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Characteristics of Porphyry Cu-Au Systems in the Ordovician Macquarie Arc of NSW

Characteristics of Porphyry Cu-Au Systems in the Ordovician Macquarie Arc of NSW. Bruce Mowat & Stuart Smith. Outline. Introduction History of exploration and research Distribution of systems Review key geological aspects of the major deposits Characteristics of the Temora porphyry systems

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Characteristics of Porphyry Cu-Au Systems in the Ordovician Macquarie Arc of NSW

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  1. Characteristics of Porphyry Cu-Au Systems in the Ordovician Macquarie Arc of NSW Bruce Mowat & Stuart Smith

  2. Outline • Introduction • History of exploration and research • Distribution of systems • Review key geological aspects of the major deposits • Characteristics of the Temora porphyry systems • Future challenges of exploration

  3. History • 1976 Geopeko/North identifies porphyry style Cu-Au in the Goonumbla area • Initial Research, (Paul Heithersay, John Walshe) • AGSO, NSW DMR (Doon Wyborn) • Newcrest identify Cadia Hill porphyry system • AMIRA P425 (Gregg Morrison, Phil Blevin) • SPIRT (Dave Cooke, Tony Crawford, Dick Glen) • Ongoing research by Newcrest team

  4. Sydney Melbourne Macquarie Arc • Macquarie Arc is a component of the Lachlan Orogen • Ordovician to early Silurian Volcanic Province

  5. Macquarie Arc • Four Separate Belts • Junee-Narromine (JNVB) • Molong (MVB) • Rockley-Gulgong (RGVB) • Kiandra (KVB) JNVB MVB RGVB KVB

  6. Distribution of Systems • 24 porphyry systems • Most (22) occur within definable districts • 17 including all operations within Cadia and Northparkes • Districts defined by coherent geological character • Clustered Northparkes District Cowal District Cadia District Rain Hill District

  7. Northparkes District Copper Hill Cargo Macquarie Arc – Summary Time-Space Plot Temporal Distribution Cadia Lake Cowal

  8. Macquarie arc - Australia's only economic porphyry province

  9. Key Features of the Districts • Higher proportion of intrusive rocks • More complex (but not unique) magnetic signatures - most related to intrusive activity • Gravity lows • Overall more felsic • Overall more potassic

  10. Age of the Systems • 455 Ma • E43, Cargo, Copper Hill, • Low K, dacite association • adakites • 440 Ma • Cadia District, Northparkes, Rain Hill • Medium to High K, monzonite association

  11. LFB Late intrusive shoshonites (monz) 440 Evolved shoshonitic lavas suites Copper Hill-type adakitic dacite-gdt suites 450 Middle Ord high-K to (higher) shoshonitic lavas Narromine and Cowal Middle Ord Intrusive Monzodiorites etc (hi-K CA) 465 480 Nelungaloo Volcs and Mitchell Fmn- - Hi-K calc-alk and shoshonitic

  12. Igneous Character • Macquarie Arc dominated by basalts and andesite compositions • Productive districts tend to be more felsic on average • E43, Cargo and Copper Hill low-K Calc-alkaline • Dacite porphyry association (adakites) • Rain Hill District medium to high-K Calc-alkaline • Cadia and Northparkes districts are high-K to shoshonitic in character, the most potassic regions in the arc • Monzonite, syenite, latite, trachyte

  13. Alteration • Core: • potassic (biotite-mt; orthoclase-qtz-sulphide-hematite) • calc-sodic (act-mt-ab) • Phyllosilicate (sericite, hm, ab) • Distal: • propylitic (chl-carb-epi-ab-hm) • sodic (ab-chl-tm) • Phyllosilicate (sericite, albite) • Late faults: • phyllic (QSP-carbonate-base metals) • Distinctive pink rock hematite alt of intrusions & volcanics

  14. GOONUMBLA Schematic Intrusives - Alteration - Mineralisation G Morrison & P Blevin 3/96 Ap MZp MMZa MMZp ALTERATION MMZm MMZp K Feldspar-quartz MZD K Feldspar destructive MMZc Sericitic Kf network + biotite spots MMZp MMZc DI MMZa MZD GRp

  15. Northparkes potassic alteration

  16. Northparkes Potassic Alt

  17. Sericite Albite Alteration • Cadia East, Ridgeway, E26, E48 have sericite and/or albite bearing zones • These can be • Central and directly associated with ore • Proximal and directly associated with ore • Peripheral and not associated with ore • Minor associated with narrow fault zones • Distinguishing these is critical but can be very difficult

  18. Cadia East • Extensive alb-ser-tour-py-hem zone • Above and peripheral to orebody • Obscures outcrop of the orebody After Tedder et al., 2001

  19. Albite Sericite Tourmaline Alt

  20. E26 • Widespread and generally high level qtz-ser-py-alb • Highly bleached Unaltered or Propylitic Qtz-ser-py-alb Weak K-fs Strong K-fs Weak mt-bi Strong mt-bi

  21. Central Sericite Alteration • E26 & E48 both have a core zone of magmatically derived sericite +/-albite, alunite • Associated directly with bornite, chalcocite, covellite, digenite, tennantite, enargite Generally > 2%Cu

  22. E48 Proximal He-Se-Carb

  23. Propylitic Alteration • One of the greatest unknowns in Macquarie Arc porphyries • Cadia has both distal and proximal • Northparkes, possibly has distal • Strong and very widespread regional assemblage that is definitely unrelated to mineralisation • Use with extreme caution

