.00 0.40 0.10 n.d. 1.24 SrO n.r. H 20 + 0.12 1.20 0.16 0.17 n.r. 0.1 0.78 H20" 0.04 0.35 0.14 n.r. n.r. n.r. 0.29 T i 0 2 0.10 0.23 0.30 tr 0.24 tr n.r. P2°5 2.00 0.03 3.26 1.94 1.22 0.4 0.20 so 3 0.02 0.22 0.06 n.r. 0.89 n.r. n.r. tr - trace n.r. - not reported n.d. - not detected 81 TABLE 16 (continued) .1 .2 .3 .4 .5 .6 .7 Cl 0.00 0.01 0.02 n.r. n.r. n.r. n.r. F 0.15 n.r. n.r. n.r. n.r. n.r. 0.03 S 0.09 n.r. 0.42 n.r. n.r. n.r. 0.04 Nb 20 5 t r 0.30 0.80 n.r. 0.21 0.01 n.r. (Ce,Y) 2Q 3 t r 0.40 n.d. n.r. 0.42 t r n.r. Total 99.63 100.20 100.81 100.32 100.56 99.4 (calculated) 99.98 t r - trace n.r. - not reported n.d. - not detected Explanation 1. Magnet Cove, Arkansas (Erickson and Blade, 1963). 2. Chilwa Series, Nyasaland (Smith, 1953) 3. Fen D i s t r i c t , Norway (Brogger, 1921) 4. Alno, Sweden (Von Eckermann, 1948) 5. Mrima H i l l , Kenya (Coetzee and Edwards, 1959) 6. Seabrook Lake, Ontario (location B) 7. Spitzkop, Transvaal (Strauss and Truter, 1951) 82 Alno Island, Sweden. (Von Eckermann, 1948) The complex, which i s located approximately 200 miles north of Stockholm, i s c i r c u l a r i n plan aid has a diameter of approx-imately 13,000 feet. The outer zone consists of metasomatic nephe-l i n e f e n i t e , syenitic fenite and nepheline syenite fenite a l l of which were derived from the country rock migmatites. The above order i s from the margin towards the centre of the complex. Mel-t e i g i t e , malignite and jacupirangite form an arcuate zone on the south and north flank of the complex. Intruding into these rocks are u r t i t e , j u v i t e and foyaite. The carbonatites intrude a l l these rocks and are present as dykes and irregular masses. They are con-centrated i n the south, west and north parts of the complex. Min-e r a l o g i c a l l y these carbonatites are highly variable but consist es s e n t i a l l y of c a l c i t e , dolomite, b i o t i t e , apatite, aegirine-augite, hedenbergitic-augite, p y r i t e , f l u o r i t e , serpentine, n a t r o l i t e , sphene, melanite, ilmenite and s e r i c i t e . Table 16, Column 4 pre-sents a chemical, analysis of one of the carbonatite dykes. Figure 42 i l l u s t r a t e s a trend from j u v i t e and i j o l i t e to carbonatite. The regularity of t h i s trend suggests processes of d e s i l i c a t i o n as sug-gested by Von Eckermann (1948) or possibly f r a c t i o n a l c r y s t a l l i z a t i o n and d i f f e r e n t i a t i o n . Chilwa Series, Southern Nyasaland. (Smith, 1953) The complex i s situated on Chilwa Island. The units 83 CaO+COg+FegO^ +MgO NagO+I^OfA^C^ Figure 42. Triangular composition diagram for the igneous rocks at Alno, Sweden. The samples plotted are ae followsi 1. Juvite 2. Foyaitic ijo l i t e 3. Melteigite U.Melanite melteiglte Malign!tic melteiglte 6. Biotite,pyroxene sovite 7. Biotite jacupirangite 8. Pyroxene>biotite sovite 9. Biotite sovite 10.Biotite sovite 11.Sovite 84 present are: syenite, nepheline syenite, i j o l i t e , dykes of phono-l i t e and nephelinite and volcanic vents occupied by carbonatite and brecciated fenite. These volcanic vents form a v e r t i c a l pipe-like zone into which the alkali-carbonate magma intruded. The gneisses surrounding these vents are altered and appear to closely resemble the fenites described by Brogger. The r e l a t i o n of carbonatites to feldspathic intrusions i s complex. Carbonatites are of particular interest as they show intrusive relationships with other rocks, as well as constituting the main constituent of the vent agglomerates and breccias. The mineralogy of these carbonatites i s as follows: c a l c i t e , apatite, orthoclase, quartz, p y r i t e , magnetite, pyrochlore, r u t i l e , anatase, zircon, synchysite, b a r i t e , mica, f l u o r i t e and f l o r e n c i t e . Table 16, Column 2 presents a t y p i c a l chemical analysis of the Chilwa carbonatite. Mrima H i l l , Kenya. (Coetzee and Edwards, 1959) Mrima H i l l l i e s on the coastal p l a i n of Kenya approx-imately 40 miles southwest of Mombasa. within the general area of the complex three rocks are found: carbonatites, lamprophyres and f e n i t i z e d sediments. Since the outcrop i s extremely