Geology of the Wirrda Well IOCG Deposit

Transcription

1 Geology of the Wirrda Well IOCG Deposit 10 th Annual SA Exploration and Mining Conference BHP Billiton Olympic Dam and CODES (University of Tasmania) 28 th November 2013

2 Acknowledgments Stuart Shelf Exploration Team: Mick Carew, Stuart McCallum, Dean Lovett, Glen Collins, Glen Diemar, Riccardo Guerra, Matthew Cusick University of Tasmania: Jocelyn McPhie, Dima Kamenetsky, Maya Kamenetsky, Jesse Cotterill (Honour s 2012) Slide 2

3 Disclaimer Reliance on Third Party Information The views expressed herein contain information that has been derived from publicly available sources that have not been independently verified. No representation or warranty is made as to the accuracy, completeness or reliability of the information. This presentation should not be relied upon as a recommendation or forecast by BHP Billiton. Forward Looking Statements This presentation includes forward-looking statements within the meaning of the U.S. Securities Litigation Reform Act of 1995 regarding future events and the future financial performance of BHP Billiton. These forwardlooking statements are not guarantees or predictions of future performance, and involve known and unknown risks, uncertainties and other factors, many of which are beyond our control, and which may cause actual results to differ materially from those expressed in the statements contained in this presentation. For more detail on those risks, you should refer to the sections of our annual report on Form 20-F for the year ended 30 June 2012 entitled Risk factors, Forward looking statements and Operating and financial review and prospects filed with the U.S. Securities and Exchange Commission. No Offer of Securities Nothing in this presentation should be construed as either an offer to sell or a solicitation of an offer to buy or sell BHP Billiton securities in any jurisdiction. Non-GAAP Financial Information BHP Billiton results are reported under International Financial Reporting Standards (IFRS). References to Underlying EBIT and EBITDA exclude any exceptional items. A reconciliation to statutory EBIT is contained within the profit announcement, available at our website Slide 3

4 Contents Wirrda Well Summary Basement Geology Alteration Mineralisation Comparisons with Olympic Dam Conclusions Slide 4

5 Wirrda Well Summary Located ~25 km SE of Olympic Dam Total of 51 holes drilled over 16 km 2 area WRD1 drilled Aug 1980 by WMC 26 diamond holes from diamond holes from IOCG mineralisation located in two zones intersecting structural lineaments brecciated megacrystic granite mafic to ultramafic dykes transition from mag-py-cp at depth to hem-bncc assemblages open laterally and at depth Less evolved hydrothermal breccia system compared to Olympic Dam more akin to Carrapateena localised breccia pipes within larger geophysical footprint Acropolis Falcon Gravity Image Residual Olympic Dam Wirrda Well Snake Gully Slide 5

6 Wirrda Well Basement Geology Divided into North and South Zones Differences in alteration and mineralisation characteristics. General observations Cu-Au mineralisation hosted in megacrystic granite (Donington; 1850 Ma) and mafic dykes (also 1850 Ma) Medium-grained granite to NE interpreted at Hiltaba-suite (~1590 Ma) Felsic volcanic rocks in the north and south-west interpreted as GRV (~1590 Ma) Alkaline picrite dykes (~1590 Ma) Slide 6

7 Pre-mineralisation Felsic Intrusions Diagnostic characteristic: megacrystic K-feldspar Donington Suite granitoid (1850 Ma) Slide 7

8 Pre-mineralisation Mafic Dykes Diagnostic characteristic: ppm Cr, olivine-phyric Dykes are a few centimeters to 10s of meters wide Tournefort Metadolerite (?), may be much younger Slide 8

9 Pre-mineralisation Ultramafic (?) Dykes Diagnostic Characteristic: >1000 ppm Cr, olivine-phyric Dykes are a few centimeters to 10s of meters wide, often assoc with mafic dykes Tournefort Metadolerite (?), may be much younger Slide 9

10 Pre-mineralisation Deformation Mylonitisation of granite and mafic dykes, most intense in mafic dykes Zones are a few centimeters to <20 meters wide. Slide 10

11 Pre- to Syn-mineralisation Felsic Volcanics Diagnostic Characteristic: ~ ppm Zn and Pb Weak to strong hematite alteration Correlated with GRV Slide 11

12 Pre- to Syn-mineralisation Alteration Diagnostic Characteristic: anomalous Cr (mafic ultramafic) Weak to intense potassic (± magnetite) alteration along microfractues Often incorrectly logged as felsic dykes because of presence of K-feldspar Slide 12

13 Brecciation of Granites and Dykes Simple breccias, granite and/or mafic dyke clasts Brecciation often initiates along dyke-granite contact This example has intense hem-ser alteration of earlier mag-chlorite-carbonate Slide 13

14 Syn- to Post-mineralisation Alteration WRD34 Diagnostic Characteristics: up to ppm Cr, but with anomalous V and Ti Intense sericite to sericite-hematite alteration of mafic-ultramafic dykes Often incorrectly logged as felsic dykes Slide 14

15 Post-mineralisation (?) Alkaline Picrites WRD33 Diagnostic Characteristics: ppm Cr, fresh biotite Olivine-phlogopite-phyric dykes are a few to <20 meters wide Chromite inclusions in olivine, abundant apatite (dated ~1590 Ma) Slide 15

