En petrografisk och mineralogisk studie av en komplex gång bestående av metadiabas och kvartskeratofyr i Kiirunavaaragruvan

Sammanfattning: Summary: The purpose of this paper is to give a petrographic and mineralogic description of a dyke complex, consisting of a metadiabase and a quartz keratophyre. The type locality is the northern part of the Kiirunavaara apatite-rich iron ore, situated in northern Sweden. In this part of the mine, the dyke complex has caused problems with rock stabil ity and water inflow. The dyke is well mapped from drill holes and mining. It cuts the ore body from the footwall to the hanging wall, striking N 75-80 W and dipping 50-60 S. Within the 2-8 m thick metadiabase, the quartz keratophyre appears as a thin, unconform "dyke in dyke", i.e. it does not penetrate the main dyke at all levels. Sections in both the footwall and hanging wall show large balls of the metadiabase, enclosed in the quartz keratophyre. Consequently, the latter formed late in the Kiirunavaara ore region. The metadiabase has a 'blast ophitic' texture, mainly consisting of albite and biotite with accessory amphibole (act-trem). The quartz keratophyre consists of albite and quartz phenocrysts in a matrix with partly granophyric intergrowth of albite and quartz. The albite phenocrysts show chessboard twinning. The same type of amphibole (act-trem), appears as an accessory mineral. Whole rock, trace element and mineral analyses are presentated in tables and in the following text. There is no evidence of a magmatic differentiation trend, neither for the metadiabase, nor for the quartz keratophyre (variation diagrams, Figs. 11 and l2). The contents of Na (mean value 4.19 wt%) and especially K (mean value 2.82 wt%) are high for a basalt (Tab. 2a). However, the mineralogy of the metadiabase is of secondary origin. The albites and biotites look altered, are full of inclusions and grow into each other. To summarize, the mineralogy coincides with that of a spilitic rock, as follows: l. The major minerals consists of highly sodic plagioclase and - as augite is absent - actinolite, chlorite and epidote or chlorite and hematite. Olivine is typically missing (Turner and Verhoogen, 1960). 2. Mineral analyses of albites, biotites, amphiboles and epidotes from the metadiabase are given in Tables 4, 5, 6 and 7. It is notable that the major- and trace elements (Tables 2 and 3) give inconsistent classifications. For example: In a total alkali-SiO2 diagram (Fig. 6) the metadiabase is plotted in the field of alkali basalts, but a plot in a Zr-Y diagram (Fig" 7) results in a tholeiitic magma. Although the metadiabase was formed by continental rifting, it has a spilitic mineralogy. From the mineralogical composition given above, Na-metasomatism is suggested to be important. This is apparent from Fig. 5 ( O the metadiabase), especially when comparing the metadiabase with the felsic samples in the same diagram (Fig. 5, X quartz keratophyre, ' samples from the hanging wall and ¤ granophyre). The samples from the quartz-keratophyre dyke are very low in K (mean value 0.78 wt%, SiO2 mean value 71.4 wt%), but their amphiboles are high in Mg. Mineral analyses of albites and amphiboles from the quartz kerathophyre are given in Tables 8 and 10. Two samples from one dyke of 'granophyre' (Geijer, 1931) in the southern part of the mine, 51 and 52, differ in their alkali contents, i.e. normative albite and orthoc1ase are nearly equal (Tab. 2b and Fig. 12). It might be that the dykes of granophyre in the southern part of the mine represent the original mineralogy of the quartz-keratophyre dyke. However, this suggestion is based on rather unsufficient evidence. Geijer (1910), made a description of a dyke of diabase, found in a drill ho1e south of Jägmästarn (in the southern part of the mine). This diabase consisted mainly of plagioclase, augite and magnetite, the plagioclase is probab1y a labradorite (page 4) .