Orebody geometry in lode gold deposits from Zimbabwe: implications for fluid flow, deformation and mineralization T.G. Blenkinsop School of Earth Sciences, Economic Geology Research Unit, James Cook University, Townsville, Queensland 4811, Australia Received 23 September 2002; received in revised form 24 January 2003; accepted 14 March 2003 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 ¼ (U/V 2 1)/(V/W 2 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. q 2004 Elsevier Ltd. All rights reserved. Keywords: Orebody; fluid flow; mineralization; gold; self-similar; Zimbabwe 1. Introduction The geometry of a hydrothermal orebody depends on the interactions between fluid flow, host rock types and structures. There may be additional feedback between deformation, chemical reactions and mineralization for syntectonic orebodies. In homogeneous host rocks, the relation between orebody geometry, fluid flow and defor- mation may be relatively simple, and the geometry of the orebodies may therefore give insights into mineralising processes on scales of hundreds of metres. These scales are orders of magnitude larger than those accessible from laboratory measurements of permeability and structure, which commonly underpin conceptual models of fluid flow in deformation zones (e.g. Caine et al., 1996; Evans et al., 1997). Orebodies can be classified into discordant and con- cordant bodies, and further into regular and irregular shapes (e.g. Evans, 1995). Regular orebodies are subdivided qualitatively into tabular and tubular shapes. Orebody geometry (shape and orientation) needs to be well under- stood and described quantitatively in order to be useful in investigations of fluid flow and mineralization, and to understand the structural controls on mineralization. The major aim of this paper is to propose a method for describing some orebody geometries simply and accurately, based on familiar concepts in structural geology. The method has direct applications to exploration, and it may be useful in fluid flow modelling and ore deposit studies. Two examples of epigenetic gold deposits in Zimbabwe (the Arcturus and Shamva deposits; Fig. 1) are used to illustrate the methods of describing orebody geometry. The results have some implications for fluid flow in mineralizing shear zones and the evolution of orebodies. 2. A simple method to describe orebody geometry At some level of detail, all orebodies have complex geometries. One of the challenges of understanding a structurally-controlled orebody is to be able to describe the orebody simply enough to relate it to structures, yet in a quantitative way, that can allow comparison to be made between different deposits. A three-dimensional approach is necessary: in the general case, horizontal or vertical sections will be arbitrary planes through an orebody. Measurements made from sections, such as strike length, strike orientation (e.g. from level plans), or down-dip length and dip amount 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2003.11.010 Journal of Structural Geology 26 (2004) 1293–1301 www.elsevier.com/locate/jsg E-mail address: thomas.blenkinsop@jcu.edu.au (T.G. Blenkinsop).