Abstract - Vein arrays occur in a variety of tectonic settings and lithological sequences. The aim of this talk is to describe the effects of host-rock stratification on both vein distributions and the associated strains based on a large database of line samples from outcrop and core.
Arrays of parallel veins in layered sequences, such as sandstone/shale sequences, are usually confined to one layer, i.e. stratabound. Stratabound veins are regularly spaced (anticlustered) and thickness distributions are non power-law. The scaling of such vein systems is governed by the thickness of the mechanical unit in which they occur.
Vein arrays in massive rocks, such as granite or homogeneous limestone, are, at the scale of outcrop observations usually non-stratabound. The veins are spatially clustered, with a cumulative frequency distribution of vein spacing which is generally lognormal. However, datasets which include even microscopic veins have spatial distributions which are approximately power-law (i.e. fractal) and it is likely that lognormal distributions simply reflect the limited resolution of many datasets. Non-stratabound veins have cumulative frequency distributions of vein thickness which are power-law, with an exponent usually in the range of -0.5 to -1.0. The fractal nature of non-stratabound vein arrays occurs because vein dimensions are not limited by the thickness of the host layer.
A new parameter, termed the mass function, is a measure which incorporates and combines the distributions of both thickness and spacing of veins. The mass function quantifies the way in which strain across vein arrays changes with the scale of observation. Results obtained using this technique confirm that strain is homogeneous in stratabound veins above the scale of the vein spacing, while strains across non-stratabound vein arrays show a systematic power-law decay away from vein clusters.
Ore-mineralised veins consistently show the spatial clustering and power-law thickness populations typical of non-stratabound vein arrays. Non-stratabound veins may be inherently more prone to ore mineralisation as they occur in clusters and individual veins have much larger dimensions than stratabound veins, allowing rare elements to be concentrated from very large host-rock volumes.
Abstract of talk given to:
Structural Control and Genesis of Economic Resources (Minerals and Hydrocarbon deposits), Dave Johnston Memorial Meeting, Trinity College Dublin, November 1997