poor, d i s t r i -bution of these rocks i s unknown. The carbonatites are of four types: s o v i t i c , b i o t i t i c , dolomitic and agglomeritic. Surrounding these rocks i s an irregular aureole of metasomatically altered arenaceous sediments. At some distance away from the carbonatite -85 sediment contact aegirine - augite has developed along j o i n t planes. Closer to the carbonatite the more altered sediment consists of c h l o r i t i c clay with authigenic orthoclase laths . The mineralogy of the carbonatite i s as follows: c a l c i t e , dolomite, amphibole, pyrox-ene, o l i v i n e , orthoclase, b i o t i t e , phlogopite, c h l o r i t e , epidote, magnetite, hematite, barit e , pyrite, pyrrhotite, pyrochlore and trace amounts of galena, sphalerite, apatite, ilmenite, r u t i l e sphene, mon-a z i t e , brookite, zircon, perovskite, anatase, m e l i l i t e and scapolite. The complex is covered by a thick mantle of residual s o i l which i s enriched i n pyrochlore. Table 20, Column 5 presents a chemical anal-ys i s of one of the carbonatite dykes. (b) Comparison of chemical composition of carbonatites. This section w i l l present chemical data on carbonatites from around the world. A, detailed survey of the l i t e r a t u r e yielded chemical analyses from the following l o c a l i t i e s ; Alno Island, Sweden; Fen D i s t r i c t , Norway; Magnet Cove, Arkansas; Spitzkop, Trans-vaal; Magnet Heights, Transvaal; Mrima H i l l , Kenya; Mbeya, Tanganyika; and Chilwa, Nyasaland. These chemical analyses and an average Sea-brook Lake analysis (present study, p a r t i a l chemical analyses with t h i n section calculations) are plotted on a ternary diagram using the following parameters: s i 0 2 + A-12°3: C a 0 + C 0 2 : F e2°3 + F e 0 + MgO + MnO (Figure 43). In each example a plotted point represents more than 95 per cent of the bulk composition of that carbonatite. 86 This comparison, of chemical composition, shows a general concentration around the CaO + C O 2 apex, and more s p e c i f i c a l l y i t shows that 70 per cent of the carbonatites plotted are bounded by the following compo-s i t i o n a l boundaries: 757= CaO + C02> 157= Fe 20 3 + FeO + MgO + MnO, and 10/o Si02 + A^2^3 • The Alno carbonatites, which plot outside these l i m i t a t i o n s , are considered t r a n s i t i o n a l between carbonatites and a l k a l i c rocks. Since Nb 20 5, SrO, BaO, P 20 5 T i 0 2 and rare earth oxides are usually present i n s i g n i f i c a n t amounts they w i l l be discussed b r i e f l y to complete the chemical picture. The amount of Nb20^ i n carbonatites may range from a few hundred p.p.m. to several per cent, but as a rule the average Nb20^ value rarely exceeds 0.5 per cent. Figure 44 pre-sents nine average Nb20^ analyses from l o c a l i t i e s throughout the world and compares these to the average sixteen determinations from Seabrook Lake, Ontario. The average from the Seabrook area i s 1200 p.p.m. while the average from the other l o c a l i t i e s i s 2200 p.p.m. SrO is generally present and ranges from an unusual 150 p.p.m. at Alno to approximately 20,000 p.p.m. at an unspecified l o c a l i t y (Pecora, 1956). As with Nb2^5 the SrO values at Seabrook Lake are similar to other carbonatite l o c a l i t i e s (Figure 45). According to Higazy (1954) and Russell, et a l . , (1954) SrO is generally present i n amounts greater than BaO. Values for SrO reported by these authors range from 3,000 - 9,200 p.p.m. and for BaO 100 - 2,180 p.p.m. At Alno the trend Sr0>Ba0 is reversed. The work at Seabrook Lake agrees with the Sr0>Ba0 trend. Phosphate Figure 43. Triangular composition diagram illustrating the similarity of carbonatite composition. Plots are from the following localities: 1. Magnet Cove, Arkansas(Erickson and Blade,1963) 2. Fen District, Norway(Brogger,1921) 3.Sovite-Alno, Sweden(Von Eckermann,19lt8) Ij.Pyrite sovite-Alnd, Sweden(von Eckermann, 191|8) 5. Eiotite sovite,Alno, Sweden(Von Eckermann,I9I48) 6. Pyroxene sovite-Alno, Sweden(Von Eckermann, 191*8) 7. Biotite,pyroxene sovite-Alno, Sweden(Von Eckermann,19U8) 8. Pyroxene,biotite sovite-Alno. Sweden(Von Eckermann,19U8) 9. Chilwa I, Nya3aland(Smith,1953) 