16 Mineralisation: ~0.5% Cu (py-cp) Magnetite-chlorite (Mg)- siderite/calcite-apatite initially occurs as microveinlets. Py-cp grains occur in the microveinlets. The frequency and size of the microveinlets increases as the magnetite increases. Slide 16

17 Mineralisation: +1% Cu (cp-py) Intense Fe-oxide alteration of mafic-ultramafic ± weak brecciation Magnetite-chlorite (Mg)- siderite/calcite-apatite, hematite pseudomorphs magnetite Interstitial py-cp grains, often in carbonate microveinlets Slide 17

18 Mineralisation: +1% Cu (bn-cc) Clast-rich breccia, with megacrystic granite and/or mafic-ultramafic dyke clasts Hematite alteration, bn-cc grains in matrix Weak zones of supergene oxidation (e.g. covellite rims on idaite) Slide 18

19 Wirrda Well Northern Zone 0 L -200 L WRD44 WRD17 WRD33 WRD53 WRD50A/WRD43 WRD1 WRD52 WRD35/WRD45 Interpreted GRV associated with postmineralisation block faulting(?) Dykes typically thicker compared to southern zone -400 L Donington granite -600 L Mafic dykes (undifferentiated) Picrite Cu-Au mineralisation commonly associated with dyke/granite contacts -800 L L Gawler Range Volcanics Cover sequence (undifferentiated) Hydrothermal breccia L >1 % Cu >0.5 % Cu L Lower extent of cc/bn Top of py/cpy zone deeper in north compared to southern zone L L L Post-mineralisation picrite Upper extent of py/cpy Drill Hole (out of section envelope) Drill hole trend introduction of magnetite Depth of late oxidation Slide 19

20 Wirrda Well: Southern Zone WRD12 WRD29 WRD15 WRD20WRD30 WRD22 0 L Donington granite Mafic dykes (undifferentiated) Picrite -200 L Gawler Range Volcanics Cover sequence (undifferentiated) Hydrothermal breccia -400 L >1 % Cu >0.5 % Cu -600 L Lower extent of cc/bn Upper extent of py/cpy Drill Hole (out of section envelope) Drill hole trend introduction of magnetite Chalcocite zone less prominent in southern zone -800 L L Depth of late oxidation Larger interval of >1 % Cu in south compared to northern zone L Slide 20

21 Fe, Cu, U3O8, Au Cu (wt %) Au (ppm) Conditional Means Plots Highest Cu and U 3 O 8 grades associated with Fe content between 40 to 60 % Cu grades on average higher in southern zone compared to north relative to Fe Fe (wt %) Cu (wt %) Au and U 3 O 8 higher in south compared to north relative to Cu U 3 O 8 (ppm) U3O8 (ppm) Northern Zone Southern Zone Suggests higher Cu, Au, U 3 O 8 tenor relative to Fe in south not a factor of barren hematite in north Fe (wt %) Cu (wt %) Slide 21

22 Wirrda Well Timing unconformity unconformity v v v v v v v v v v Stuart Shelf Cover Sequence Gawler Range Volcanics (1590 Ma) mag-ap-cb-py-cp microveins brecciation + /- sulphides K-feldspar-quartz veins post-mineral alkaline picrite dykes megacrystic granite (1850 Ma) pre-mineral mafic dykes Slide 22

23 Similarities and Differences to Olympic Dam Key Similarities Dominant host-rocks are felsic intrusions Abundance of mafic dykes Vertical zonation of sulphides py cp bn cc physiochemical changes (P,T, ph, fo 2, SO 2 ) Gangue mineralogy mag/hem-chl-ser-cb-qtz-flu-bar Uranium mineralogy coffinite, uraninite, brannerite Paragenesis broadly similar early sericite-chlorite Fe oxide and assoc Cu-Au mineralisation FeCO 3 -K-Ba-F-rich alteration assemblages (OD paragenesis is more complex) Key Differences OD: Pre- and syn-mineralisation ultramafic dykes deep mantle-tapping structures WW mafic dykes not source from same crustal depths OD: lower proportion of mag-rich assemblages magnetite does increase with depth at OD WW: localised monomictic brecciation with higher clast:matrix ratio brecciation confined to the N and SE zones High grade (>1% Cu) focussed within subvertical breccia pipes OD: intense brecciation /high-grade Cu more pervasive/continuous within geophysical footprint Name, Position, Department, Day Month Year Slide 23

24 Wirrda Well Summary and Conclusions Wirrda Well less evolved end-member of the IOCG family High-grade Cu mineralisation in the north and southeast associated with intersecting structural lineaments elevated gravity (>6 Mgal) intense/larger brecciation zones Lower-grade Cu mineralisation associated with veins/breccia zones focussed along zones of weakness in host rock (e.g. contacts between granite and mafic-ultramafic dykes) Grade distribution at Olympic Dam more continuous/pervasive than at Wirrda Well mag-cp-py hem-bn-cc Gelling : An operational challenge due to mineralogy 28 th November 2013 Slide 24

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26 Wirrda Well: Mineral Paragenesis Slide 26