Geochemistry & mineralogy of late-metamorphic shear zones:

Disseminated gold in the Otago Schist, New Zealand

Dave Craw

Geology DepartmentUniversity of OtagoDunedin, NZ

in collaboration with:D.J. MacKenzie, D.A.H. Teagle, I. Pitcairn, R.J. Norris

Otago Schist:

Mesozoic metasedimentary rocks

Greenschist facies

Historic alluvial Au province: 8 M oz

Minor historic Au-quartz vein mining: <400 k oz

Subgreenschist facies rocks

Lower greenschist facies rocks

Upper greenschist facies rocks

FaultShear zone

Caples Terrane

20 km

N

HMSZ

RSSZTorlesse Terrane

Dunedin

South IslandNew Zealand

TasmanSea

AlpineFault

SouthPacificOcean

45°S

170°E

Subgreenschist facies rocks

Lower greenschist facies rocks

Upper greenschist facies rocks

FaultShear zone

Caples Terrane

20 km

N

HMSZ

RSSZTorlesse Terrane

Dunedin

South IslandNew Zealand

TasmanSea

AlpineFault

SouthPacificOcean

45°S

170°E

Shear zones:

At boundary betweenupper and lower greenschist facies

Boundary is post-shear low-angle normal faults

Au mineralisation but little quartz veining

Subgreenschist facies rocks

Lower greenschist facies rocks

Upper greenschist facies rocks

FaultShear zone

Caples Terrane

20 km

N

HMSZ

RSSZTorlesse Terrane

Dunedin

South IslandNew Zealand

TasmanSea

AlpineFault

SouthPacificOcean

45°S

170°E

SWHyde Macraes Shear Zone

ShagValley

Garnet-Biotite Zone Schist

?

?

?

Chlorite Zone Schist

Syn-MineralisationThrustingMacroscopic Folding

Shear zone

Footwall Fault

NE

?

Chlorite Zone Schist

Biotite Zone Schist?

?

SW NE

Rise & Shine Creek

0 1 2 3km

No vertical exaggeration

100 metres

Altered, sheared, folded schist

Macraes mineNearly 2 million ounces Au produced since

1990 startup of new mineProduction >170 000 oz/year, from ore with 1.5 g/tonne AuTotal resource > 5 million oz

Shear zone

Rolling grassland, little outcrop

titanite => rutile + calcite + quartzepidote => kaolinite + siderite + calcite

methane + carbon dioxide + mature kerogen => graphiteFe-silicates + As, S in solution => pyrite + arsenopyrite

Fe-silicates + S in solution => chalcopyrite + sphalerite + galena

metamorphic muscovite => hydrothermal muscovite => illiteMineral transformations

Au bisulfide complex + graphite => native gold

Subtle hydrothermal alteration

Difficult to detect in outcrop or hand specimen

Well-defined shears are black (graphite, sulphides)

0.01 mm 0.01 mm

GOLD

GOLD

1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000 1000000

Si

Al

As

Fe

KCa

NaMg

Ti

C

Ba

ZrCr

Zn

VRb

Ni

PbTh

Cr

P

La

Sr

Host rock, ppm

Min

eral

ised

rock

, ppm

U

NbCu

Uniform element

Macraes distinctive element As

Si

S

Subtle geochemical signature

0

1020

3040

5060

70

10 100 1000 10000 100000As, ppm

Sb, p

pm

host rocks

Macraes mineralised rocks

A

100 µm

Sb locally enriched with As

PIXE image: C. Ryan

0.1 mm

PyriteArsenopyrite

Micaceous schistGraphitic microshears

0.1 mm

Pyrite, arsenopyrite, graphite

Recrystallised quartz, muscovite lenses separated by graphitic microshears

Intense late metamorphic alteration and brittle/ductile deformation

4000 3000 2000 1000Wavenumber, cm-1

Standard graphite

Macraes mine graphite

Lower greenschist faciescarbonaceous materialOtago Schist

Kerogen C-H-O lines

Infra-red spectroscopy

Graphitic host rock?

or hydrothermal addition?

CH4 + CO2 = 2C + 2H2O

0

200

400

600

800

1000

1200

1400

0 1 2 3 4 5 6 7 8δ15N(per mil)

Nitr

ogen

con

tent

, ppm

Macraes mineralised schist

Otago Schist metamorphic phengite

Elevated N content of micas

Subgreenschist facies rocks

Lower greenschist facies rocks

Upper greenschist facies rocks

FaultShear zone

Caples Terrane

20 km

N

HMSZ

RSSZTorlesse Terrane

Dunedin

South IslandNew Zealand

TasmanSea

AlpineFault

SouthPacificOcean

45°S

170°E

SWHyde Macraes Shear Zone

ShagValley

Garnet-Biotite Zone Schist

?

?

?

Chlorite Zone Schist

Syn-MineralisationThrustingMacroscopic Folding

Shear zone

Footwall Fault

NE

?

Chlorite Zone Schist

Biotite Zone Schist?

?

SW NE

Rise & Shine Creek

0 1 2 3km

No vertical exaggeration

100 metres

Altered, sheared, folded schist

titanite => rutile + calcite + quartzepidote => kaolinite + siderite + calcite

methane + carbon dioxide + mature kerogen => graphiteFe-silicates + As, S in solution => pyrite + arsenopyrite

Fe-silicates + S in solution => chalcopyrite + sphalerite + galena

metamorphic muscovite => hydrothermal muscovite => illiteMineral transformations

Au bisulfide complex + graphite => native gold

addition of ankerite

Subtle hydrothermal alteration, like Macraes

No graphite formation during mineralisation

More prominent carbonate alteration than Macraes

1

10

100

1000

10000

100000

1000000

1 10 100 1000 10000 100000 1000000

Si

Al

As

Nb

Fe

KCa

NaMg

Ti

C

Ba

ZrCr

Zn

VRb

Ni

PbTh

Cr

P

La

Sr

Host rock, ppm

Min

eral

ised

rock

, ppm As

C

U

U

Th

NbCu

Uniform element

Macraes distinctive element

Rise & Shine distinctive element

As

As

Si

SS

Rise & Shine shear: geochemically subtle, but different from Macraes

N, O, S isotopes same as host rocks

schist host

0 +10-10

magmatic

+20

meteoricMacraesR&S

schist hostmagmatic(MORB)

kerogen

Macraes

0 +10-10 +20

metamorphic rocks

schist host

0-20-40 +20

bacteriogenic

Macraesseawater

magmatic

δ34S,‰

δ18O,‰

δ15N,‰

FLUID

SULFIDES

ROCKS

CO2 + CH4 = 2C (graphite) + 2H2O (isotopically light hydrogen)

-100 -50-150 0

meteoric magmatic

graphitisation Macraes

δD,‰ FLUID

schist host

schist host

0 +10-10

magmatic

+20

meteoricMacraesR&S

δ18O,‰ FLUID

schist host

-10 -5-15 0

graphite equilibrium

magmatic

MacraesR&S

δ13C,‰ FLUID

Conclusions• Late metamorphic shears without quartz veins are

important exploration targets for Au, BUT:

• Alteration is geochemically subtle

• Two nearby shear zones are geochemicallydifferent

• Arsenic is the most prominent indicator, but it is localised in the most prominent shears only

• Reactions in common:Formation of rutile from titanite, andFe-carbonate from epidote