10. Chilwa II, Nyasaland(Smith,1953) 11. Mbeya I, Tanganyika(Fawley and James,1955) 12. Mbeya II, Tanganyika(Fawley and James,1955) 13. Mrima Hi l l I, Kenya(Coetzee and Edwards,1959) III.Mrima H i l l II, Kenya (Coetzee and Edwards,1959) 15.Magnet Heights, Transvaal(Strauss and Truter,195l) lo.Spitzkop I, Transvaal(Strauss and Truter,195l) 17.Spitzkop II, Transvaal(Strauss and Truter,195D l8.Spitzkop III, Transvaal(Strauss and Truter,195l) 19.Seabrook Lake, Ontario 87 i s present i n apatite or more rarely as rare earth phosphates. Its content, which ranges between 0.3 - 7.7 per cent, i s shown on Figure 46. The average of s l i g h t l y less than two per cent agrees with some of the approximations from Seabrook Lake. According to Pecora (1956) TiO^ ranges from traces to 4.5 per cent. At Seabrook Lake the values for TiO^ are less than 100 p.p.m. (Parsons, 1961). The occurrence of high concentrations of rare earth elements i s s t r i k i n g for both carbonatites and a l k a l i c rocks. Data for the rare earths are scarce, but values of several hundred p.p.m. are not uncommon. At Mountain Pass, C a l i f o r n i a (Olson, et a l . , 1954) the rare earth content i s exceptional and ranges from less than 1 per cent to 38 per cent. At Seabrook Lake the rare earths present are Ce and Y and occur i n amounts not greater than a few hundred p.p.m. The rare earths(La, Ce, Pr and Nd), which are present at Mountain Pass, show a d i s t i n c t chemical a f f i n i t y for P, F and CO^, but at Seabrook Lake and at other l o c a l i t i e s they are associated with pyrochlore. 88 10,000 8,000 U N o C M 6,000 a. a. 1,000 2,000 6 10 Figure 44 * Comparison of Nb2°$ i° carbonatites from the following localities: 1. Magnet Cove,Arkansas(Erickson and Blade,1963) 2. Magnet Cove,Arkansas(Erickson and Blade,1963) 3.Isoka District,Rhodesia(Reeves and Deans,195k) h. Chilwa, Nyasaland (Smith,1953) 5. Mriaia Hill,Kenya(Coetzee and Edwards, 1959) 6, Average of 9 presented 7.Seabrook Lake,Ontario(average of 16 analyses) 8. Firesand River,Ontario(Parsons,1961) 9. Mbeya,Tanganyika(Fawley and James,1955) 10.Fen District,Norway(Br6gger,1921) 89 8,000 o u to E 6,000 il,000 2,000 Figure 45. Comparison of SrO i n carbonatites from the following l o c a l i t i e s : 1. Alnb, Sweden(Von Eckennann,19U8) 2. Chilwa, Nyasaland(Smith,1953) 3.Seabrook Lake, Ontario(average of 16 analyses) Ii.Loolekop, Transvaal(Russel et als.,195U) ^.Magnet Cove, Arkansas (Erickson and Blade,1963) 6. Flresand River, Ontario(Parsons,196l) 7. Average of previous six 90 8.0 6.0 1 2 3 - 4 5 6 7 8 9 10 Figure 46 . Comparison of PgO-j in carbonatites from the following localities: 1. Chilwa II, Nyasaland(Smith,1953) 2. Alnd, Sweden(Von Eckermann,19U8) 3. Mrima H i l l , Kenya(Coetzee and Edwards,1959) L l o r o r o Hills, Uganda (Davies, 19SU) 5.Magnet Cove, Arkansas(Erickson and Blade,1963) 6.Seabrook Lake, Ontario( 2 determinations) 7. Mbeya, Tanganyika(Fawley and James,1955) 8. Chilwa I, Nyasaland(Smith,1953) 9. Aln6, Sweden(Von Eckermann,19U8) lO.Spitzkop, Transvaal(Strauss and Truter,195l) 91 III. Petrogenesis of the Seabrook Lake complex The Seabrook Lake complex i s believed to have been formed by d e s i l i c a t i o n and metasomatism of fractured and brecciated granite by a soda-iron-rich carbonatite magma. This mode of o r i g i n is postulated since a l l the rocks of the complex, with the exception of the carbonatite, have been derived by extensive soda, ir o n and carbonate metasomatism. The only available source for the metaso-matic f l u i d s appears to be the carbonatite. The existence of a carbonatite magma r i c h i n a l k a l i e s has been postulated by Von Eckermann (1948) at Alno, Sweden and esta-blished by Dawson (1964) at the Oldoinyo Lengai volcano i n Tangan-y i k a . Von Eckermann (1948) postulated the existence of a primary potassic carbonatite magma and derived a l l the s y e n i t i c fenites and nepheline-bearing rocks by i n s i t u metasomatism of granite gneiss. The a l k a l i e s necessary for metasomatism are derived from the primary carbonatite magma. Geologists were reluctant to accept Von Eckermann1s hypothesis because the existence of an a l k a l i c