MT PERCY GOLD DEPOSIT THE ROLE AND SIGNIFICANCE OF PORPHYRY - PowerPoint PPT Presentation

MT PERCY GOLD DEPOSIT THE ROLE AND SIGNIFICANCE OF PORPHYRY INTRUSIONS IN THE GOLD MINERALISATION PROCESS Daniel Sully Research supervisor: Steffen Hagemann Co-supervisor: Paul Duuring

1. Gold Industry Fourth Largest Commodity Sector in WA Gold price currently >US$ 1,350 p/oz. (>AU$ 1,800)  Important safe haven during times of crisis  Easy deposits have been found in mature districts  Need to do things differently to find new deposits 

Mineral Exploration Driving Discoveries for a Growing World Exploration geologists are responsible for finding the mines of the future Employ a wide range of technologies  Work in varied and remote locations  Changing roles and responsibilities  We commonly use deposit case studies to  guide exploration

Archean Orogenic Gold Deposits Magmatic or Metamorphic? Porphyry stocks, sills and dykes are present in many Au  deposits, particularly in Canadian deposits Spatial association of these intrusives with mineralisation  implicates them in deposit genesis The Mt. Percy gold deposit contains three types of porphyry  stocks and dykes (Groves et al., 1998)

The Kalgoorlie Gold Camp >120 years of Continuous Mining  One of the largest gold mining districts in the world (>1,200t Au); currently producing >800,000 oz. per annum  Two ultramafic-mafic volcanic sequences, deposited between 2715 and 2690 Ma  Intruded by ultramafic to felsic rocks  Two dominant mineralisation styles: Fimiston and Charlotte (ca. 2640) (Vielreicher et al., 1998)

Mt. Percy Deposit Geology (10t Au) A Complex History Sir John Pit  Ore mined from three open pits (1985-1992)  Two main stages of gold mineralisation(~2640 Ma)  Three types of intrusive porphyry  Key structures trend NE to NW Mystery Pit Union Club Pit

Early Events Formation of the Rock Pile  Deposition of Kalgoorlie sequence (2715-2690 Ma) 1km Black Flag Group  Intense alteration of HLS and DCB  Intrusion of the Williamstown (2,696±5 Ma) and Golden Mile Dolerite Paringa Basalt Golden Mile (2680±9 Ma) Dolerites Williamstown Dolerite Devon Consols Basalt  Intrusion of FQP (2670±5 Ma) and HAP(?) stocks (2650±6 Ma) followed by intense alteration Hannans Lake Serpentinite 100m FQP 2cm 2cm KS WD 2cm 2cm HLS DCB 2cm

Stage 1 Mineralisation Shear Zone Hosted Gold Mineralisation  GM thrust fault forms with the Kalgoorlie anticline as a hanging-wall anticline.  Development of stage 1 shear zone-hosted gold mineralisation during late D1 early D2  Regional NE-SW compression leads to tilting of stratigraphy during D2  Kersantite dykes are intruded ca. 2642±6 Ma 200m 500m N D 2 D 1

Stage 2 Mineralisation Brittle Vein Hosted Gold Mineralisation  NE trending, sub-vertical to NW dipping dextral-oblique-slip faults  Sheeted vein network mineralization forms ca. 2640 Ma 0.5m 200m N D 4 1cm

Intrusive Phases Temporal Association with Mineralisation Temporal Association Porphyry Type Age^ In Core with Mineralisation* Feldspar-Quartz 2670±5 Ma None Porphyry Stocks U-Pb zircon 2cm Hornblende-Albite 2650±6 Ma Overlap Possible Porphyry Stocks U-Pb zircon 2cm Kersantite 2642±6 Ma (Lamprophyre) Synchronous U-Pb zircon Dykes 2cm ^All ages from Vielreicher et al. 2010 *Gold mineralisation ca. 2640

Implications for Exploration Porphyries as an Indicator of Prospectivity?  Temporal and spatial correlations support the possibility of a genetic link between porphyries and gold mineralisation  At the mine scale porphyries are volumetrically too small to have been the only source of gold  Geochemical similarities to the Mt Shea intrusive complex, which underlies part of one of the best endowed gold belts in the world (after Mueller, 2007)

Key Learnings and Impacts Research Critical to Improved Exploration Outcomes Applied research teaches valuable skills and encourages critical and lateral thinking  Mineralisation is spatially and temporally associated with mineralisation at Mt Percy  Characterising porphyry types could help significantly in understanding the  prospectivity of gold belts Additional research now needs to be undertaken to support a genetic link 

Thank You for Listening Questions? Thanks to: Nick Hayward for advice and feedback;  Research Supervisor: Steffen Hagemann;  Co-Supervisor: Paul Duuring;  Dave Nixon  KCGM: Project Funding.  UWA, CET, UTAS and CMCA staff;  LA-ICPMS Instruction (CODES, UTAS): Leonid Danyushevsky 

References  Groves DI, Goldfarb RJ, Gebre-Mariam M, Hagemann SG, Robert F (1998) Orogenic gold deposits: a proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geology Reviews 13: 7-27  Mueller (2007) Copper-gold endoskarns and high-Mg monzodiorite – tonalite intrusions at Mt. Shea, Kalgoorlie, Australia: implications for the origin of gold – pyrite – tennantite mineralisation in the Golden Mile. Miner Depos 42:737 – 769  Bateman RJ, Hagemann SG, McCuaig CT, Swager CP (2001) Protracted gold mineralisation throughout Archaean orogenesis in the Kalgoorlie camp, Yilgarn Craton, Western Australia: structural, mineralogical, and geochemical evolution. In: Hagemann SG, Neumayr P, Witt WK (eds) World-class gold camps and deposits in the eastern Yilgarn Craton, Western Australia, with special emphasis on the Eastern Goldfields Province, Western Australia. Geol Surv Record 2001/17:63 – 98  Bateman RJ, Hagemann SG (2004) Gold mineralisation throughout about 45 Ma of Archaean orogenesis: Protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia. Miner Depos 39:536−559  Vielreicher NM, Groves DI, Snee LW, Fletcher IR, McNaughton NJ (2010) Broad synchroneity of three gold mineralisation styles in the Kalgoorlie Gold Field: SHRIMP, U-Pb, and 40 Ar/ 39 Ar geochronological evidence. Econ Geol 105:187 – 227  Vielreicher NM, Groves DI, NJ McNaughton (2016) The giant Kalgoorlie Gold Field revisited. Geoscience Frontiers 7: 359-374

Regional Correlations Similarities to Mt Shea  Oxidised, calc-alkaline, negative Nb anomalies, REE patterns, trace element patterns, ages and a wide range in: SiO 2 content, chromium, nickel, barium and Sr/Y ratios

Laser-ablation-ICPMS  LA-ICPMS analysis undertaken at the ARC Centre of Excellence in Ore Deposits (CODES), UTAS  Pyrite from stage 1 mineralisation contains high As (up to 1500 ppm) and shows very similar Ni- Co-As ratios to Fimiston style gold 100µm 100µm mineralisation  Pyrite from stage 2 mineralisation contains much lower As (up to 220ppm) and contains higher Ni-Co ratios

Porphyry Geochemistry Porphyry stocks and dykes are collectively:  of I-type origin  oxidised  calc-alkaline The porphyry-REE pattern suggests formation through fractional crystallisation from a monzodiorite parent magma