The Paleoproterozoic Hitura Ni-Cu deposit in western Finland is in an
ultramafic igneous complex composed of two subvertical plug-shaped
bodies embedded in intensely migmatised felsic gneisses. The ultramafic
complex is not far from a north-south trending graphitic and sulphidic
gneiss zone interpreted to be a primary sulphide-graphite unit in the
metasedimentary sequence. The sulphide accumulation exists as a zone
parallel to the contact of the northern ultramafic body. The ultramafic rock types, now mainly serpentinites, have primarily
been olivine adcumulates in the core grading to olivine orthocumulates
and pyroxenites towards the margins. Coarse-grained felsic pegmatite
veins injected serpentinite where they now exhibit prominent zoned
alteration rims of chlorite, antophyllite and talc. The serpentinite core of
the body was primarily an olivine adcumulate derived from a tholeiitic or
high-Mg basalt parental magma with about 11 wt % MgO.
Metamorphism and alteration have totally changed the igneous mineral
compositions and the rock types are serpentinites (from olivine
adcumulates), ambhibole-bearing serpentinites and amphibole-chlorite
rocks (from pyroxenites). As the result of CO2 alteration talc is an
accessory mineral throughout the serpentinite. Like silicates, also
sulphide minerals are altered. In the core of the body the low-grade
sulphide dissemination still reveals primary igneous interstitial forms. The
core sulphides are now mackinawite, pentlandite and valleriite instead of
primary igneous pyrrhotite and pentlandite derived from Mss. At the
marginal zone the heavy dissemination and net-textured sulphides display
pyrrhotite-pentlandite-magnetite intergrowths with magnetite films along
cleavage of sulphides, mackinawite in pentlandite and exsolution textures
of different low-temperature pyrrhotite phases as the result of alteration
processes. Mechanically weak rocks, complicated structures and textures of the
ore, magnetite and mackinawite intergrowths in pentlandite and
readily-floating MgO silicates cause problems in exploitation of the
deposit. Solution to problems come from selective stoping, three-step
classification in grinding, froth-washing technique and on-line process
control based on ore type determination from process data using an expert
System