carbonatite magma was not proved. However, i n 1960 and 1961, soda-rich carbonatite was extruded i n the form of lava flows from the Oldoinyo Lengai volcano (Dawson, 1964) . These flows are thought to represent an uncontaminated primary carbonatite magma which has escaped d e s i l i c a t i o n of the metasomatic envelope, beneath the volcano, by e a r l i e r a l k a l i c carbonatite i n -92 trusions. At least two possible processes can be offered to explain the present absence, at Seabrook Lake, of s i g n i f i c a n t soda and iron i n the observed carbonatite. The f i r s t process involves d i f f e r -entiation of an o r i g i n a l homogeneous a l k a l i c carbonatite magma into a lower c a l c i t i c zone and an upper zone r i c h i n soda and ir o n . Fractionation of the magma may have taken place by coordination of soda and iron to upward moving v o l a t i l e s as suggested by Kennedy (1955). As a resul t of later erosion, to the c a l c i t e zone, the only evidence of the soda-iron carbonatite magma i s i t s metasomatic effects on the country rock. The second process involves depletion of a carbonatite magma i n soda and iron by outward d i f f u s i o n to form fenite and i j o l i t e . Dawson (1964) postulated that the preferential a c t i v i t y of soda over lime and magnesia may be due to i t s low i o n i -zational p o t e n t i a l . Diffusion from the observed carbonatite i s not considered probable as the carbonatite i s late and does not possess soda-iron-rich halos . The close s p a t i a l relationship between the exposed carbonatite and the extensive carbonate-iron metasomatism, of the mafic breccia zone, suggests that the observed c a l c i t e - r i c h carbonatite probably was emplaced along the same channelways as the highly reactive soda-iron carbonatite. A carbonatite magma r i c h i n a l k a l i e s and iron may form by di f f e r e n t i a t i o n of a carbonate-rich a l k a l i c magma, or by magmatic 93 assimilation of sedimentary limestone. Formation of carbonatite by d i f f e r e n t i a t i o n of a carbonate-rich a l k a l i c magma has been proposed by Erickson and Blade (1963) at Magnet Cove, Arkansas; by King (1949) at Napak, Uganda; and by Brogger (1921) at the Fen D i s t r i c t i n Norway. This hypothesis i s supported by the experimental studies of Watkinson and Wyllie (1965) on the NaALSiO^ - CaCC>3 - 25% H^ O system. These authors did not study the d i s t r i b u t i o n of soda i n the residual carbonatite l i q u i d . Assimilation of sedimentary limestone i n a magma has been suggested to explain the close association of carbonate and undersaturated a l k a l i c rocks. This process i s one of d e s i l i c a t i o n and can be i l l u s t r a t e d as follows: 1 imes tone ma gma CaC03 + S i 0 2 = CaSi0 3 + C0 2 c a l c i t e s i l i c a wollastonite carbon dioxide magma limestone NaAISi 0 Q + 2CaCC- = 2CaSi0 o + NaAlSiO + 2C0o 3 o 3 3 4 2 a l b i t e c a l c i t e wollastonite nepheline carbon dioxide The early formation and removal of wollastonite, perhaps to the bottom of the magma chamber, has been postulated to form an undersaturated magma. Additional assimilation, with later differen-t i a t i o n , i s believed to form carbonatite. Watkinson and Wyllie (1966) 94 rejected this hypothesis as phase relations i n the NaAISi 0 " CaCO 3 8 3 - Ca (0H)2 - system suggested that about 20 weight per cent lime-stone must be assimilated before a feldspathic melt can be s u f f i c i e n t l y d e s i l i c a t e d to produce nepheline. Even i f enough superheat was a v a i l -able to permit solution of so much limestone, assimilation causes c r y s t a l l i z a t i o n before d e s i l i c a t i o n proceeds very far, and the for-mation of feldspathoids i n quantity results only from subsolidus reactions (Watkinson and Wyllie, 1966). The o r i g i n of the proposed soda-iron-r i c h carbonatite at Seabrook Lake, is not known since neither a l k a l i c magma nor sedimentary limestone i s present. The emplacement of the exposed carbonatite was by i n j e c t i o n into fractures and shears that were opened and cleared by previous injections of carbonatite magma. This suggestion i s