G2: what does it mean?

Transcription

1 G2: what does it mean? David Giles Centre for Mineral Exploration Under Cover Discipline of Geology and Geophysics School of Earth and Environmental Sciences The University of Adelaide With thanks to John Foden,, Martin Hand, Roger Skirrow and Martin Fairclough GEOLOGY AND GEOPHYSICS UNIVERSITY OF ADELAIDE, 2006 Expanding (record) undergraduate numbers ~200 1 st years in 2006, >20 honours students Similar numbers likely in 2007 Lots of post-graduate students (30-ish ish) Record numbers of post-doctoral fellows (9) New staff appointments: Steve Hill, Galen Halverson Continuing Australian Research Council success -33 Discovery Grants, 1 Infrastructure Grant and 1 Linkage Grant funded for 2007 Well-equipped, equipped, cutting-edge analytical equipment Increased collaboration with industry and PIRSA 1

2 CMXUC 2006 Full time staff from 0 to 4 Integration with University of Adelaide staff $600K, ARC Linkage grant success with G.Heinson Heinson,, K.Selway Research with: BHPB, Teck, Heathgate, Maximus, Oxiana, Dominion, Adelaide Resources PACE proposal West Gawler Craton NCRIS seismic proposal North Gawler Craton Bouguer Gravity O Driscoll,, E.T.S (1986) Phil. Trans. R. Soc. London, 317;

3 Wise, D.U (1986) Geological Society of America Bulletin, 93; Where is G2? 3

4 Why should we care? Prominent Hill Olympic Dam Carrapateena Punt Hill Because, G2 has been proposed as a fundamental control on the distribution of ore deposits. Why should we care? 4

5 Why should we care? Long (and longlonglived) deeply penetrating structures focus magmatic and hydrothermal energy and make deposits What is G2? No single structure but but a series of structures that bound gravity ridges and troughs 5

6 What is G2? Magnetic fabric continues through gravity troughs but subdued (ie (ie.. Buried!) What is G2? Gravity troughs correspond to deeper magnetic basement beneath Neoproterozoic sediments 6

7 Officer Adelaide What is G2? Gravity troughs correspond to deeper magnetic basement beneath Neoproterozoic sediments So If the gravity and magnetic signature that defines G2 is a collage of Neoproterozoic (and younger) structures: What relevance does the concept have to the IOCG world at 1590 Ma? Can we reconcile this with our understanding of the regional geology? 7

8 The lineament concept requires fundamental structures (or zones of weakness) that can be reactivated again and again through time and can propagate into successively younger sequences. Can we test this on G2? 8

9 Olympic Dam SW NE 200km Seismics and Magnetotellurics show some very interesting big structures under OD but, we don t t know their age or their geometry in 3D Profile is quite oblique to G D MT Project With BHPBilliton and Teck Cominco 3D crustal structure and mineral systems 9

10 All this is fine, however If G2 is such a long-lived lived zone of weakness, why has OD been preserved pretty much intact for 1590 million years? In fact, all the evidence suggests OD sits on a piece of crust that was protected from multiple periods of deformation and metamorphism. Nawa P&D Olarian aged metamorphism and deformation Blinman Curnamona Yorke Barossa 10

11 Neoproterozoic extension and Delamerian shortening Olarian and Delamerian deformation and metamorphism and Neoproterozoic extension was actually focused to the east of OD. Why? 11

12 Ma Hiltaba Ages Ma 1578 ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 7 Thanks to John Foden Hiltaba Suite: U vs Pb (Foden and Stewart) 80.0 PPM U PPM Pb 238U/204Pb = = second stage of the Stacey-Kramer two stage crustal lead evolution curve which supports the composition of common Pb (e.g. Broken Hill Pb-ore) W. and N. Gawler Domains Moonta/Cleve/Lincoln Domains Thanks to John Foden 12

13 Perhaps OD was only on the margin of the really hot, juicy Hiltaba magmatism and benefited by way of preservation. Which leads us to another interesting point 13

14 There are two groups of fluid inclusions in the Olympic Dam area: Moderate temperature (<200 C), moderate salinity (<30 wt%), oxidised,, surface waters High temperature (>400 C), very high salinity (>30, up to 60 wt% NaCl equiv!), relatively reduced, deeply derived Which one has the Copper? From Bastrakov, Skirrow and Barovich,, GA website 14

15 From Bastrakov, Skirrow and Barovich,, GA website From Raymond, Fletcher and McNaughton,, GA website 15

16 OD Distribution of previously unmetamorphosed volcano-sedimentary sequences potentially containing evaporites at 1590Ma Curnamona Moonta- Wallaroo To Summarise: G2 is a collage of Neoproterozoic and younger structures The jury is still out concerning the possibility of a pre-existing existing zone of weakness Don t t discount the importance of other, more quantifiable, geologic controls (eg( eg. magmatism, evaporitic source rocks) But, hold on Dave, what about the tectonic setting? 16

17 younging The Gawler Orocline 17

18 Why should we care? Long (and long- lived) deeply penetrating structures focus magmatic and hydrothermal energy and make deposits OD Distribution of previously unmetamorphosed volcano-sedimentary sequences potentially containing evaporites at 1590Ma Curnamona Moonta- Wallaroo 18

19 To Summarise: SA IOCGs are in an arc parallel belt (G2 as an arc parallel structure is plausible but not proven) IOCGs occur where magmatism intersects a basin containing evaporites (we are not in the arc) Magmatism moves inboard of the arc at 1590 Ma due to a plume(?) 19