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Hannan South |
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Western Australia, WA, Australia |
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Au Cu
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Hannan South Au-Cu skarn deposit is located 12 km southeast of Kalgoorlie and the Golden Mile gold-telluride lode system in the 2.7 Ga Eastern Goldfields Orogen of the Archaean Yilgarn Craton, Western Australia.
Archaean Yilgarn Craton may be broadly subdivided into the 2.73 to 2.64 Ga Eastern Goldfields orogenic belt, and an older continental foreland to the west characterised by 3.0 Ga greenstone and 3.7 to 3.2 Ga gneiss-quartzite belts. The craton is eroded to a mid-crustal level exposing 2.99 to 2.60 Ga composite granite batholiths, which underlie more
than 75% of the present surface. Hannan South lies within 20 km of the western margin of the Orogen.
Hannan South is one of at least four epidote-calcite-magnetite±actinolite skarns associated with the Mount Shea high Mg monzodiorite-tonalite intrusive complex. The Hannan South orebodies are hosted by granodiorite porphyry and have been classified as oxidised endoskarns
The Mt Shea intrusions cut folded ultramafic-mafic volcanic rocks and greywackes in an antiform bounded by sinistral strike-slip fault structures of the Boulder Lefroy-Golden Mile system. In the deposit area, the intrusive complex comprises a NNW-oriented core of seriate monzodiorite that is approximately 250 m wide and >1 km long within a larger mass of granodiorite porphyry. The intrusive complex has a generally NNW-SSE trending contact with 85° northeast dipping carbonaceous metagreywackes some 50 to 100 m west of the seriate monzodiorite contact with the intervening granodiorite porphyry. At surface this contact is represented by a 65 to 70°NE dipping D3 reverse fault. The immediate footwall of this fault is occupied by a 50 to 100 m thick zone of sericite-quartz-ankerite schist with 80° NE to vertically dipping foliation. The D3 fault and schist (alteration) zone are both displaced by a north-south trending D4 structure.
The porphyry complex, dated at 2662±3 and 2658±3 Ma (zircon U-Pb), post-dates D1 folding, regional greenschist-facies metamorphism, and the main phase of D2 strike-slip faulting by ~20 m.y., and is part of a high K and high Mg calc-alkaline suite of I-type intrusions clustered along the regional NNW-SSE to north-south oriented D2 faults. They display a spatial association with gold deposits such as the Golden Mile, Mount Charlotte, Victory-Defiance and Hunt, all of which are regarded as typical "orogenic lodes" with low base metal contents and moderate to high Au/Ag ratios.
The Hannan South Au-Cu deposit comprises two separate orebodies located 450 m apart. Both are controlled by the same 65 to 70°NE dipping D3 reverse fault, which cuts granodiorite at the margin of the composite porphyry stock. The fault displaces, and the skarns overprint, the sericite-chlorite-quartz-carbonate alteration along the porphyry-greywacke contact.
The gold ore in the northwest open pit comprised a supergene blanket in saprolite centred on enriched sulphide ore in leached epidote skarn. From the surface to the pit bottom, this skarn decreased in length from 140 to 35 m, in thickness from 20 to 3 m, and in grade from 5.4 to 3.5 g/t gold, and pinched-out completely at 95 m below surface and along strike.
The SE skarn comprises a sub-economic orebody that is 8 to 13 m thick and remains open at depth.
Both skarns have a gangue composed of 15 to 30 vol.% epidote; 10 to 30% calcite; 20% quartz; 10% chlorite and 5% phengite, and the same principal oxide-sulphide assemblage of 5 to 15% magnetite and 2 to 5% pyrite. The SE skarn contains relatively high ~0.5 wt.% Cu and low <10 ppm Bi, 63 ppm Co and 25 ppm As, while the NW skarn has low <0.1 wt.% Cu but is enriched in the other elements, with 130 to 240 ppm Bi, 140 ppm Co 230 ppm As. The Au:Ag ratio of the SE skarn (0.35 g/t Au, 6 g/t Ag) is lower than that of the primary NW skarn.
The deposit was mined by open cut between 1986 and 1988 for 0.256 Mt of mixed oxide-sulphide ore at a recovered grade of 4.98 g/t Au for 1.275 t of gold.
This summary is mostly drawn from, and in places closely paraphrases, Mueller et al., 2012.
The most recent source geological information used to prepare this decription was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Mueller A G, Lawrance L M, Muhling J and Pooley G D, 2012 - Mineralogy and PTX Relationships of the Archean Hannan South Au-Cu (Co-Bi) Deposit, Kalgoorlie, Western Australia: Thermodynamic Constraints on the Formation of a Zoned Intrusion-Related Skarn : in Econ. Geol. v.107 pp. 1-24
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