  24. Outer Propylitic Inner Propylitic Albite-pyrite Potassic Calc-Potassic Garnet-silica Propylitic Alteration Ridgeway Cadia East 200m After Tedder et al., 2001 After Wilson et al., 2003

  25. eg. Cadia East Extensive alb-ser- tour-py alteration

  26. Regional vs Distal Porphyry • Some clues - but a lot more work needed • Fracture control • Overlap with most distal magnetite-biotite • Any low level Cu • Prehnite/actinolite • Distal Porphyry ep-chl-preh • Regional ep-chl-calc

  27. Fe-OxideDistribution

  28. Magnetite Distribution • Magnetics is the second most common targeting tool (behind simple Cu & Au geochemistry) • How well do we understand the controls on magnetite distribution and therefore the types of signatures to expect • What are the controls • Primary magnetite • Magnetite constructive alteration • Magnetite destructive alteration

  29. Alteration Magnetite • Magnetite constructive alteration • Occurs in ALL systems, but location is not always the same • All Macquarie arc systems share an early mt alteration stage • Associated with early intrusions - can be widespread -several 100 m from intrusions Distal magnetite-biotite

  30. Magnetite & Alteration • Cadia systems • Ridgeway - direct association with ore Ridgeway Cross Section Contoured Magnetic Susceptibility values; 10-5SI After Harper, 2000

  31. Northparkes • Fundamentally different character • In all known systems the ore-bearing stage overprints and destroys earlier magnetite constructive stage • Amount of early magnetite AND the degree of magnetite destruction is variable • Mt alteration is in part function of host rock composition • Intermediate hosts develop large mt halos • In felsic hosts low 1o Fe content results in lesser mt

  32. Ore-stage Mt Destruction • E26 • Major ore stage is associated with intense K-feldspar alteration • This overprints and destroys much of the magnetite-biotite alteration

  33. E26 - magnetite destructive K-feldspar Weak Remnant bi-mt alteration Moderate Intense

  34. Magnetite and Ore • Directly associated with Ore • Ridgeway • Magnetite destruction with Ore • E26 • Felsic host less Mt • Northparkes • Mafic to intermediate host more Mt • Cadia Region

  35. Metal Zoning • Cu-Au • Pipe-like systems (eg NPM, Ridgeway) show a strong zoning with Au increasing toward cores • Can be used as an exploration tool - slight systematic increase in Au:Cu should encourage further drilling

  36. Metal Zoning • Systems have traditional Cu, Zn zoning From Heithersay & Walshe, 1995 • Cu anomaly much larger than the systems • Lows within major ?peripheral Zn anomaly

  37. Preservation • Remarkably intact, little deformation • Northparkes • Intrusives vertical, 30 degree dip volcanics • Cadia • Intrusives vertical, stratigraphy flat • Cowal • intact • Rain Hill • Devonian shear zone overprint

  38. Temora Porphyry District • Goldminco Corporation holds majority of District • Junee-Narromine Volcanic Belt • 6 identified systems so far • The Dam, Mandamah, Culingerai, Estoril, Harold Bell, Yiddah

  39. TemoraGeology

  40. TemoraMagnetics

  41. Temora Porphyry Characteristics • Porphyry mineralisation clustered around margin of Rain Hill Monzodiorite • Similar setting to Northparkes • Medium to high-K calc-alkaline • Mineralisation associated with high level porphyritic monzodiorite dykes and plugs • 435 Ma age on syn to post mineral dyke • Andesitic volcanics and volcaniclastics • No felsic volcanics • Qtz poor volcanics and intrusives

  42. Temora Porphyry Mineralisation • Mineralisation • Early classic qtz-mt-py-cpy seam veins • Late coarse qtz-carb-chl-cpy veins • Alteration • Core mt-hm-biot-chl±K-feldspar • Distal phyllic ab-ser-py • Late propylitic chl-ep-carb • Devonian ser-py shear overprint

  43. Estoril porphyry Au-Cu system Qz-mt-ksp-cpy veins Chl-mt-bi alt volc Qz-mt-cpy seam vein Ep-chl overprinting Early mt-ksp alt

  44. Estoril porphyry Au-Cu system Qz-mt-cpy veins in Diorite host rock Sheeted qz-mt-cpy veins Andesite and MZDR Intrusive host rock

  45. Qz-mt-ksp-cpy veins He-mt alt MZDR Local intense He-mt Alteration MZDR

  46. Similarities to other systems • Geological Setting • Similar to Northparkes setting • Age • Late Ordovician early Silurian • Similar alteration facies • Inner Potassic and overprinting phyllic • Igneous character • Oxidised High-K intrusives • Mineralisation • Qtz-mt-cpy seam veins • Alteration and ore stage mt

  47. Differences to current economic systems • No Felsic rocks • Lack of the felsic suites (monz, trach, latite) • Limited hematite • Much less alteration hematite than Northparkes • Post mineral tectonics • Overprinted by Devonian shear zones

  48. Future Exploration • Ordovician Systems • Current model prefers the current 4 productive districts (tightly held) • Under cover Narromine-Junee • Variations on current model (Less oxidised systems) • Other Ages • Siluro-Devonian Systems (Yeoval, Bald Hill, Vic, Bushranger)

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