Canadian Mineralogist Vol. 15, pp. 117-120 (1977)

DAWSONITE.FLUORITE RELATIONSHIPS AT -AREA LOCALITIES

JOHN S. STEVENSON AND LOUISE S. STEVENSON Department of Geological Sciences and Redpath , McGill University, Montreal, Que.

ABSTRACT quarry, Mount St. Hilaire, (Fig. 1). In a recent publication Clark (1972, p. 128- Dawsonite and fluorite, crystallized together, have 129) stated: "Dawsonite, first described from been collected recently from three localities in the Montreal area, where they were found in particular Montreal, may be the result of hydrothermal dykes or sills which were only briefly exposed by changes more nearly akin to weathering than to quarrying and excavation. The intergrowth of the metamorphism." We believed a study of this dawsonite and fluorite crystals and their frequent new material might contribute further evidence association with vein quartz gave evidence of hy- in support of a truly hydrothermal origin for drothermal origin. The fine-needle dawsonite in the the Montreal dawsonite. assemblages is similar to dawsonite which occurs in fluid inclusions in gold-quartz veins. Field relation- DA WSONITE-FLUORITE LOCALITIES ships support the conclusion that the Montreal-area dawsonite and fluorite are low-temperature hydro- The Montreal Trans- Highway daw- thermal minerals related in origin to the alkalic sonite-fluorite was found in April, 1973, near rocks of the area. the University Street entrance. This mineraliza- tion appeared to occur in a continuation of the SOMMAIRE feldspathic dyke associated with nepheline sy- On a recolte recemment de la dawsonite et de la enite (feldspathoidal monzonite of Gelinas 1972) fluorite, cristaIlisees ensemble, dans trois localites of the McGill University type locality (Steven- de la region de Montreal. Ces mineraux ont ete trou- son & Stevenson 1965), although dawsonite and ves dans des dykes et des sills exposes temporaire- fluorite crystallized together had not been found ment, au cours des travaux, dans des carrieres et at the type locality. Construction work has made des excavations. L'intercroissance de cristaux de further collecting at the Trans-Canada site im- dawsonite et de fluorite et leur frequente associa- possible. tion avec du quartz filonien indiquent une origine In 1974-75 several specimens of dawsonite hydrothermale. La dawsonite aciculaire de ces as- and fluorite were collected from sills in the sociations ressemble It la dawsonite que I'on trouve dans les inclusions fluides des filons de quartz auri- Francon quarry, St-Michel, Montreal. This fere. Les observations faites sur Ie terrain etayent la quarry has gained prominence as the type locality conclusion que la dawsonite et la fluorite de la re- for weloganite and as a source of other rare gion de Montreal sont des mineraux formes a minerals, including dresserite (Sabina et al. 1968; basse temperature, par voie hydrothermale, dont I'origine est liee a celie des roches aIcalines de cette region.

INTRODUCTION Although both dawsonite and fluorite have i been collected separately from several localities in the Montreal area, occurrences are few where these two minerals have crystallized together and where their petrogenetic relationships may be studied. Our interest in this relationship first began with the collection of excellent specimens of well-crystallized dawsonite and fluorite from rock near University Street in Montreal which was being excavated for an underground portion of the Trans-Canada Highway. Later, good spe- cimens were obtained from the Francon Quarry, FIG. 1. Location of dawsonite-fluorite occurrences in St-Michel, Montreal, and from the De-Mix the Montreal area. 117 1r8 THE CANADIAN MINERALOGIST

Jambor et al. 1976; Sabina 1976), although the That the dawsonite and fluorite were crvstal- specimenswe studied did not contain these rare lized together is suggested by the intergrowths minerals. illustrated in Figure 4, where delicate needles Dawsonite-fluorite material was collected of dawsonite bridge space between fluorite crys- from the De-Mix quarry, Mount St. Hilaire, 48 km east of Montreal, n L974 and 1975. These specimenswere assosiatedwith syenite breccia (Chao et al. 1967; Perrault & Mandarino 1972). At all tlree localities, the dawsonite-fluorite is concentrated in relatively small areas of partic- ular sills or dykes, as Sabina (1976) has noted at the Franson quarry.