supported by the highly fractured and brecciated nature of the rocks and by the well developed f o l i a t i o n and lack of replacement textures i n the car-bonatite. The development of fractured and brecciated zones occurred during many episodes by increase of v o l a t i l e pressure, at the top of the magma, as the carbonatite was emplaced into higher crustal l e v e l s . Whether the zones formed by explosive a c t i v i t y or a gradual increase and decrease of pressure i s not known. The presence of car-bonatite fragments i n the mafic breccia zone suggests that emplace-ment of carbonatite occurred during as w e l l as after the formation of the zone. The temperature of the country rocks, into which the 95 carbonatites were emplaced, must have been moderately high as c h i l l e d margins are not found i n the carbonatite. The temperature of c r y s t a l l i z a t i o n of the presently exposed carbonatite can be experimentally shown to range from approximately o o 900 C. to s l i g h t l y less than 600 C. This range i s incomplete as many minerals are known to have formed both above and below these l i m i t s . The upper l i m i t of the temperature range i s the maximum s t a b i l i t y of magnesioriebeckite. The lower l i m i t i s established by the exsolution texture i n magnetite. The temperature range indicated i s consistent with both mineral paragenesis and experimental work (Wyllie and Tuttle, 1960). The age of the Seabrook Lake complex i s believed to be Proterozoic. This i s suggested by the K-Ar date of 1090 m i l l i o n years of Nemegosenda, Ontario (Figure 3). Origin of the Seabrook Lake Complex The Seabrook Lake complex was formed by d e s i l i c a t i o n and metasomatism of fractured and brecciated granite by a soda-iron-rich carbonatite magma of unknown o r i g i n . Fractionation of the magma into an upper soda-iron-zone and a lower c a l c i t i c zone occurred during the i n i t i a l stages of magmatic development. Upward emplace-ment of the diff e r e n t i a t e d magma fractured the granite, which 96 resulted i n d e s i l i c a t i o n and soda-iron metasomatism to form f e n i t i z e d granite and i n zones of intense metasomatism, i j o l i t e . As the magma advanced into the f e n i t i z e d zone extensive refracturing and brec-c i a t i o n occurred. The mafic breccia was then formed by extensive carbonate-iron metasomatism of the highly fractured and brecciated f e n i t i z e d granite. Injection of the lower c a l c i t e - r i c h carbonatite followed the emplacement and metasomatism of the upper soda-iron carbonatite magma. 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Rend. Acad. S c i . Paris, volT 228, p. 1232. 100 Powell, J . L., 1962, Isotopic composition of strontium i n carbon-a t i t e s , Nature 196, p. 1085 - 1086. 87 Powell, J . L., 1965, Low abundance of Sr i n Ontario carbonatites, Am. Min., v o l . 50, p. 1075 - 1079. Ramdohr, P., 1953, Ulvospinel and i t s significance i n titaniferous iron ores: Ec. Geol., v o l . 48, no. 8, p. 677 - 688. Rankama, K., and Sahama, Th. G., 1949, Geochemistry: Univ. Chicago Press, 912 p. Reeves, W. H., and Deans, T., 1954, An occurrence of carbonatite i n the Isoka d i s t r i c t of N. Rhodesia; Col. Geol. and Min. Res., v o l . 4, p. 271 - 281. Rowe, R. B., 1958, Niobium deposits of Canada, Geol. Sur. of Canada Ec. Geol. Ser. No. 18. Roy, N. N., 1965, The mineralogy of the potassium-barium feldspar series, Min. Mag. V o l . 35, p. 508 - 518. Russell, H. D., Hiemstra, S. A., and Groeneveld, D., 1954, The min-eralogy and petrology of the carbonatite at Loolekop, Eastern Transvall: Geol. Soc. South A f r i c a Trans, and P r o c , v o l . 57, p. 197 - 208. Sandell, E. B., 1959, Colorimetric Determination of Traces of Metals, 3rd ed., Interscience Publishers, Inc., New York. Sellmer, H. W., 1966, Geology and petrogenesis of the Serb Creek i n -trusive complex near Smithers, B. C, M. Sc. thesis, Uni-v e r s i t y of B.C. Shand, S. J . , 1931, The granite-syenite-limestone complex of Palabora, Eastern Transvaal and the associated apatite deposits: Geol, Soc. South A f r i c a Trans, and P r o c , v o l . 