Coldwell

Coldwell is an approximately circular complex 25 km in diameter of over 1500 km2, the southern third of which lies beneath Lake Superior. Coldwell is one of the largest intrusive alkaline complexes in North America. It is emplaced into metavolcanic and metasedimentary rocks of the Schreiber - White River greenstone belt of the Archaean Superior Province. The complex appears to have been intruded about three separate centres and to have complex contacts, part faulted and part showing the effects of assimilation and rheomorphism, with the development of a thermal aureole up to 2 km wide that reaches pyroxene hornfels grade. The overall and internal structures are discussed in some detail by Currie (1980, pp. 27-9). All the igneous rocks of the complex show some degree of layering, which is generally concentric to the margin, and varies from nearly vertical at the margins to inward dips of 40-50 . The earliest rocks, about Centre 1, are gabbros, which form the border rocks in the north and east, and ferroaugite syenite. The gabbros are texturally very variable, often layered, and include numerous xenoliths of country rock. They consist of labradorite, clinopyroxene, olivine (Fo43-67), biotite, magnetite and ilmenomagnetite, sulphides and apatite. Brown amphibole occurs in some varieties and orthopyroxene is reported, but there is debate as to its primary nature. The ferroaugite syenites are the most extensive rock types in the complex, and one shore section has been described in detail by Mitchell and Platt (1978). In earlier literature they were referred to as 'larkivites' because the feldspars exhibit a schiller. They cut the gabbros and are commonly layered, with cross bedding and trough banding sometmes evident. Mafic-rich layers include olivine, pyroxene and Fe-Ti oxides, but most rocks contain 70% alkali feldspar (Or35-50). The pyroxenes range from diopsidic hedenbergite to aegirine-augite. Amphibole rims pyroxene and olivine and forms intercumulus crystals; varieties of hornblende, actinolite, edenite and ferrorichterite are recorded, while minor amounts of riebeckite and arfvedsonite also occur. Olivine (Fa82-94) and Fe-Ti oxides are cumulus phases and aenigmatite is a late intercumulus mineral. The rocks of Centre 2 comprise alkali biotite gabbro and nepheline syenite. The gabbros vary from melanocratic varieties towards melasyenites, some of which contain nepheline. The more gabbroic types contain labradorite, diopsidic pyroxene, olivine, poikilitic brown hastingsitic amphibole, brown biotite, which is found principally in the feldspar-rich varieties, and magnetite. K-feldspar is present in most rocks and this, together with other mineralogical features, suggest the term gabbro may be inappropriate. The nepheline syenites are very variable due to textural variations, abundant and varied xenoliths, layering and the presence of extensive breccia zones which, together with alteration and metasomatism, obscure the relationships with the other rock types. The nepheline syenites comprise perthite, zoned hastingsitic amphibole, occasional pyroxene which is usually mantled by amphibole and varies from diopsidic varieties towards hedenbergite and finally aegirine, olivine (Fa74-77), which is restricted to mafic bands, biotite, nepheline, natrolite, hydromuscovite, sodalite, titanomagnetite, ilmenite, fluorite, zircon, apatite and sphene. Metasomatic nepheline-plagioclase intergrowths have been described from xenoliths in nepheline syenite by Mitchell and Platt (1979b). The rocks of Centre 3, which are the least known, vary from syenites through quartz syenites to granites, and there are considerable differences in the categorization and mapping of these rocks between Currie (1980, Fig.17) and Platt and Mitchell (1982a, Fig. 1). They characteristically contain sodic amphiboles and abundant xenoliths of hornfels, volcanics and syenite. Fenites occur in several parts of the complex and some rocks have been interpreted as mobilized fenites (Currie, 1980, p. 7). There are numerous dykes both within and around the complex including lamprophyres of various types, syenites and tinguaites. Significant concentrations of lamprophyric dykes occur in the west around McKellar Harbour and near Marathon and Heron Bay in the east, the latter having been named the Marathon Dikes by Platt and Mitchell (1982b). Numerous rock and mineral analyses of Coldwell will be found in Currie (1980) and Mitchell and Platt (1978, 1982). A gravity survey (Mitchell et al., 1983) indicates that a significant volume of dense material, interpreted as a differentiated basic intrusion, lies beneath the principally felsic rocks of the uppermost 3-5 km.