PETRocENETtcStuptns In the syenite breccia at Mount St. Hilaire, Fro. 4. Scanningelectron photomicrographs dawsonite-fluorite of daw- lines the walls of vugs which sonite and fluorite: (A) several needlesof daw- range from about 1 cm to 2 m in diameter. Vugs sodte bridging spacebetween crystals of fluorite. at other localities are generally smaller, L cm to University Street. (B) crystals of dawsonite, 2 cm in dia'meter, and seem to represent filled sjlow1ns{100} and {110}, joining crystalsof or partly filled vesicles or amygdules, with flow- Iluonte. t,ranconquarry. age of groundmassplagioclase of the dyke rock around the vesicles (Fig. 2). Most dawsonite tals. These photographs occurs a$ closely packed blades, some as large show interesting simi- larities to the SEM photographs as 1 cm long by 1 mm wide, ranging down to of the Chihua- hua geode minerals studied 2,4 mm long and O.04 mm wide, frequently in- by Finkelman ef a/. (L972), in timately intergrown with fluorite in cubes or which delicate euhedral and subhedral crystals were interpreted precipitated parts of cubes up to 2 mm on the edge (Fig. 3). as having simultaneously. Although dawsonite usually occurs as closely Both the dawsonite packed blades, some of it occurs as needles, and fluorite are inter- grown in places associatedwith the blades, up to 0.8 mm long with minor amounts of coarse calcite crystals and coarse-grained and 0.09 ,mm wide; the sma.llest needles ranse hypidiomor- phic quartz characteristic quartz. down to a size so small that the individual crv-s- of vein Another common occurrence tals are barely detectable with the electron mi- of the assem- croscope. blage dawsonite-fluorite-quartz-calcite is as gash veins from O.025 mm to 2 mm wide within the grey feldspathic dykes (Frg. 5). Much of tle daw-