34, p. 81 - 155. Smith, W. Campbell, 1953, Carbonatites of the Chilwa series of Sou-thern Nyasaland: B r i t i s h Museum (Nat. Hist.) B u l l . , Min-e r a l . , v o l . 1, p. 77 - 119. Smith, W. Campbell, 1956, A review of some problems of African car-bonatites: Geol. S o c London Quart. Jour., v o l . 112, p. 189 - 219. Smith, J . S., 1960, Phase diagrams for a l k a l i feldspars, 19 - 22, pt. 21, p. 185 - 192. 101 Stevenson, J.S., 1954, Determination of Nb i n ores by X-ray f l u o r -escence, Am. Min., v o l . 39, p. 436 - 443. Strauss, C.A., and Truter, F.C., 1951A, The a l k a l i c complex at Spitzkop, Sekukuniland: Geol. Soc. South A f r i c a Trans., v o l . 53, p. 81 - 125. 195IB, Post-Bushveld ultrabasic, a l k a l i c and carbonatitic eruptives at Magnet Heights, Sekukuniland, Eastern Transvaal: Geol. Soc. South A f r i c a Trans., v o l . 53, p. 169 - 190. Swift, W.H., 1952, The geology of Chishanya, Buhera d i s t r i c t , Southern Rhodesia: Edinburgh Geol. Soc. Trans., v o l . 15, p. 346 - 359. T i l l e y , C.E., 1958, Problems of a l k a l i c rock genesis, Geol. Soc. Q. Jour., v o l . 113, p. 323 - 360. Turner, J.F., and Verhoogen, J., 1960, Igneous and metamorphic petro-logy: 2 ed., New York, McGraw-Hill Book Co., Inc., 694 p. 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Wyllie, P.J., and Tuttle, P.F., 1959, Synthetic carbonatite magma: Nature, v o l . 183, p. 770. 1960, Experimental v e r i f i c a t i o n for the magmatic o r i g i n of carbonatites: 21st Internat. Geol. Congress Norden Rept., pt. B, p. 310 - 318. 102 A P P E N D I X Methods of Quantitative and Qualitative Analysis This particular series of studies was undertaken to deter-mine the rather unusual geochemistry and mineralogy of the Seabrook Lake carbonatites. The methods used are as follows: ( i ) X-ray fluorescence (Nb, Sr, Fe, Ce, Y and Ba), ( i i ) Emission and absorption flame photometry (Na, K, Mg), and ( i i i ) Colorimetric geochemistry (Cu, Mn). Also included i n this section i s an X-ray study of microcline compositions. ( i ) X-ray fluorescence X-ray fluorescence (spectroscopy) was used to quanti-t a t i v e l y determine ^£0,., SrO and t o t a l i r o n . During preliminary scanning Ce, Y, Ba and Mn were q u a l i t a t i v e l y detected. The procedures used i n this study are si m i l a r to those used by Stevenson (1954) and Claisse (1961). Equipment and operation constants The unit employed i n this study i s the P h i l l i p s , type 12096, X-ray spectrograph. The operating constants for both q u a l i t a t i v e o and quantitative analysis are: 50Kv, 20 Ma, 1 /minute scan time, H.V. 900, c t s . 1600, scale factor 8 and time constant 4. 103 Sample Preparation Rocks were f i r s t broken into small chips with a hammer and then ground by hand i n a large corundum mortar. After the mat-e r i a l was ground to smaller than 70 mesh the samples were quartered and further ground i n a mechanical pulverizer. In order to obtain the minimum grinding time necessary for consistent reproducible re-sults a single sample was divided into four parts and ground re-spectively for 10, 20, 30 and 40 minutes i n the mechanical pulverizer. The A I (intensity above background) of NbK^ ^, SrK ^ ^ and MnK ^ ^ were then determined on the X-ray spectrograph. The results are plot-ted on Figure 47 and show that the carbonatite sample must be ground for at least t h i r t y minutes i f the niobium values are to be consis-tent. With this grinding time ninety per cent of the sample i s f i n e r than 200 mesh. The consistency of A I for Sr and Mn i s indicative of a s o l i d solution relationship within c a l c i t e . For the f i n a l determination 1.5 gms. of the sample was mechanically mixed with 0.5 gm. of the appropriate internal standard. The material was mixed for ten minutes to ensure homogeneity of the sample with the internal standard. Preparation and choice of the standards Accurate X-ray fluorescent determinations depend on making the chemical composition of the standards as close as possible to the material that i s being analysed. From thin section, as well 104 as preliminary X-ray fluorescent data, chemically pure Nb^O^, SrCO^ and -?e2^3 w e r e chosen for the preparation