Geochemistry & mineralogy of late-metamorphic shear zones:

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1 Geochemistry & mineralogy of late-metamorphic shear zones: Disseminated gold in the Otago Schist, New Zealand Dave Craw Geology Department University of Otago Dunedin, NZ in collaboration with: D.J. MacKenzie, D.A.H. Teagle, I. Pitcairn, R.J. Norris

2 Otago Schist: Mesozoic metasedimentary rocks Greenschist facies RSSZ Caples Terrane Shear zone Fault Subgreenschist facies rocks Lower greenschist facies rocks Upper greenschist facies rocks Torlesse Terrane HMSZ Dunedin N 20 km South Island New Zealand Alpine Fault Tasman Sea 45 S South Pacific 170 E Ocean Historic alluvial Au province: 8 M oz Minor historic Au-quartz vein mining: <400 k oz

3 RSSZ Torlesse Terrane HMSZ N 20 km Caples Terrane Shear zone Fault Subgreenschist facies rocks Lower greenschist facies rocks Upper greenschist facies rocks Dunedin South Island New Zealand Alpine Fault Tasman Sea 170 E 45 S South Pacific Ocean

4 Shear zones: At boundary between upper and lower greenschist facies Boundary is post-shear low-angle normal faults RSSZ Caples Terrane Shear zone Fault Subgreenschist facies rocks Lower greenschist facies rocks Upper greenschist facies rocks Torlesse Terrane HMSZ Dunedin N 20 km South Island New Zealand Alpine Fault Tasman Sea 45 S South Pacific 170 E Ocean Au mineralisation but little quartz veining

5 SW Hyde Macraes Shear Zone Shag Valley NE Shear zone Footwall Fault Syn-Mineralisation Thrusting Macroscopic Folding Chlorite Zone Schist Garnet-Biotite Zone Schist km No vertical exaggeration SW 100 metres NE Altered, sheared, folded schist Rise & Shine Creek Chlorite Zone Schist Biotite Zone Schist

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7 Macraes mine Nearly 2 million ounces Au produced since 1990 startup of new mine Production > oz/year, from ore with 1.5 g/tonne Au Total resource > 5 million oz Shear zone Rolling grassland, little outcrop

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9 Mineral transformations metamorphic muscovite => hydrothermal muscovite => illite titanite => rutile + calcite + quartz epidote => kaolinite + siderite + calcite methane + carbon dioxide + mature kerogen => graphite Fe-silicates + As, S in solution => pyrite + arsenopyrite Au bisulfide complex + graphite => native gold Fe-silicates + S in solution => chalcopyrite + sphalerite + galena Subtle hydrothermal alteration Difficult to detect in outcrop or hand specimen Well-defined shears are black (graphite, sulphides)

10 0.01 mm 0.01 mm GOLD GOLD

11 Subtle geochemical signature Si Mineralised rock, ppm As Nb U Ni Cr La Cu Pb Th S V Rb Zn Ba Sr Zr Cr C P Host rock, ppm Ti Ca Fe Mg Na K Uniform element Al Macraes distinctive element Si As

12 Sb locally enriched with As Sb, ppm PIXE image: C. Ryan 100 µm As, ppm host rocks A Macraes mineralised rocks

13 Arsenopyrite Pyrite 0.1 mm Micaceous schist Graphitic microshears

14 Intense late metamorphic alteration and brittle/ductile deformation Pyrite, arsenopyrite, graphite 0.1 mm Recrystallised quartz, muscovite lenses separated by graphitic microshears

15 Infra-red spectroscopy Lower greenschist facies carbonaceous material Otago Schist Graphitic host rock Kerogen C-H-O lines Macraes mine graphite or hydrothermal addition CH 4 + CO 2 = 2C + 2H 2 O Standard graphite Wavenumber, cm -1

16 Nitrogen content, ppm Elevated N content of micas Macraes mineralised schist Otago Schist metamorphic phengite δ 15 N(per mil)

17 RSSZ Torlesse Terrane HMSZ N 20 km Caples Terrane Shear zone Fault Subgreenschist facies rocks Lower greenschist facies rocks Upper greenschist facies rocks Dunedin South Island New Zealand Alpine Fault Tasman Sea 170 E 45 S South Pacific Ocean

18 SW Hyde Macraes Shear Zone Shag Valley NE Shear zone Footwall Fault Syn-Mineralisation Thrusting Macroscopic Folding Chlorite Zone Schist Garnet-Biotite Zone Schist km No vertical exaggeration SW 100 metres NE Altered, sheared, folded schist Rise & Shine Creek Chlorite Zone Schist Biotite Zone Schist

19 Mineral transformations metamorphic muscovite => hydrothermal muscovite => illite titanite => rutile + calcite + quartz epidote => kaolinite + siderite + calcite methane + carbon addition dioxide + of mature ankerite kerogen => graphite Fe-silicates + As, S in solution => pyrite + arsenopyrite Au bisulfide complex + graphite => native gold Fe-silicates + S in solution => chalcopyrite + sphalerite + galena Subtle hydrothermal alteration, like Macraes No graphite formation during mineralisation More prominent carbonate alteration than Macraes

20 Rise & Shine shear: geochemically subtle, but different from Macraes Si Mineralised rock, ppm As As Th Nb U Nb U Ni Cu Pb Th S Cr La S V Rb Zn Ba Sr Zr Cr C P C Host rock, ppm Ti Ca Fe Mg Na K Uniform element Al Macraes distinctive element Rise & Shine distinctive element Si As As

21 N, O, S isotopes same as host rocks kerogen magmatic (MORB) schist host Macraes metamorphic rocks -10 δ 15 N, ROCKS +20 magmatic meteoric R&S schist host Macraes δ 18 O, FLUID +20 bacteriogenic magmatic schist host Macraes seawater δ 34 S, SULFIDES

22 CO 2 + CH 4 = 2C (graphite) + 2H 2 O (isotopically light hydrogen) meteoric graphitisation schist host Macraes magmatic δd, FLUID magmatic meteoric schist host R&S Macraes δ 18 O, FLUID +20 graphite equilibrium schist host R&S magmatic Macraes δ 13 C, FLUID

23 Conclusions Late metamorphic shears without quartz veins are important exploration targets for Au, BUT: Alteration is geochemically subtle Two nearby shear zones are geochemically different Arsenic is the most prominent indicator, but it is localised in the most prominent shears only Reactions in common: Formation of rutile from titanite, and Fe-carbonate from epidote