Tower Hill gold deposit, Western Australia: an atypical, multiply deformed Archaean gold-quartz vein deposit

The Tower Hill gold deposit is distinguished from most Archaean lode deposits of the Yilgarn Craton by virtue of its formation early in the regional deformation history and its consequent deformation. The deposit is located in ultramafic schist, adjacent to the contact with a small pluton of biotite monzogranite that intrudes pervasively foliated granodiorite, the dominant component of the Raeside Batholith. Gold, accompanied by local concentrations of bismuth minerals and molybdenite, occurs in a number of quartz vein 'packages' Mineralised quartz Veins at Tower Hill lie within an envelope of potassic alteration (talc-biotite-chlorite-pyrite schist). up to several hundred metres wide. They are spatially and temporally associated with the biotite monzogranite and felsic porphyry intrusions, and their deformed equivalents. The deposit lies in a broad zone of ductile deformation (the Sons of Gwalia Shear Zone). Within the altered ultramafic schist, thin units of felsic schist, derived from biotite monzogranite and felsic porphyry, provided sites of contrasting competency that localised quartz vein formation. The mineralised quartz veins were subsequently deformed during alternating periods of shortening and extension, probably related to the syntectonic, solid-state emplacement of the Raeside Batholith. These deformations pre-dated strike-slip movement on the Cemetery Fault, which truncates the ductile fabrics of the Sons of Gwalia Shear Zone, south of Tower Hill. In terms of the regional deformation history, gold mineralisation at Tower Hill formed during early Dp (regional upright folding): subsequent deformation of the orebody pre-dated D-3 (strike-slip movement on the Cemetery Fault), The nearby Sons of Gwalia and Harbour Lights deposits also probably formed at an early stage, in contrast to most rode gold deposits in the Yilgarn Craton, which formed during or after D-3.