of the standard working curves. J u s t i f i c a t i o n for this choice i s based on the presence of Nb20g i n pyrochlore, Sr i n s o l i d solution i n c a l c i t e and Fe i n hema-t i t e and magnetite. Mixing of the standards with the chemically pure c a l c i t e matrix was done with the mechanical pulverizer. After completion of this step each standard was checked for consistency prior to mixing with the internal standard. Choice of the internal standards The choice of an internal standard f i r s t involves the determination of s p e c i f i c elements i n the carbonatite, and secondly a comparison of absorption edges of both the reference element and the element to be determined. Internal standards for niobium, strontium and iron are pure MoO^, RbCO^ and CoO. The mixing procedure for the internal standards with the c a l c i t e matrix was the same as for the standards. The internal standards were a l l checked for reproducible results and were found to be satisfactory (Figure 48) . Determination of working curves Once the standard for each element was prepared i t was diluted to concentrations appropriate for the preparation of a standard working curve. Following this step each standard concentration was thoroughly homogenized with the appropriate internal standard. The r a t i o 105 O •o- -o- -OMnK. 10" Grinding time(minutes) F igure 47. Optinium gr ind ing curves fo r Nb,Sr,and Mn. T h i r t y minutes es tab l i shed as the mininium gr ind ing time necessary fo r cons is tent reproduc ib le r e s u l t s . 106 of A I were then determined on the X-ray spectograph for each s p e c i f i c concentration and plotted on the standard working curve. In preparing these standard working curves a l l A I values were double checked to eliminate compaction problems. This source of error i s n e g l i g i b l e . The mixing and checking procedures for the unknown samples followed the same r i g i d controls as i n the preparation of the standards. Once the r a t i o s , A l NbK^ l A I S r K ^ A IFeK ^  i and A I MoK^ x A I Rbk^ A l CoK^ for known element amounts, were determined the raw data were plotted and subjected to a regression analysis. Once this was completed a l i n e of best f i t i s established from which analyses of unknown samples can be made. Figures 49, 50 and 51 are working curves of Nb, Sr and Fe respectively. The results of the X-ray fluorescent analysis are tabulated i n Table 17. Precision calculations were made to determine the approximate amount of error i n the X-ray fluorescent determinations. A co e f f i c i e n t of va r i a t i o n was found to range from 0.7 - 2.2 per cent (Calder, 1960) . This minimum error i s attributed to d i f f i c u l t i e s i n compaction, mixing and instrumentation. The actual error i s d i f f i c u l t to determine but i s approximately + 5 - 1 0 per cent. This value i s i n keeping with the check results determined by X-ray Assay Laboratories Ltd. of Toronto for the niobium values from the 107 60n 50-—--o- - o -O MoK* < ho-30-20 -O RbK« 10 o-— - o -o- -O CoK, *1 I L_ L . I I . 1 2 3 U 5 D i f f e r e n t determinations of same sample Figure 48. Check of the homogeneity of the Co,Rb and Mo i n t e r n a l standards with d i f f e r e n t runs and compactions. 108 Seabrook Lake carbonatite. ( i i ) Emission and absorption flame photometry Emission flame photometry was used to determine Na and K, while absorption atomic spectroscopy was employed to determine Mg. One gram of powdered carbonatite was accurately weighed and dissolved using the following procedure: ( 1 ) H C 1 concentrated 1 0 ml. - evaporate to dryness ( 2 ) H C 1 concentrated 5 ml. - i f no effervescence evap-orate to dryness and proceed with step number 3. I f effervescence add more H C 1 u n t i l no effervescence noted. (3) H F (49%,) 15 ml. + 2 ml. cone. H 2 S 0 4 . evaporate slowly to dryness. (4) H F (49%) 1 0 ml. evaporate slowly to dryness. (5) H N O 3 (concentrated) 1 2 ml. evaporate slowly to dryness. ( 6 ) Dissolve white powder i n 2 ml. perchloric acid and make to 50 ml. with d i s t i l l e d water. After completion of the above procedure brookite was the only mineral that was not dissolved. Na, K and Mg standards were kindly supplied by the Department of S o i l Sciences. Figures 52, 53 and 54 are the standard working curves for Na, K and Mg respectively. 