Orebody geometry in lode gold deposits from Zimbabwe: implications for fluid flow, deformation and mineralization

Abstract

The geometry of some orebodies can be described simply and accurately by three orthogonal axes, U≥ V≥ W. The ratios between these axes can be expressed as a parameter j=( U/ V−1)/( V/ W−1), and represented by a graph of U/ V plotted against V/ W, analogous to the treatment of strain ellipsoids. The orientations of orebodies can be plotted simply on projections using the UVW axes. Measurements of ore bodies from two examples of lode gold deposits from the Zimbabwe craton show that most of these orebodies are oblate. However, orebodies can have significant U/ V ratios, implying a component of pipe-like fluid flow during mineralization. Pipe flow is demonstrated to be orders of magnitude more conductive than flow in planar veins and faults. There are significant variations in orebody geometry between deposits and within different sections of a single deposit. W values appear to be influenced by host rock: more permeable rocks have higher W. A negative trend of j value with orebody volume indicates that orebodies do not evolve in a self-similar way, but tend to more oblate shapes with increasing volume.