Archaean gold mineralisation in a normal-motion shear zone at Harbour Lights,Leonora, Western Australia

Although many Archaean greenstone-hosted mesothermal gold deposits are in steep, reverse-motion fault zones, other fault geometries are prospective for mineralization. Harbour Lights is one of a number of deposits at Leonora hosted in a normal-motion shear zone, probably related to movement off the adjacent domal granitoid. The deposit is also atypical in that mineralization predates the last deformation to affect the mine sequence, but formed by similar processes to other mesothermal deposits in all respects other than the detail of shear zone geometry, kinematics and timing. Gold mineralization at Harbour Lights is related to D1 quartz veins parallel to a well-developed gently NE- to E-dipping D1 cleavage, both of which are deformed in steeply dipping and later extensional shear bands (D2). Gently dipping quartz veins, as at Harbour Lights, must have formed at extremely high fluid pressures, capable of holding the weight of the overlying crust. In the gently dipping normal-motion shear zone continued reactivation and veining was possible only with extremely high fluid pressures, and steeply dipping structures, such as the D2 extensional shear bands, were initiated as the fluid pressure dropped after the mineralizing event. The rarity of gold mineralization hosted in normal-motion shear zones is due to their being linked to steep structures which pump fluid upwards and prevents the build-up of extremely high fluid pressures. At Harbour Lights it appears that these links were (atypically) absent, probably because deformation was a result of granitoid doming, and was subparallel to strata.