10.0 U CO M M <] <1 8.0 6.0 _ lt.0 2.0 1,000 2,000 3>000 U,000 5,000 p. p. m. Sr 6,000 7,000 8,000 Figure 50. Working curve for strontium. 12 .0 IS Tf <D O fe O <l < 10.0 8.0 6.0 h.O 2.0 1.0 2.0 3.0 1.0 $ Percent. Iron Figure 51 . Working curve f o r i r o n . 112 TABLE 17 Summary of Qualitative and Quantitative Fluorescent Data from the Carbonatites at Seabrook Lake. Data i n per cent Sample Number SrO Nb 20 5 Fe Ba Mn Ce Y 19 0.17 0.01 1.8 d d n.d. d 43 0.18 0.17 3.4 d d d d 44 0.20 0.17 2.7 d d p.tr. d 46 0.18 0.05 1.9 d. d n.d. d 50 0.27 0.11 2.7 d d d d 51 0.30 0.16 3.1 d d d d 54 0.30 0.06 2.0 d d p.tr. d 55 0.18 0.14 2.6 d d d d 60 0.25 n.d. 1.0 d d p.tr. d 66 0.18 0.15 3.0 d d n.d. d 78 0.27 0.06 1.8 d d d d 82 0.30 0.01 2.3 d d n.d. d 86 0.25 0.42 3.6 d d n.d. d 94 0.67 0.27 4.8 d d d d 102 0.24 0.06 1.4 d d d d 104 0.27 0.06 1.8 d d n.d. d Average 0.26 0.12 2.4 Detected (d) Not Detected (n.d.) Possible Traces (p.tr.) 113 Each value on the graph was run twice to check r e p r o d u c i b i l i t y . The results are l i s t e d i n the oxide form on Table 18. ( i i i ) Colorimetric geochemistry The biquinoline method (Ward, et a l . , 1963) was used for the Cu determinations and potassium periodate method (Sandell, 1959) was employed for the Mn analysis. Table 19 gives the Cu '(p.p.m.) and MnO (7o) determinations of seven carbonatite l o c a l i t i e s from Seabrook Lake. X-ray d i f f r a c t i o n study of microcline composition The purpose of this study was to determine to Or (ortho-clase molecule) composition of the microcline i n the granite, f e n i -t i z e d granite and carbonatite. X-ray powder photographs as w e l l as thin section examination confirmed the t r i c l i n i c nature of the three K-feldspars. Compositions of the microclines were determined by X-ray methods involving measurement of the angle between a known standard peak and the peak of the 201 r e f l e c t i o n of a l k a l i feldspar. The (101) r e f l e c t i o n of chemically pure KBrO^ was used as the standard or reference peak. Since the position of the r e f l e c t i o n of a l k a l i feldspar varies l i n e a r l y with the weight per cent of the orthoclase 114 p. p. m. Na Figure 52. Working curve for sodium Intensity (arbitrary scale) 8 8 S o 911 117 TABLE 18 Results of Emission and Absorption Flame Photometry of Carbonatite Samples from Seabrook Lake . Sample Number Na20 K 20 MgO M-44 0.58 0.38 2.12 M-54 0.19 0.10 1.63 M-82 0.41 0.21 1.96 M-86 0.44 0.55 2.32 M-94 0.15 0.27 2.72 M-104 0.25 0.03 1.74 TABLE 19 Cu and MnO Determinations of Seven Carbonatites from Seabrook Lake Sample Number Cu MnO M-44 2.5 0.32 M-54 5 0.57 M-66 5 0.32 M-82 5 0.48 M-86 9 0.19 M-94 13 0.19 M-104 15 0.32 118 molecule the difference - A 20 - or 20 (201)0r - 29 (101) K Br0 3 i s d i r e c t l y proportional to the weight percentage orthoclase. The a l k a l i feldspar compositions i n this study were determined with Parsons' (1965) plutonic a l k a l i feldspar curve. Table 6 gives the A 2 0 values of each microcline and the equivalent weight per cent orthoclase. The preparation of samples, subsequent heat-treatment and A 20 determinations that were used i n this study are described i n d e t a i l by Sellmer (1966) . The results (Table 20) clea r l y show that the microcline within the carbonatite complex i s more sodium-rich than the micro-c l i n e of the granite. The anomalous result (M-25A) has been double checked and found to be correct. Its ambiguity does not detract from the general conclusion regarding sodium metasomatism. The reason for the ambiguity i s not clear, but may be related to i r r e g u l a r i t i e s i n Parsons' determination curve. 119 TABLE 20 Composition of Microcline Before and After Heating with Corres-ponding Approximate Weight Per Cent Orthoclase Rock type A 20 (201) K-feldspar Weight % Or heated unheated heated unheated Granite (M-25A) Fenitized granite Carbonatite 0.65 0.84 0.77 0.65 0.85 0.75 Or 108 Or 108 Or 89 Or 90 Or 98 Or 97 

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