Cu-Co Mineralization and Geotectonic Evolution of the Zambian Basin.

Transcription

1 Cu-Co Mineralization and Geotectonic Evolution of the Zambian Basin. Steve Roberts 1 School of Ocean and Earth Science, University of Southampton, Southampton, UK Plus: Mike Richards; Ross McGowan; Robin Bernau; James Nowecki; Adrian Boyce; Bruce Nesbitt; Alistair Beach Brussels 2010 Geological Society London 1

2 The Zambia Copperbelt and Domes Region Lumwana Nchanga 2

3 Geotectonic Setting Part of the 900km long Lufilian Arc a Ma Fold Belt formed during the collision of the Angola-Kalahari and Congo- Tanzania plates 3

4 Copperbelt 4

5 Geology of Nchanga Mine Produced > % Cu. Reserves in region of % Cu.

6 Nchanga Mine 6

7 Feldspathic Quartzite Ripple Marks Phlogopite Rich Fault Zone 7

8 8

9 Nchanga Mine D D. Shale overlying lower orebody C C. Sample of lower ore-body B A 2 1 B. Lower arkose in vicinity of mineralization, Fe-oxide staining and development of malachite A. Lower arkose, coarse feldspars with shear zones evident within samples. 9

10 Nchanga Mine D A C D. Sericite quartz alteration of Upper Orebody in NE511 B A C. Carrollite bearing upper arkose with variable amounts of dolomite B. Sample of weakly mineralised Pink Arkose. A. Biotite/phlogopite alteration front preserved within arkose in lower section of NE511 Borehole 10

11 Ore Petrology and Hydrothermal Alteration

12 Ore Grade Distribution and Metal Enrichments Depth (m) Depth (m) Dolomite Schist UBS Enriched TFQ BSS LBS Arkose Granite Depleted Total Copper (%) Total Cobalt (%) Down-hole grades of copper and cobalt mineralization for borehole NOP 741, Nchanga Open Pit (assay grades from mine data) and Grant Diagram. 12

13 Phlogopite Key Points (1) Phlogopite replaces earlier detrital Muscovite. (2) Up sequence variation in Mg# Ratio. (3) Phlogopites are F-rich Wt% F Phlogopite Mg/Mg+Fe Arkose Min Arkose PQ TFQ Min TFQ Granite SZ DolSchist SZ BandSS SZ Arkose Mica PQ Mica TFQ Mica 13

14 NE511-6P A NOP723-13P B Dol 100µm 50µm NE532-1P C NE511-5P D Dol Dol Dol 50µm 100µm NOP723-19P E NE554-6P F Mal Mal 100µm 100µm 14

15 Carbonate Stable Isotope Data Upper Roan Dolomites Shear Zone dolomite Upper Orebody dolomite alteration 4! 13 C (PDB) ! 18 O (SMOW) From Selley et al

16 Carbonate Sr Isotope Data Sr/ Sr WR Arkose > Malachite Upper Orebody Dolomitic Schist Intial Ratio at 880Ma Upper Orebody Dolomitic Schist Data from Muchez et al Neoproterozoic seawater /Sr 16

17 Sulfur Isotope Data 17

18 Fluid Inclusion Data Nchanga study compared to fluid inclusion data from the Chambishi and Musoshi deposits, Zambia (Annels, 1989; Richards et al., 1988), some Irish Pb-Zn orefield deposits, and classic Mississippi Valley Type fluids (Trude & Wilkinson, 2001). 18

19 Geology Chingola C Geology of the Chingola C deposit where a NE-verging recumbent anticline structure has developed where basement schists are thrust over Lower Roan stratigraphy 19

20 Distribution of mineralization at Chingola B & C The distribution of mineralization at Chingola C (based on copper grade boundaries) plotted on to the section geology. High-grade mineralization is strongly related to thrust structures. The distribution of mineralization at Chingola B (based on copper grade boundaries) plotted on to the section geology. Mineralization is strongly related to the fault-propagation folds and controlling thrusts, as well as basal detachments. 20

21 Nchanga Giant geochemical anomaly. Lithostratigraphic and structural controls on distribution of ore. Evidence for hydrothermal alteration (phlogopite/k feldspar/dolomite). Evidence for thermochemical sulfate reduction in the generation of ore bodies. Passage through the system of highly saline brines Evidence for interaction of mineralized fluids with basement lithologies Characteristics of ore minerals as a result of initial mineralization and subsequent oxidation. 21

