Fluid flow in shear zones: insights from the geometry and evolution of ore bodies at Renco gold mine, Zimbabwe

Abstract

The geometry of mineral deposits can give insights into fluid flow in shear zones. Lode gold ore bodies at Renco Mine, in the Limpopo Belt, Zimbabwe, occur as siliceous breccias and mylonites within amphibolite facies shear zones that dip either gently or steeply. The two sets of ore bodies formed synchronously from hydrothermal fluids. The ore bodies are oblate, but have well-defined long axes. Larger ore bodies are more oblate. High-grade gold ore shoots have long axes that plunge down dip; this direction is perpendicular to the long axes of the lowgrade ore bodies. The centres of the high-grade ore bodies align within the low-grade ore bodies along strike in both gently and steeply dipping groups. The range of sizes and shapes of the ore bodies are interpreted as a growth sequence. Geometrical models are proposed for the gently and steeply dipping ore bodies, in which individual ore bodies grow with long axes plunging down dip, and merge to form larger, more oblate ore bodies. The models show that when three or more ore bodies coalesce, the long axis of the merged ore body is perpendicular to the component ore bodies, and that ore bodies in the deposit may have a range of shapes due to both growth of individual ore bodies, and their coalescence. The long axes of the high-grade ore bodies are parallel to the shear directions of both the gently and steeply dipping dip slip shear zones, which were the directions of greatest permeability and fluid flow. The larger, lower grade bodies, which may have formed by coalescence, are elongate perpendicular to these directions.

An integrated approach for identifying aquifers in transcurrent fault systems: The Najd shear system of the Arabian Nubian shield

“…Because shear zones by definition are wide in comparison to faults, they are potentially a more significant aquifer compared to faults. Brittle shear zones, faults, and mélanges are generally more permeable than their surroundings, and thus they can potentially act as aquifer rocks, and as conduits enhancing groundwater flow; less likely as barrier systems impeding fluid flow (Blenkinsop and Kadzviti, 2006;Goddard and Evans, 1995). In contrast, dykes intersecting wadis at high angles are more likely to act as barriers to groundwater flow (Babiker and Gudmundsson, 2004).…”

Structural Controls on Groundwater Flow in Basement Terrains: Geophysical, Remote Sensing, and Field Investigations in Sinai

“…1a). Favorable settings for hosting and channeling groundwater within the basement complex include areas of enhanced porosity where faults, fractures, and/or shear zones (meters to tens of meters wide) intersect (Goddard and Evans 1995;Blenkinsop and Kadzviti 2006;Sultan et al 2008).…”

Chapter 1: Structural Geology Applied to the Evaluation of Hydrothermal Gold Deposits

“…Clear shear sense indicators are seen on planes parallel to the lineation and perpendicular to the foliation, giving a reverse sense of shear in the north part of the mine (Kisters et al, 1998). The U axes of ore bodies plunge generally to the southeast (Blenkinsop and Kadzviti, 2006), parallel to the lineations and perpendicular to the inferred vorticity vector (Fig. 7).…”

“…The oblate shapes of these ore bodies, and their orientation parallel to the lineation, can be explained by higher permeabilities within the shear zone and parallel to the shear direction. Variations in the ore body orientation may reflect a process of ore body growth by coalescence (Blenkinsop and Kadzviti, 2006).…”