Geometry and P– V– T– X conditions of microfissural ore fluid migration: the Mokrsko gold deposit (Bohemia)

Abstract

Mokrsko, the largest gold deposit in the Bohemian Massif, is characterised by a dense set of sub-parallel gold-bearing quartz veinlets in granodiorite and the surrounding volcano-sedimentary rocks of the Jı́lové belt. Studies of the 3D geometry, compositions of fluid inclusions, and mineral paragenesis have been carried out to determine P– V– T– X conditions of ore fluid migration. Ore deposition resulted from the superimposition of three main stages of fluid migration through a series of dense and regular sets of extensional structures (veinlets and microfissures). Pre-ore stage is characterised by the formation of thick quartz veins and brines of probable magmatic origin, and is followed by moderate ductile deformation. During the main ore stage (E–W compression), parallel and regularly spaced quartz veinlets formed and were filled by quartz, pyrite, pyrrhotite, and arsenopyrite. Corresponding fluids belong to the C–H–O–N system and probably resulted from fluid–rock interactions within the metamorphic series at high P– T conditions ( T≈450–550°C and P≈250–400 MPa). Fluids have interacted with the vein wall rocks, partially altering the magmatic amphiboles and biotites to secondary biotites that are richer in Mg (±microcline). In addition, pyrite and arsenopyrite precipitated within the altered magmatic Fe–Mg silicates. The quartz veinlets and the granodiorite are, in turn, affected by a succession of brittle deformations responsible for the development of sub E–W striking microfissure sets indicating a nearly constant EW compressional regime. Associated fluids are enriched in volatiles, especially CH 4, and were trapped at relatively high pressure and temperature (350–450°C and 120–180 MPa). This stage occurs after significant uplift of the Hercynian basement, but under rather high thermal gradients if fluid pressure is considered as lithostatic. These fluids are responsible for the partial alteration of the earlier Fe–Mg minerals into chlorites. The late ore stage is characterised by intense microfracturing of the rock which results in (i) sub E–W striking fluid inclusion planes (FIP), and then (ii) dense sets of sub N–S striking FIP, which cross-cut all previous structures. Gold grains are found in microfissures where they cut earlier sulphides, but are also intimately associated with Bi minerals and chlorites (±carbonates), indicating that part of the economic mineralisation is linked to this stage. These FIP contain aqueous inclusions with decreasing T m ice (salinities from 8.5 to 2.5 wt.% eq. NaCl) and decreasing homogenisation temperatures ( T h from 320±20°C to 160±20°C). Intermediate and late ore fluids were mobilised during the Hercynian in response to abnormal heat flows and local enhanced fissural permeability, associated with the long lived deformational activity of the regional boundary which characterises the Jilové belt. These fluids are equilibrated with metamorphic host rocks (H 2O–CO 2 fluids) and minor meteoric fluids deeply infiltrated in the upper crust (Au stage). The general P– T path implies a significant difference in the structural level between early and intermediate stages (4–7 km), and an enhanced geothermal gradient during the intermediate stage.