Multiple S and Fe isotope composition of detrital pyrite as a tool

for provenance analysis

Axel Hofmann, Andrey Bekker, Olivier Rouxel, Doug Rumble and Sharad Master

Witwatersrand Basin

• the world’s largest gold field• ca. 50 000 t of gold produced since 1886• significant quantities of uranium as

byproductCentral Johannesburg, 1890’s Central Johannesburg, today

Frimmel, 2005

Mineralisation

• quartz pebble conglomerates (“reefs”) 0.1 – 2 m thick

• 3 – 25 g/t Au; 270 g/t U (average of mined reefs)

• micron-sized Au particles; rounded pyrite and uraninite grains

• strong hydrothermal overprint and secondary mineral growth

NASA Visible Earth

Soweto

JoburgCBD

Main Reef

Pyrite

Uraninite

Rounded uraninite

Rounded pyrite

Pyrite

Pyrite

Pyrite

Au

Mineralisation models

• placer model– gold (+pyrite/uraninite) are detrital in origin

• hydrothermal model– gold (+pyrite/uraninite) are hydrothermal in

origin• “the most disputed issue in the history of

economic geology” (Davidson, 1965)

Kirk 2002

Mineralisation models

• modified placer model– gold (+pyrite/uraninite)

is of detrital origin– short-range mobilisation

and recrystallisationduring metamorphism, hydrothermal alteration and hydrocarbon migration between 2.7 and 2.0 Ga

Kirk 2002

Geologic O2 Indicators

Holland (1994), from J. Kasting

Rounded pyriteWits Huronian Jacobina

• rounded pyrite formed as a result of hydrothermal sulphidation of Fe-Ti oxides

• uraninite is hydrothermal• Au, pyrite and uraninite are associated with bitumen,

which is of secondary origin

Objectives

• characterise the multiple-S and Fe isotope composition of rounded pyrites

• determine the origin of the pyrites (hydrothermal vs detrital)

• if detrital, determine the provenance of the pyrite (e.g. VMS, shale-hosted etc)

Sulphur mass-independent fractionation data

Farquhar et al., 2007

∆33S ≈ δ33S - 0.515*δ34S

Fe isotopes

Rouxel et al. 2005

Dauphas and Rouxel 2006

Pyrite types

• rounded massive py• rounded porous py• authigenic py

Frimmel 2005

Belingwe belt• Upper Greenstones, Ngezi Group

(ca. 2.8? - 2.65 Ga)

• Lower Greenstones, MtshingweGroup (ca. 2.9 - 2.8 Ga)

Manjeri Formation

Manjeri Formation, Jimmy Member

Grassineau et al. 2006

Cheshire Formation

Dominion Reef (3.08 Ga)

0.16cubic pyrite in amygdaloidal lavaDR-3

0.36several rounded pyrite grainsDR2

-0.17chalcopyrite in quartz vein in basement granitoidDR-1

δ56Fe

Mozaan Contact Reef (2.95 Ga)

-1.12PO-RP2

-1.09PO-RP1

δ56Fe

Kimberley Reef (2.85 Ga), VCR (2.72 Ga), Black Reef (2.64 Ga)

Kimberley Reef Ventersdorp Contact Reef

Kimberley Reef (2.85 Ga), VCR (2.72 Ga), Black Reef (2.64 Ga)

Kimberley Reef (2.85 Ga), VCR (2.72 Ga), Black Reef (2.64 Ga)

Conclusions

• rounded pyrites are detrital in origin• Cheshire pyrites are derived from Manjeri

massive sulphides• Witwatersrand pyrites have mixed provenance

– some are igneous/ high-T hydrothermal (e.g. Dominion Reef)

• lack of NMDF signal, near crustal δ56Fe values (0.1±0.2‰)

– most are sedimentary in origin• Fe isotope fractionation with or without NMDF of S