PARAGENESIS OF THE PAROO FAULT, MOUNT ISA, QUEENSLAND, AUSTRALIA

The sigmoidal 23km long Paroo Fault forms the footwall to the Mount Isa copper orebodies which lie within a silica and dolomite halo. Orebodies are predominantly located directly above the fault in the Proterozoic (c. 1650 Ma) Urquhart Shale which the fault juxtaposed against the metabasalts and psammites of the Haslingden Group (c.1750 Ma), though some are “perched” tens to hundreds of meters above the fault. The Paroo Fault is a probable conduit for ore-forming fluids but the paragenesis of the fault rocks has never been studied. Here we present results of transects across it from the basement to the overlying shales using petrography, microstructural analysis and cathodoluminescence (SEM-CL).

The first generation of quartz (Q₁) is brightly luminescent and consists of angular to sub-rounded clasts (10-100μm), formed during early faulting. A subsequent deformation caused fracturing and rounding of Q₁ clasts. Then a weekly luminescent generation of quartz (Q₂) formed angular to sub rounded clasts around Q₁. Cross cutting Q₂ are fine (10-30µm) veins of moderately luminescent quartz (Q₃) containing sulphide. Coarse carbonate (C₁) and some sulphide mineralisation formed (in the fault) and were subsequently deformed. Q₄ is luminescent quartz which forms large veins (300-1000μm) and is cross cut by the final stage of quartz (Q₅) comprising small (1-5μm) brightly luminescent veins. C₂ carbonates form along the edge of the large recrystalised quartz (Q_1-5) grain boundaries in association with microfracturing and grain size reduction. C₂ is associated with chlorite and carbonaceous material and is prevalent at the edges of the Paroo Fault.

Reverse movement is recognized on the Paroo fault from macro-scale observations and from fluid inclusion plane (FIP) analysis. Normal movement is recognized from gouge marks on graphitic mylonite and quartz fish, drag structures, and the FIPs. Normal postdates reverse movement as the lineations are best preserved. Although the reverse movement determined from FIPs postdates Q_1-3, this movement is consistent with the flat part of the Paroo Fault acting as a dilatant bend. We therefore concluded that the quartz (Q₃) and carbonate (C₁) veins which are associated with sulphide mineralisation may have been caused by reverse movement creating a dilatational jog during D₃.