Fluid evolution of the late Archaean Rämepuro gold deposit in the Ilomantsi greenstone belt in eastern Finland

Abstract

The late Archaean (ca. 2.7 Ga) Ilomantsi greenstone belt hosts a large number of small mesozonal gold occurrences. The Ramepuro deposit is spatially related to a feldspar porphyry dyke which has intruded the contact between metagreywackes and intermediate metavolcanic rocks. It consists of gold-bearing quartz–tourmaline–sulphide veins located mainly within an intensively altered and sheared zone about 20–30 m in width. Two types of fluid inclusions were distinguished in the quartz veins: (1) H2O–CH4 (≤9 equiv. wt% NaCl), and (2) H2O–CO2 (≤12 equiv. wt% NaCl). The two compositionally different fluid-inclusion types occur in separate veins. The compositions of decrepitate residues indicate that the type 1 fluid inclusions consist predominantly of Na, S, Ca, Cl, and in lesser proportions of K and Fe. The residues of the type 2 fluid inclusions are dominated by Na, Ca, Cl and minor K. For the type 1 and type 2 fluid inclusions, microthermometric experiments indicate average homogenisation temperatures of 310–350 and 220–250 °C respectively. Both types of fluid inclusions contain 1–3 anisotropic solids (Ca/Mg carbonate?, nahcolite?, tourmaline?). Sulphide daughter minerals occur only in the type 1 fluid inclusions. This fluid type is also often associated with short trails of sulphide inclusions which suggest mobilisation and redistribution of ore constituents. Some type 1 fluid inclusions display distinct morphological features ("implosion textures") related to post-trapping re-equilibration. The presence of the fluid-inclusion re-equilibration textures in some auriferous quartz veins and their absence in other gold-bearing quartz veins suggest an age difference between them. This conclusion is also supported by the compositionally different fluid regimes associated with the different quartz veins, indicating a change from reducing (CH4) to a more oxidising (CO2) fluid environment. The fluid-inclusion evidence suggests two separate gold mineralisation events, one related to late Archaean regional metamorphism, the other to Palaeoproterozoic thrusting and metamorphism.