COMMISSION GEOLOGIOUE - arkisto.gsf.fi

On the prospecting and geology of the Kemi chromite deposit, Finland 25
It is difficult to detect the local anomaly caused by the ore from the
original map, as it is masked by the strong regional anomaly resulting
from the basic massif.
The linear elimination of the regional anomaly, as shown in Fig. 11,
has fairly clearly brought out the effect of the ore and the situation of
the ore can be determined quite easily by means of this map. The effect
of the ore appears far more distinctly in the residual map proper, Fig. 10.
In addition, it brings out another zone running parallel with the main one
150 m to the northwest and corresponding to the chromite veins met with
at the bottom of the channel.
DENSITY DETERMINATIONS FROM DRILL CORES
In order to verify the interpretation of the results given by the gravity
observations, a large number of density determinations were made from
the drill cores.
As is weIl known, it is rather difficult to obtain a picture of the mean
density through density determinations made from individual sampies, particularly
in the case of non-homogeneous rocks or ore-bearing zones. Upon
the density's changing at short intervals within a relatively large range, a
poorly selected group of sam pIes is liable to lead to erroneous results. In
order that the determinations might give a more reliable average picture
of the distribution of the density, the Geological Survey starting in 1954
has used a device, in which the density of the drill core can be determined
over a length of 80 cm at one and the same time. In this way, a continuous
spectrum of the distribution of the density can be obtained, whenever
desired, throughout the length of the drill hole by measuring the cores
of the entire hole in a single continuous operation. This device is represented
in Fig. 13.
The device consists of a balance equipped with effective eddy current
damping, and with a range of 0- 2 kg, the sensitivity being 0.2 g. The
drill cores are stacked in an aluminum trough 80 cm in length, and the
weighing is performed first in air and then in water. Several troughs are
used and they are identical so that their loss of weight in water can be
compensated for by a single constant additive weight.
Density determinations have been made from the cores of six drillholes
over a total length of some 1 040 meters. These determinations are
concentrated mainly in the drill-holes of profiles I-II and III- IV. The
density of the ultrabasic rock occurring as the country of the ore varies
between 2.7 3 and 2.90 and that of the albite rock between 2.60 and 2.65.
4 10866-60