Fro. 2. (left) Completely enclosed smooth-walled vug containing intergrown dawsonite (D), fluo- rite (F), Suartz (Q) and calcite (C). Francou FIc. 5. Gash veins (Vn) of dawsonite and fluorite. quarry. Crossed polars. Francon quarry: (A) outline and cross-cutting nature of the veins. Plane-polarized Ught. (B) Flc. 3. (right) Coarsebladed dawsonite(D) inter- Enlargement of portion of vein in A, lower rlght grown with fluorite (F) in open vug. Francon showing intergrown dawsonite @) and quartz quarry. Clossedpolars. (Q). Crossedpolars. DAWSOMTE-FLUORITE RBLATIONSHIPS tt9 sonite exposed in broken dump material repre- feldspar phenocrysts by fluorite and quartz sents the coatings of these vein walls. These (Lindgren 1900, Fig. 10). The Cripple Creek oc- veinlets have all the characteristics of filled, cunences are similu to tle replacementsin the sha4r-walled tension fractures. At Francon and Montreal-area dawsonite-fluorite. At Cripple Trans{anada" they cut sharply across the feld- Creek the fluorite is considered to be a low- spar phenocrysts and flow-aligFed groundmass temperature hydrothermal minssal of magma- lath feldspar of tle enclosing dyke rock. They tic origin. Likewise, the Montreal'area dawson- frequently contain very jagged and irregular ite, by virtue of its close association with fluorite, fragments of the wall rock. In places the frag- is viewed as a low-temperature hydrothbrmal ments are almost sufficiently numerous to con- mineral related in origin to the alkalic rocks of stitute microbreccia, and in others the veinlets the area. are close and form a micro-stockwork pattern. For dawsonite to form, it is necessary that Where breccia fragments are absent, both the the requisite chemical constituents be present dawsonite and quartz are normal to the vein and that the rigbt pH and pressure be main- walls, approximating a comb texture, and the tained, but these requirements can be satisfied fluorite occurs interstitial to both and also fills by a variety of geological environments. For the medial parts of the veinlets. example, in the Montreal and similar occur- Because these dawsonite-fluorite veinlets are rences, an alkaline environment relatively rich confined to the dykes and do not extend into in soda and alumina and deficient in the strong the wall rock, and becauseof their habit and tex- acidic radicals such as Cl and SOa would be tural features, they are thought to represent natural in an area of nepheline syenite. Further filled contraction fractures formed shortly after alumina may also be contributed from the de- the consolidation of the dykes. Their origin is struction of the feldspars. therefore similar to that of ladder veins in dykes On the other hand, a completely different but (Lindgren 1933). proper environment for the formation of daw- It is apparent from thin-section study that the sonite is in the saline soils of Olduvai Gorge, filling of these veinlets occurred at the same Tanzania (Hay 1963), in the marine strata of time as the filling of the vugs and amygdules. the Berry Formation of the Sydney Basin, New The veinlets extend into vugs, join vugs to amyg- South Wales (Goldbery & I-oughnan 1970), and dules, and connect amygdules almost impercep- also in the oil shalesof the Green River Forma- tibly with hair-like stringers of dawsonite and tion, Colorado (Smith & Milton 1966; Milton occasionally fluorite. 1976). Another type of occurrence has been de- Thus the formation of the dawsonite-fluorite scribed by Aikawa et al. (L972) from the Izumi is seen as a hydrothermal mineralization in vug Group in southwest Japan, where rnudstones fillings and in contraction fractures in the rocks, with marine fossils are associatedwith layers of accompanied by widespread metasomatic re- tuff and with mineral springs abnormally rich placement within the rock, all very shortly after in NaHCOs. consolidation of the enclosing igneous bodies. Dawsonite in fluid inclusions in gold-quartz The hydrothermal solutions were apparently veins of the Alleghany district, California (Cove- related to the alkalic igneous activity of the ney & Kelly 1971) is similar in habit to the -Mount St. Hilaire syenites. Montreal-area fine-needle dawsonite. Coveney & Kelly suggest that the general rarity of daw- sonite as a vein mineral may be due to its high on DewsoNtrE,eNp FruontrE, Onlcrx solubility under normal hydrothermal conditions, Fluorite