COMMISSION GEOLOGIOUE - arkisto.gsf.fi

On the prospecting and geology of the K emi chromite deposit, Finland 47
In addition to the aforedescribed peridotitic rocks, olivine has been
found in the vicinity of Elijärvi, as an intercalation in the ore, on the foot
wall side of the ore belt. l The rock consists of serpentine and olivine, the
former occurring as an unoriented mass, in which the olivine is present in
the form of small, round grains. The simultaneous extinction in groups
of the olivine grains in polarized light and the figures formed by chromite
present in accessory amounts indicate the rock to have originally been
relatively coarse-grained dunite. The higher interference color of the olivine
points to its larger faialite content in comparison with the olivines of the
aforementioned peridotitic rocks.
As the degree of metamorphism increases, serpentine becomes the dominant
mineral of the peridotitic rocks. Rocks dominated by serpentine occur
as harder intercalations in the strongly metamorphosed zone of the area.
Although the increasing serpentinization cannot be followed as a gradual
alteration process in a transverse direction in relation to the zone, it is
nevertheless possible, on the basis of samples taken from various points, to
obtain aseries in which, starting with the aforedescribed olivine-bearing
rocks, the rocks gradually become richer in serpentine. The next degree
after the aforementioned peridotites is the total serpentinization of the
olivine content of the rock. The rhombic pyroxene has likewise become
throughout transformed into fine-grained, oriented serpentine. The relictic
structural feature deriving from the different mode of occurrence of serpentine
easily reveals, on the one hand, the originally olivinic and, on the
other, the bronzitic portions. Tremolite, to which we shall return in the following,
is the prevalent mineral component, along with serpentine, of the
majority of the metamorphosed rocks of peridotitic origin. Likewise, the
chlorite occurring as an interstitial mineral, referred to already in connection
with the less metamorphosed peridotites, remains unaltered in considerably
metamorphosed rocks, thereby in part also exhibiting the texture of the
original rock.
Relatively common in the metamorphic zone are, furthermore, serpentine
rocks, in which other mineral components occur only in accessory
amounts. In numerous instances their peridotitic origin can be noted from
the relictic structural feature reflecting the different appearance of serpentine
(Fig. 7). However, in a large number of such serpentinites, no such
structural features can any longer be seen, but the entire rock consists
throughout of homogeneous, unoriented serpentine. Considering their origin,
one is inclined to identify them as alteration products of dunites.
1 It is, however, most probable that thc prevalence of Tocks containing olivine is greater
than that observed. In the examinations of drillcorcs carried out in routine fashion, such rocks,
being almost impossible to identify macroscopically, could easily have passed unnoticed.