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Page 60 text:
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Report on Field Trip to Study Soifie Features of 1 the Coldwell Alkali Complex Sept. Sth-llth, 1970 by Pat Fung Party: Director: Dr. H. Loubat Members: 3rd year students, Lakehead University A. Chan, P. Fung, J. Kasarda, B. Paske, J. Scott Our field trip was intended to study some interesting features of tte Coldwell Alkali complex. The main stops we made, together with a general geological map of the area, are given in the map below. This complex was intruded into the Archean Greenstone belt in Keeweenawan time (+1065 m.y. to 1225 m.y.). At stop 4 the gabbro of the complex comes into gradual contact with the impure arkose of the Greenstone belt, forming a contact metamorphic aureole of gneiss. At some places lenses and veins of quartz, aplite and calcite, with pockets of syenite were found in the gabbro; and in some outcrops, magmatic bandings in the gabbro were seen. These features indicate an intrusion of a mainly gabbroic magma with local varieties and perhaps a later intrusion of syenite. The contact at stop 9 was much simpler. Walking east we passed from highly metamorphosed (granulite facies?) we ll-bedded claystone, through lenses and veinlets of syenite which increased in content until, through a few tens of metres, it was pure syenite. 58
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Page 61 text:
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In several outcrops we saw xenoiiths of different sizes and composition, mainly basalt. For ex- ample, at stop 2, just off the highway, was a large xenolith of basalt, and at stops 3 and 4 there were xenoiiths of basalt and breccia in the syenite. Most of them showed evidence of slow sinking. F. Puskas suggested that these were from the Coubran Lake volcanic cap and that the intrusion was a lopolith where the present level of exposure is near roof. The consistent fracture and shear zones might also expose some interesting structures. For instance, at stop 4 there was a domelike structure with pre-Coldwell rock sheared and fractured on top of the Coldwell gabbro. Also, at stop 9 near the western margin of the complex, there were two main directions of fracture - one vertical and one horizontal in the south-east direction. The rocks in the complex show complicated structural and age relationships as well as diversified composition. The oldest was the gabbro, followed by laurvakite, syenite and related rocks. But at stop 4, within a few feet the gabbro changed in composition from common to leucocratic gabbro due to an increase in acidic content. This change might be due to the interaction of syenite with gabbro (hybridi sation). The laurvakite also shows variations in composition - at stop 1 it was much altered, with plenty of quartz, calcite and pegmatite veins. The laurvakite is also rich in olivine in molybdenite, and native Mo., but in areas such as stop 8 it is very fresh and approaches the composition of a syenite. Most impressive of all are the variations in the syenite. In stops 2 and 8, the feldspars are very pinkish, probably rich in hematite due to alteration. At stop 2 it was intruded by the rhombopyre dyke near the Bamoos Radio Station, implying an older age relative to the rhombopyre dyke. At these stops, the syenite shows an intersertal texture of the potassium feldspars in a matrix of mainly mafic minerals, with evidence of agpaiitic sequence of crystallization. The syenite is much altered whereas the rhombopyre is rather fresh. At stop 4 we actually saw some xenoiiths of syenite in the gabbro, but at stop 5 the nephelene was very prevalent in the syenite, which is rather fresh with alteration only of nephelene to zeolite. Finally at stop 5 » on our way down the railway road near Port Coldwell, we saw the fresh nephelene syenite dying out into the altered, pinkish normal type of syenite - and yet at stop 7 two series of syenite veins cut each other, in diabase. These observations imply that the intrusion is not as simple as F. Puskas has anticipated. It can only be explained by the occur- rence of multi-stage crystallization and local variation in the order of crystallization. Generally, the main part of gabbro crystallized out first, then the normal syenite which became altered either before or during the intrusion of the rhombopyre dyke, followed by the intrusion of the syenite by the dyke. A second and significant portion of syenite rich in nephelene crystallized out, breaking up some older syenite and the rhombopyre dyke. Somewhere another, but insignificant, portion of gabbro crystallized out and engulfed some of the older syenite. The laurvakite generally fits into the pattern after the crystallization of the first portion of gabbro, but another part might have formed later - or the change in composition and amount of alteration can be attributed to local magmatic differentiation and environment alone. Ae seen at stops 1 and 5» the pegmatites and others in various rock types were always of late origin. They usually occurred in veinlets of the parent rocks and showed similar mineralogy, but their grain size and composition altered relative to the distance from contact. For example, the pegmatite at stop 6 can be attributed to the late magmatic portion of magma rich in volatiles, crystallized in fractures in the already solid rock. Since the whole complex is generally rich in rare minerals and elements, it might be interesting to do a chemical analysis of these pegmatites. There are also some features of geochemical interest, one being the agpaiitic sequence of crystal- lization in the altered intersertal syenite. Another is the interaction among various rock types evident in the xenoiiths and surrounding rocks, and the contact between various veins. To mention a few, the xenoiiths of gabbro in syenite (stop 7)» remained fresh and retained sharp boundaries; the rhombopyres and breccia at the same stop showed similar features. But several hundred feet away, the two series of syenite veins in the so-called nephelene syenite veins showed where they cross-cut each other in a complete fusion of their contact. This can be explained by the similarities and differences in the composition of the host and foreign material. When syenite veins meet and cut each other, it is not hard chemically for solution and recrystallization near the contact, whereas the reaction between a gabbro and a syenite is much more difficult, if not impossible. The alteration in nephelene syenite to colourful zeolite might be worth studying too. Mineralization occurred in several places. Common ores such as pyrrhotite, pyrite, chalcopyrite and iron oxides were seen in many outcrops in small amounts. But at stop 1 the molybdenite associated with laurvakite is rather rare. Further study might reveal yet other interesting elements and minerals such as titanium. At stop 8 large amounts of magnetite were seen either in nearly pure forms or injected into the surrounding rocks such as syenite and gabbro. 59
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