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ON THE GEOLOGY OF THE CHROMITE DEPOSIT AT KEMI, NORTH FINLAND BY V ALTO VELTHEIM ABSTRACT The chromite deposit is associated with a zone of ultrabasic rocks. The continuous ore belt is approximately 3.5 km long. The thickness of the belt varies considerably measuring at its thickest weIl over a hundred meters. The dip of the ore bodyaverages 65 0 NW. The ore is known to continue at least to a depth of 250 m in roughly the same thickness. The chromite occurs in the ore as either dense or sparse impregnations. The impregnations are generally evenly distributed, although there are also ore types in which oreless flecks of former olivine grains appear in an evenly distributed impregnation. The grain-size of the chromite remains within a few tenths of a millimeter. At its richest the ore contains ca. 40 % Cr 20 3 • It is assumed that a portion rich in chromium separated from the parent magma at an early stage. The parent magma penetrated into its present sill-like position where it differentiated into anorthosite and p eridotite. The chromium-rich portion then penetrated into the peridotite. After the placement of the ore, hydro thermal solutions penetrated into the same zone producing a metasomatic change in the parent rock of the ore and on both si des of the ore zone. CONTENTS THE GENERAL GEOLOGY OF THE REGION.. .. .... .... .. .. .. .. .. . . .. .. .. 32 THE GEOLOGY OF THE MINERALIZED ZONE ........... ... ... . .... . ..... 34 THE ORE OCCURRENCES INVESTIGATED .. ......... ... . .. .. ...... . .. .. .. 37 WEST END ORE .. .... . ..................................... .. .... .. 38 NUOTTIJÄRVI ORE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 ELIJÄRVI ORE ...... . .... . ............ . .. . ...... .. ...... . . ........ . . 40 VIIANMAA ORE . .. ... . ... . .... . . . . .. ................... . ..... . ...... 41 Page
32 Bulletin de la Commission geologique de Finlande N: 0 194. PETROGRAPHIC DESCRIPTION .................... . ...... . . .. . .. ... .. .. . PERIDOTITE . .. ... .. .... ... . . .... .... ... . . .......... . . . ...... .. .. . . . PYROXENITE . . ..........•..• . ... . . ..... ... . ..... ......• .... •... .... GABBRO . . .. ......... . ...... . .................. . .. . .. .. . .•.• . . ... . . CHLORITE ROCK ... . .... ............ . . ..... . . .. . . .. ........•• ..... .. CHROMITE.PERIDOTITE AND CHROMIUM ORE ... . ......... . . . . . . • .•..... . ALBITE ROCK .. .............. ..... . .. .....•..................•...... DISCUSSION .......................................... . . ..... .. .. ..... . SUPPLEMENTARY NOTE .... ... .. .. . ........... . .. . ........ .. ......... . ACKNOWLEDGEMENTS .. ... .. ....... .... . .. ..... .. . . . .. ............... . REFERENCES ............. . ..... . ........................... . .. ... ... . . THE GENERAL GEOLOGY OF THE REGION Page 42 42 50 53 53 54 59 61 63 65 66 The stratigraphy and structural geology of the Kemi area has been investigated by Härme (1949). The geological map (Fig. 1) appended to Härme's publication shows the zone of intrusive rock running northeast from the town of Kemi. This »anorthosite-serpentine series» is about twelve kilometers long and, in its southwestern part, about a kilometer wide, the width increasing toward the northeast so that ne ar the other end of the formation, at Elijärvi, it is approximately three kilometers. Another formation described as consisting of the same rock occurs, however, at a distance of so me eight kilometers to the northeast from Elijärvi, where it runs in a northeasterly direction as a belt two or three kilometers wide and about twenty-one kilometers long. The Kemi-Elijärvi anorthosite-serpentine sequence borders at its southeastern end, as shown in the map, on a broad stretch of migmatite granite. Facing the formation on its northwestern side there is greenstone as weil as, at about the middle of the sequence, quartzite. According to Härme, the different rocks mentioned would rank in the following order, starting with the oldest: the older component of the migmatite granite (gneiss granite) - quartzite - greenstone - anorthosite-serpentine sequence - the younger component of the migmatite granite (potash granite). The greenstone occurs in two varieties: amygdaloidal and hypabyssal. Judging by the primary structures, the trend of the base of the quartzite is toward the southeast. The general dip of the formation is NW. On the basis of Härme's presentation, the main geological features of the region may be summed up as follows: 1- The weathering sediment (quartzite) has been deposited on a basis of gneiss granite. 2- During volcanic action, which took place in part also while the sedimentation process was still going on, basic supracrustallayers (amygdaloid
Page 1 and 2: GEOLOGI NE N TUTKI MUSLAITOS BULLET
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REFERENCES CAESAR, F. and KONOPICKY
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