Litho-structural controls on mineralisation at the Pillara carbonatehosted Zn-Pb Deposit, Lennard Shelf District, Western Australia

Summary

The Pillara Zn-Pb deposit is located on the northeast margin of the Canning Basin, Western Australia where it is hosted by a series north-northeast-trending, en echelon, conjugate normal faults that bound a central graben plunging gently to the northeast. Kinematic indicators on all fault surfaces indicate dominantly dip-slip movement. Extension increases from approximately 15% in the south to as much as 50% in the north of the graben structure. The complex array of faults is consistent with development in a northwest-southeast directed extensional stress field with a vertical σ₁ prior to minor tilting of the sequence to the northeast. Northwest-southeast extension was localised within a northeast-trending transfer structure.

Local controls on the development of mineralisation plunging shallowly to the northeast include dilation where faults steepen or overlap and where antithetic faults intersect the main bounding Western and Eastern Faults. Areas of greatest dilation are largely controlled by the orientation of individual fault segments relative to the local stress field and the intersection of the faults with a competent fenestral limestone unit (Unit 5) and lower platform and bank facies (Unit 1). Both units display low uniaxial compressive strengths compared to other limestone samples within the sequence. The intersection of north and northeast trending fault segments defines a steep, northerly plunge that appears to control high grade orebodies in the F10 system (A Lode and B Lode).

Mineralisation is dominantly contained within tectonic breccia zones, open-space fill along faults, and extensive hanging-wall hydraulic breccia zones localised by fault relays and intersections with antithetic faults. The latter grade outwards from rubble-breccia through mosaic breccia to crackle breccia and stockwork vein zones. Veins and breccias may show a history from early marine calcite cements through to pre-ore marcasite and calcite-cemented crackle breccia (B₀), to ore-stage breccias characterised by sulphide-cemented rubble breccias (OB₁) at times incorporating sulphide breccia clasts (OB₂), to post-ore breccias cemented by sparry calcite cement (OB₃). The ore textures and paragenetic sequence are consistent with introduction of the ore fluids during active fault movement, and rapid precipitation into open cavities created during faulting.

• Pillara Zn-Pb deposit
• Mississippi Valley-type
• Lennard Shelf