The intrusions studied represent magma that for the most part intruded as broadly concordant, sill-like bodies within a predominantly metapelitic sequence derived from turbidites. In spite of subsequent deformation and metamorphism, the age of which constrains the timing of intrusion to around 1.9 Ga, it is thus apparent that the intrusions originally had horizontal or nearly horizontal orientations.
The parent magma to the intrusions was tholeiitic, with MgO abundances of 8-11 w-% and an Al2O3/TiO2 ratio of around 10. Considerable variations in CaO/Al2O3 exist between intrusions due to variable degrees of contamination by crustal material. Olivine was the earliest and dominant cumulus mineral in all intrusions, followed by clinopyroxene in magmas relatively richer in CaO, or orthopyroxene and plagioclase in those magmas that were CaO-poor. On the basis of their respective mineralogy, the peridotites are accordingly classified as either wehrlites or lherzolites. In intrusions of gabbroic composition, orthopyroxene was the dominant cumulus phase, followed subsequently by plagioclase crystallization.
Some of the intrusions record a two-stage history, with crystallization having taken place initially within temporary magma chambers or flow conduits deeper in the crust prior to final emplacement into their present environments. Ultramafic lava flows within the metasedimentary sequence are considered to represent the eruptive equivalents of such two-stage magmatic processes, during which they became enriched in olivine.
Many of the intrusions contain Ni-Cu deposit, which is invariably situated within the stratigraphically basalmost parts of the intrusions. Separation of sulfide melt was presumably promoted by a combination of falling temperature and contamination. Evidence for contamination includes elevated LREE and Zr abundances and relatively low Nd(1.88Ga) values between 0.4±0.3 and 2.4±0.4. The mass ratio of silicate melt to sulfide melt (log R) varied from 2.29-2.97 with Dl/sil ranging from 243-530. Abundances of Pt and Pd are greatest in those intrusions which have the highest R values and in which the sulfide melt separated from a more fractionated silicate melt. Ore compositions are in many cases strongly influenced by whole-rock chemistry of the host intrusions, indicating that ore formation has occurred in situ. The sulfide melts also underwent fractional crystallization, resulting in metal zonation in many deposits. The upper parts of the ores are accordingly enriched in elements that are preferentially retained in residual melt, such as Cu, Pt, Pd and Au. The most important sulfide phases in the ores are pyrrhotite, pentlandite and chalcopyrite.
The Ni contents of magmatic olivine (with Fo contents ranging from 66.12-84.78 m-%) tend to be lower in mineralized intrusions than in barren intrusions, while a positive correlation exists between R values and olivine Ni abundances; Ni in olivine therefore has potential as an indicator of mineralization. Other potential indicators are Cu in olivine, Ni and Cu in orthopyroxene, Ni in chrome spinel, and whole-rock Ni abundances. Metamorphic olivine (with Fo compositions between 65.85-78.67 m-%) typically has higher Ni abundances than magmatic olivine in sulfide-poor rocks, whereas both types commonly have indistinguishable Ni concentrations where sulfides are present.
The Ni-Cu deposits of the Juva district closely resemble other Svecofennian deposits in Finland in terms of both overall composition and mode of occurrence, although some of the Juva intrusions have higher R values than those which are typical elsewhere. Therefore, it appears that areas other than the traditionally defined Kotalahti and Vammala Ni belts might also be highly prospective with respect to Ni-Cu mineralization potential. Most of the presently known deposits in the Juva district are rather small, which is principally a function of the generally small sizes of the host intrusions themselves.

Key words(GeoRef Thesaurus, AGI): intrusions, igneous rocks, tholeiitic composition, peridotites, gabbros, metavolcanic rocks, geochemistry, olivine, orthopyroxene, chrome spinel, electron probe data, nickel ores, mineral deposits, genesis, magmatic differentation, Proterozoic, Juva, Finland

Author's address:
Geological Survey of Finland
P.O. Box 1237
FIN-70211 KUOPIO
FINLAND

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