| Popis: |
Granitic melt, derived from migmatitic host rocks, intruded a D 3 -related fracture system in a 1.89-Ga NiCuFe-sulphide-bearing peridotite sheet, near Vammala, southwest Finland. Zones of wall-rock alteration, mainly comprising amphibole-phlogopite/chlorite, developed in the peridotite adjacent to the leucocratic dykes. The minerals in the alteration zones and in the gneissic host rocks, together with the production and intrusion of granitic melt, are consistent with metamorphic conditions of at least 600°C and 4.5 kbar. Later hydration under condtions of lower temperature and pressure resulted in partial serpentinization of the peridotite and dykes. This later transition precluded direct investigation of the earlier transition that developed the medium- to high-grade alteration zones in the peridotite at the interface with the leucocratic dykes. Because serpentinization was largely accommodated by volume increase, rather than mass loss of MgO and SiO 2 , the relative concentrations of most components in the serpentinized peridotite should mirror those in the peridotite. Based on this contention, two methods are used to document the composition-volume behaviour of the transition from now-serpentinized peridotite to alteration zones ( apparent transition), and hence infer the mass-balance trends of the igneous peridotite-to-alteration zone transition ( real transition). The methods collectively suggest that, in the alteration assemblages, K 2 O, Rb 2 O, P 2 O 5 , TiO 2 and Al 2 O 3 form a group of relatively enriched components, and that Al 2 O 3 was particularly mobile. An estimated volume increase of 10-10% accompanied serpentinization, and a calculated volume decrease of 10–15% accompanied the apparent transition; thus, a volume increase of real transition. For “best-fit” volume factors, Gresens' equation demonstrates that the apparent transition involved mass gains in SiO 2 , Al 2 O 3 , K 2 O, P 2 O 5 and TiO 2 , and mass losses in FeO, MgO, CaO, Ni, S and volatiles (loss on ignition, LOI), these amounting to net losses of 32.5 kg of volatiles and 10.8 kg of other components per 1000 kg of serpentinized peridotite. The real transition involved similar relative gains and losses, except for SiO 2 gain (slightly less), MgO loss (slightly more) and LOI loss (negligible). It respectively records inward transfer of components from a feldspar-rich source to the peridotite, and outward transfer to that source. This is consistent with metasomatic exchange between the peridotite and the probably supra-solidus granitic intrusions. During the real exchange, NiCuFe-sulphides were retained and redistributed (internally remobilized) in the outer alteration zones, but transferred into the dyke from the zone abutting it (externally remobilized). The same genetic interpretations probably apply to similar dyke and alteration systems in other NiCuFe-sulphide deposits hosted in ultramafic rocks in southwest Finland. |
