Reaction veins in marbles formed by a fracture-reaction-seal mechanism
European Journal of Mineralogy Volume 1 Number 5 (1989), p. 701 - 714
published: Nov 16, 1989
manuscript accepted: Jun 15, 1989
manuscript received: Jan 25, 1989
ArtNo. ESP147050105009, Price: 29.00 €
Abstract Numerous extension veins sealed with product minerals of chemical reactions between a silica-rich aqueous fluid and wall-rock occur in dolomite marble inclusions of the Bergell granite (Central Alps). Mineral compositions and modal analysis data are used together with textural information to suggest a multicycle growth history for the veins. There are two types of veins: a) simple veins consist of only one mineralogical zone with two product minerals (one silicate + calcite). This bimineralic zone (vein) may, however, show distinct internal zonation which results from variations in the modal abundance, the texture, and the chemical composition of the product minerals, b) composite veins consist of symmetrical sequences of more than one bimineralic zone. The different zones of composite veins may also show strong internal zonations. It is suggested that each homogeneous subsection of a given bimineralic zone of the total vein grew in one single cycle feedback loop involving deformation and chemical reaction. The proposed "fracture-reaction-seal" mechanism involves the following steps: 1) formation of extension shear fracture with very high permeability, 2) flow of silica-saturated aqueous fluid along the high permeability zone, the pressure regime changes from stressed lithostatic to hydrostatic, 3) irreversible mass transfer between the fluid and wall-rock dolomite with which the fluid is not in equilibrium, the product assemblages depend on the prevailing conditions during formation of a given vein, 4) reaction products which precipitate in the central zone gradually reduce permeability and eventually seal the vein. The system returns to a lithostatic pressure regime and shear stresses may build up. 5) the formation of the next extension fracture occurs along the sealed vein of the first cycle. Step 5 marks the beginning of a new "fracture-reaction-seal" cycle which may be identified in both, simple and composite veins as sharp modal and mineral composition discontinuities. A large number of cycles may be distinguished in a single simple vein. Composite veins are formed when the fluid of a new cycle is no longer in equilibrium with the product assemblage of the previous cycle. The product assemblage of the preceding cycle(s) is then partly replaced by a new one, thus resulting in a vein with symmetrically arranged sequences of bimineralic zones.