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Summary:
Six mineralisation styles are known in the Cadia district:
• Cadia Hill: Intrusion and volcanic-hosted, sheeted quartz vein mineralisation;
• Cadia East: Volcanic-hosted, disseminated and sheeted quartz vein mineralisation;
• Cadia Far East: Volcanic and intrusion hosted mainly sheeted quartz vein mineralisation;
• Cadia Quarry: intrusion hosted mainly sheeted quartz vein mineralisation;
• Ridgeway: intrusion and volcanic hosted quartz stockwork vein mineralisation;
• Big Cadia, Little Cadia: iron-rich skarns.
The Cadia East and Cadia Extended consist of alkalic porphyry gold-copper style mineralization and the Big Cadia deposit is a skarn-style occurrence.
The Cadia East–Far East deposit occupies a mineralised zone 2.5 km in strike length, 600 m in width and over 1,900 m in vertical extent. It is located below and to the east of the Cadia Hill deposit.
Geology
Mineralisation is developed in the FRV, and in a series of subvertical to steeply northdipping monzodioritic to quartz monzonitic dykes, that are termed the Cadia Far East intrusive complex (CFEIC). The syn-mineral nature of at least some of the intrusions is indicated by the presence of mineralised xenoliths within monzonite porphyry dykes that also host porphyry-style veining and alteration.
The Weemalla Formation has been intersected at depth, and consists of finely-bedded siltstone interbedded with basaltic volcanic rocks. Overlying this unit are five lithofacies of the FRV:
* Upper bedded unit: about 80 m thickness of finely planar-laminated feldspathic siltstone;
* Volcaniclastic unit: approximately 200 m thickness of sandy matrix polymictic conglomerate and volcaniclastic sandstones and locally volcanic breccia;
* Lower bedded unit: around 60 m thickness of bedded calcareous sandstone typically altered to skarn mineral assemblages;
* Massive volcanic rocks: about 150 m thickness of massive pyroxene phyric basalt to andesite lavas;
* Lower sequence: at least 1,100 m thickness of polymictic conglomerates and volcaniclastic sandstones.
Intrusive porphyry dykes and sills are interpreted to be co-eval with the FRV volcanic units. In the Cadia East area, the 5–30 m thick porphyry dykes appear to be stratigraphically controlled by the bedded units, and acted as feeders to overlying sills. The largest dyke has been traced for 1,500 m along strike, are coincident with a change in shape of the orebody on section 15570E, and are cross-cut by mineralised veins. Two large porphyry sills located above the lower bedded unit can be traced along the upper portion of Cadia East. Numerous smaller sills and dykes also exist in this area. The uppermost of the units termed the capping porphyry and is thickest (~70 m) in the middle of the deposit.
Alteration
Mineralisation at Cadia East is associated with an alteration system that occurs in roughly concentric zones about the core of the deposit. Within this alteration system both pervasive and selvage styles of alteration are recognised. The pervasive alteration overprints regional propylitic (chlorite–carbonate–epidote) alteration and is characterised by variably intense albite alteration.
The selvage style of alteration is largely made up of three alteration types:
* Hematite/K-feldspar alteration associated with quartz veins (generally mineralised);
* Phyllic alteration associated with faults;
* Iron carbonate–albite alteration associated with faults.
Structure
Three major groups of faults were identified, including:
1) Sericite–chlorite–clay (SCC) shears: Most common type of fault at Cadia East, and range in scale from millimetres wide to tens of centimetres wide. Infill can vary from friable sericite–chlorite to puggy clay–gouge. Pyrite is common, hence the former term of “pyrite fault”, which has been discontinued. The most continuous sub-set, the P Faults (P1, P2 and P3) are east–northeast–west–southwest-striking (parallel to the orebody) and transect the panel cave areas. These faults are anastomosing, and locally discontinuous with splays, and consist of SCC shears that are soft, present deteriorating ground upon exposure, and have a well-developed shear fabric. East–northeast–west–southwest- and east–southeast–west–southweststriking fault subsets appear to be limited to a 50–100 m strike length and do not have the continuity of the P Faults. The east–southeast-striking shears may be shear fractures that link the tensional fractures within a fault jog, hence their limited (<100 m) continuity along strike;
2) Carbonate–laumontite (Ca–La) faults: Highly fractured and veined zones to tens of metres wide. Calcite-laumontite mineralisation is common throughout the Cadia East system, but there are concentrations of calcite–laumontite on the hanging wall (Ca–La North) and footwall (Ca–La Central) sides of the higher-grade orebody (copper and gold), and on the northwest corner of the lower-grade copper zone (Ca– La West). No Ca–La zones were identified in PC2–3. Ca–La Central has not been identified above 5100 RL, and Ca-La North above 5500 RL. The faults are latestage zones of very poor ground conditions as a consequence of the hydration/dehydration of laumontite;
3) Carbonate faults: Related to late low-angle thrusts that are likely splays off the regional Gibb Fault. Characterised by zones of iron-carbonate–albite alteration and chlorite shearing around a zone of calcite veins, or intervals of calcite-rich puggy (sticky) gouge with an iron carbonate selvedge. The main Carbonate Faults interpreted to have an impact on PC2–3 are Carbonate 2 and Carbonate 5. The Cat Fault and Splay overlie PC2–3 near-surface and are zones of well-developed gouge, up to a couple of metres thick. The Cat Fault has been estimated to have at least 80 m of reverse movement. These faults will only be encountered when the PC2–3 cave reaches near surface and will not affect mining recovery.
Mineralisation
Mineralisation at Cadia East is divided into two broad overlapping zones: an upper, copper-rich disseminated zone and a deeper gold-rich zone associated with sheeted veins. The upper zone forms a relatively small cap to the overall mineralised envelope and has a core of disseminated chalcopyrite (and rare bornite), capped by chalcopyrite– pyrite mineralisation (Fox et al., 2009).
The deeper zone is localised around a core of steeply-dipping, sheeted, quartz–calcite– bornite–chalcopyrite–molybdenite veins, with the highest gold grades associated with the bornite-bearing veins. Copper and molybdenite form a mineralised blanket above and to the east of the higher-grade gold envelope.
Au:Cu values are vertically zoned. The upper, disseminated zone of volcanic-hosted mineralisation typically has low Au:Cu values (<1), whereas the envelopes of sheeted quartz-calcite-sulphide veins have higher Au:Cu values (typically >2).