22 The Zambia Copperbelt and Domes Region Lumwana Nchanga 22

23 Lumwana From Selley et al

24 Dominantly Ma qtz fs bio gneisses schists and granites - similar to Kafue Anticline in Zambian Copperbelt. Includes thrust emplaced Lower Roan and Upper Roan metasediments. 24

25 Lumwana million tonnes of ore grading 0.74% Cu, plus inferred resources of million tonnes at 0.63% Cu. 25

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27 Sulfide deformation and kyanite relationships 27

28 28

29 Chimiwungu Malundwe Ore Schist Ore Schist Hangingwall Schist Hangingwall Schist Hanging Wall Gneiss Hanging Wall Gneiss 29

30 The prograde history of the Ile d Yeu orthogneiss was marked by sharp strain localization along shear zones. During shearing, extensive fluid channelling led to a change from a Qtz+Pl+Kfs+Bt±Ms mineralogy to more aluminous micaschist assemblages made of Bt+Ms+Qtz±Ky. Mass transfers record gains in H2O, K, Mg, P, Rb, W, Sn, and losses in Ca, Na, Sr and Pb. 30

31 7 6 Sulfur Isotope Data Chimiwungu 5 Count Count Malundwe Hangingwall Gneiss Sulfide Mineralogy and Isotopes Count D34S δ S 31

32 Some Thoughts on Origin of Lumwana Mineralization 530 Ma Discrete ore-horizons within shear zones developed within granitic gneiss host rocks. Elemental mobility as anticipated for high grade shear zones Ma Thermochemical reduction of sulfate phases. Evidence suggest mineralization of basement to the Copperbelt ores rather than metamorphosed L Roan lithologies. 32

33 Katanga Supergroup Lower Roan U Roan Geological and Stratigraphic Setting of Ore Nchanga + Nkana +Chambishi etc. etc. Basement Lumwana + (Samba) 33

34 Basement Mineralization in the Copperbelt Schematic Geology of the Samba Deposit ( Wakefield, 1978) 34

35 Sericite schist and foliated porphyritic rock at Samba 35

36 Katanga Supergroup Lower Roan U Roan Kansanshi, Frontier Geological and Stratigraphic Setting of Ore Nchanga + Nkana +Chambishi etc. etc. Basement Lumwana + (Samba) 36

37 Tectonic Setting and Timing of Ore Formation 1 Nkana Brems et al

38 Tectonic Setting and Timing of Ore Formation 2 Kansanshi Key Facts Vein style deposit Postdates metamorphism Re-Os age of Molybdenite within chalcopyrite Veins Re-Os 511 +/- 1.7Ma From Torrelday et al

39 Kansanshi Mine Courtesy First Quantum Minerals 39

40 Kansanshi Qtz-Carbonate Veins in NW Pit 40

41 Kansanshi Qtz-Carbonate Vein, NW Pit 41

42 Kansanshi Grade Maps Middle Clastics Main Pit Middle Clastics NW Pit 42 Courtesy of First Quantum Minerals

43 Veins at Kansanshi 43

44 Luiswishi-Kamoto-Musonoi From Muchez et al 2008 Cross section through Luiswishi ore deposit 44

45 From Muchez et al

46 Copper Sulfides and Fluid Inclusions of the Copperbelt Musoshi T Homog. (oc) Kamoto Type 2 Kamoto Type1 Musoni/Kamoto Kansanshi Nchanga High Angle Veins Lower Orebody Upper Orebody Chambishi Salinty Wt% Equiv. NaCl 46

47 Schematic Geotectonic Setting of some Zambian Copperbelt Ore Deposits 1 Basin Rift Event Konkola/Chambishi 4 Post Kinematic Kansanshi 2 3 Basin Inversion Nchanga Basement Shear Zones. Lumwana/Samba 5 Modified by Compresional Tectonics Nkana/Chibuluma 47

48 Wessex Basin Basin setting and the mechanism of regional exhumation exerted a fundamental control on processes that determined fluid flow, heat flow and hydrocarbon formation and 48 distribution within a basin Bray et al. 1998

49 Conclusions Recent mineral exploration and research has resulted in a substantive expansion of the geological setting and characteristics of ore forming environments within the Copperbelt. These new data compel the development of an exploration paradigm which recognises this variety of settings and in particular the importance of structures which facilitate significant amounts of fluid flow, wherever these occur in the basin in terms of spatial location and timing. The timing and location of ore deposition ranges from basement to lower Roan and it is increasingly evident that world class ore deposits can form within Basement, Lower Roan and even stratigraphy of the Upper Roan and above. This further enhances the exploration potential of the Zambian Copperbelt. 49