Tectonic stacking in an Archaean greenstone sequence and its control on gold mineralisation at Dalny Mine, Zimbabwe

Abstract

The pattern of Au reefs at Dalny Mine and its satellite mines in the Chakari area of central Zimbabwe closely mimics an anastomosing network of narrow (< 5 m) shear zones characterised by highly strained silicified schist and graphitic shale horizons. These shears predated mineralisation and accommodated movements resulting in a duplex geometry of complexly stacked, lensoidal blocks of felsic and mafic greenstones, possibly reflecting thin-skinned processes early during the development of the Midlands Greenstone Belt. During mineralisation repeated reactivation of, and (fluid-pressure driven) dilation along these early shears resulted in the current distribution of Au reefs. The scale and orientation of the fault blocks in the early tectonic stack controls the size and spacing of the later Au reefs, explains the anomalous orientation of some reefs in the area and eliminates the need for complicated explanations involving multiple post-mineralisation-faults. Geometries like those at Dalny Mine could control a number of Au deposits in Zimbabwe, especially those that contain bifurcarting siliceous schist and black shale bands. Duplex geometries could have resulted from a number of processes including thin-skinned thrusting or strike-slip movements, although the first interpretation is preferred. The presence of imbricate stacks at Dalny implies that the stratigraphy of the thick greenstone succession may have resulted partly from tectonic processes.