has always been thought of as a but that the formation of dawsonite in low- typical hydrothermal mineral, and the consti- temperature hydrothermal deposits would be tuent fluorine generally accepted as a magmatic favored by loss of dissolved CO, due to fluid product (Lindgren L933; Deer et al. 1966). cooling or boiling, sharp reduction in fluid acti- Buddington (1933, p. 380) has noted: "Fluorine vity due to wall-rock reaction, and high dis- is qonsidered . . a characteristic and perhaps solved sodium and aluminum concentration. the most distinctive volatile element of alkalic Ceilainly tle special conditions mentioned magmas." The association of fluorine with al- above are met in tle Montreal occurrences of kaline rocks, such as alkaline syenite and ne- dawsonite and fluorite. Extensive wall-rock pheline syenite, is indeed widespread. Perhaps alteration of rocks rich in alkali and alumina re- the most striking association is the extensive duced the acidity and increased the alkali and occurrence of fluorite with the alkaline intru- alrrmina content of the hydrothermal fluids. The sions of Cripple Creek, Colorado, a famous gold near-surface nature of the occurrences, as at- camp. There we have extensive replacements of tested by the breccia and gash veins, provided 120 THE CANADIAN MINERALOGIST opportunity for rapid loss of pressure, loss of GELINAS, L. (1972): of Mount Royal. II/t. CO., and consequent precipitation of the daw- Geol. Cong., 24th Session, Guidebook B-12, 1-39. sonite. GOLDBERY, R. & LOUGHNAN, F. C. (1970): Oaw- sonite and nordstrandite in the Permian Berry ACKNOWLEDGEMENTS Formation of the Sydney Basin, New South Wales. Amer. Mineral. 55, 477-490. We thank Dr. Peter Tarassoff for donating HAY, R. L. (1963): Zeolitic weathering in Olduvai specimens from the Trans-Canada Highway Gorge, Tanganyika. Geol. Soc. Amer. Bill!. 7.1, University Street excavation, and Dr. Donald 1281-1286. Doell for the specimens he collected from the JAMBOR, 1. L., PLANT, A. G. & STEACY,H. R. (1976): Francon quarry. T. Ahmedali and A. Graham A dawsonite-bearing silicocarbonatite sill from kindly assisted with the SEM photographs and Montreal Island, Que. Ceol. SlIrv. Can. Pap. R. Yates with the drafting of Figure I. 76-1B, 3S7-362. LINDGREN, W. (1900): Metasomatic processes in REFERENCES fissure-veins. Trall.l'. Amer. 11/.51.Mil/il/g EI/g. ;W, AIKAWA. N., YOSHIDA, M. & ICHIKAWA, K. (1972): Sn-692. Discovery of dawsonite and alumohydrocalcitc - (1933): Mil/eral DeposiH', 3rd ed. McGraw- from the Cretaceous Izumi group in Osaka pre- Hill. fecture, southwest Japan. J. Japal/ Assuc. /'vlil/el'a/. Petrology Ecol/. Ceol. 67, 370-3gS. MILTON, C. (1976): Mineralogy of the Green River Formation of Colorado, Utah and Wyoming. BUDDINGTOK,A. F. (1933): Kinds of igneous rocks U.S.A. 11/1.Ceol. COI/g., 25th Sessiol/, 2, 581-5g2. and mineralization. In Ore Deposits of Western States. Amer. I1l.\"t.Mil/ing Met. Eng., 3S0-3gS. PERRAULT, G. & MANDARINO, 1. A. (1972): The lvlonteregian Hills: Mineralogy of Mount SI. Hi- CtlAO, G. Y., HARRIS, O. C., HOUNSLOW, A. W., laire. II/t. Ceol. COI/g., 24th Sessiol/, Guidebook MANDARINO,J .A. & PERRAULT, G. (1967): M in- B-15, 1-12. erals from the nepheline syenite, Mont 51. H i- laire, Que. Cal/. Mineral. 9, 109-123. SABINA, A. P. (1976): The Fracon (sic) quarry, a CLARK, T. H. (1972): Montreal area. Mil/isthe Ri- mineral locality. Ceol. SlIrv. Cal/. Pap. 76-1B, chesses Nat. Que. Ceol. Rep. 152. IS-19. COVENEY. R. M. & KELLY, W. C. (197\): Oaw- , JAMBOR, J. L. & PLANT, A. G. (1968): sonite as a daughter mineral in hydrothermal Weloganite, a new strontium zirconium carbonate fluid inclusions. ContI'. M il/('/"al. Petrology ;t~, from Montreal Island, Canada. Cal/. Mil/eral. 9, 334-342. 46R-477. DEER, W. A., HOWIE, R. A. & ZUSSMAN, J. (1966): SMITH, J. W. & MILTON, C. (1966): Dawsonite in AI/ II/troduction to the Rock FOl'mil/g /'vlil/erals. the Green River Formation, Colorado. Ecoll. Longman's, London. Ceol. 61, 1029-1042. FINKLEMAN. R. B., MATZO, J. 1., WOO, C C, STEVENSON, J. S. & STEVENSON, L. S. (1965): The WHITE, J. S., JR. & BROWN, W. R. (1972): A petrology of dawsonite at the type locality, Mont- scanning electron microscopy study of minerals real. Clll/. Mil/eraf. 8, 249-252. in geodes from Chihuahua, Mexico. Mil/el'al. !l4al/llscript receil'ed October J 976, emel/ded Dec- Record 3, 20S-212. ember /976.