nattanen-type - arkisto.gsf.fi - Geologian tutkimuskeskus

Front, Kai, Vaarma, Markus, Rantala, Eeva & Luukkonen, Ari, 1989. Keski-
Lapin varhaisproterotsooiset Nattas-tyypin graniittikompleksit, niiden kivilajit,
geokemia ja mineralisaatiot. Summary: Early Proterozoic Nattanen-type granite
complexes in central Finnish Lapland: rock types, geochemistry and mineraliza-
tion. Geological Survey of Finland, Report of Investigation 85. 77 pages, 26
figures and 16 tables.
The Nattanen-type granite complexes (ca. 1.8 Ga) form a group of
post-tectonic, discordant, shallow-level and multiple intrusions in the early
Proterozoic Granulite Belt and the Archaean granitic gneiss surroundings in
northern Finland. The intrusions are composed of leucocratic monzogranite
phases exhibiting zonal arrangement. Their mode of occurrence, combined with
the presence of rhyolite and quartz-porphyry dykes, miarolitic cavities and
country-rock screens, is typical of postorogenic or anorogenic granites.
The oldest phases are coarse-porphyritic or coarse-grained granites and are
followed by porphyritic and/or medium-grained granite phases. The youngest
phases are abundant quartz-porphyry and aplite dykes. Numerous texturally
variable spherulitic and flow-banded rhyolite dykes occur in the immediate
vicinity of the Nattanen stock.
The granites are two-feldspar granites with biotite (70 wt.%), total alkalies and
FeO*/MgO, and a small but regular variation in major oxides. The granites show
alkaline affinities and are slightly peraluminous, with the majority of A/CNK
values between 1.0 and 1.1. Trace elements show large variation and illustrate the
evolution of the complexes better than the major oxides. The bulk geochemistry
indicates that the Nattanen-type granites are largely of the 1-type, but there is
usually a scatter of A-type and S-type granites. The available isotope data
indicate a significant Archaean component in the source of the Nattanen-type
granite complexes.
The youngest phase of the Tepasto stock is Mo-bearing aplite granite showing
recrystallization and deformation textures. Alteration is encountered in varying
degrees of intensity and extent. Local quartz-muscovite greisens and fissures
filled with quartz and/or mica are enveloped in weak sericitization. Greisen
bodies occur as irregular (0 - 10-30 cm) zoned spheres or pipes with quartz-
rich margins and muscovite-rich centres. The aplite granite contains molybdenite
with minor chalcopyrite and pyrite as the main sulphide minerals. The bulk of
the molybdenite occurs as dispersed rosettes (0 -0.5-2 cm). Sphalerite,
hematite, covellite and bornite occur incidentally. Other Mo showings in
late-stage phases of the Nattanen-type complexes with molybdenite rosettes
similar to the Tepasto occurrence contain less molybdenite, with no or only weak
greisenization, and less voluminous host aplite granites. Being richer in Li, Cu,
Cs, Ta and U, and poorer in F, Co, Zr, Ba, La and Th, the Tepasto aplite
granites differ from the other Nattanen-type late-stage phases in evolution of
trace elements. The greisen bodies have anomalously high Rb, Li, Nb, Ga, Sn,
Th and F.
Despite local specialization and greisenization, the Nattanen-type granite
complexes are not tin granites. The mineralization is also different from the
Svecokarelian porphyry-type occurrences in Southern Finland. Many features
suggest that the mineralization in the Tepasto aplite granite is a local concentra-
tion of coarse-grained molybdenite rosettes at the top of a shallow-level, barren
granite stock.
Key words: granites, rhyolites, dikes, mineral composition, geochemistry, pet-
rophysics, molybdenite, greisen, Proterozoic, Nattanen, Sodankylä,
Kittilä, Finland
Kai Front
Technical Research Center of Finland
Geotechnical Laboratory
SF-02150 Espoo, Finland
Ari Luukkonen
University of Helsinki
Department of Geology
P. 0. Box 11.5, SF-00171 Helsinki, Finland
Eeva Rantala
Plancenter Ltd
Opastinsilta 6
SF-00520 Helsinki, Finland
Markus Vaarma
Geological Survey of Finland
SF-02150 Espoo, Finland
ISBN 95 1-690-329-0
ISSN 0781-4240