Bulyanhulu: Anomalous gold mineralisation in the Archaean of Tanzania. Claire Chamberlain, Jamie Wilkinson, Richard Herrington, Ettienne du Plessis

Transcription

1 Bulyanhulu: Anomalous gold mineralisation in the Archaean of Tanzania Claire Chamberlain, Jamie Wilkinson, Richard Herrington, Ettienne du Plessis

2 Atypical Archaean gold deposits Groves et al., 2003

3 Regional geological setting After Borg & Shackleton, 1997

4 Sukumaland geology

5 Bulyanhulu geology 85ºNE

6 Bulyanhulu geology 85ºNE Y Reef 1 vein X Pyrite Zone Co-volcanic porphyry intrusions Y X Mafics Felsics Mafics

7 Main-stage veining FW: Basalt HW: Pyrite Zone Quartz stringers 1 m

8 Pyrite Zone Occurs between main argillite unit and felsic volcanic rocks Spatial relationship with argillaceous sediments No Au association No Cu-Pb-Zn association 1 cm 1 cm 1 cm

9 Syn-volcanic pyrite chemistry 1 28 Pyrite Zone: No anomalous Cu or Au (or Pb-Zn) δ 34 S: per mil S and Fe (atomic %) S Fe As Au Ag Zn Co Cu Trace elements (atomic %)

10 Timing of metamorphism and veining 1 cm 1 cm 1 cm Basalts Rhyodacites Qtz-eye porphyries

11 Gold related to late quartz

12 Location of gold Au Qtz Au Cpy Qtz Sph Cpy Py Au 0.1 mm 0.1 mm

13 Accessory sulphides Cpy Bi-Te Bi-Te Po Py (main-stage) Po Cu Py (O) Au Po Py (overgrowth) Qtz Py 100 um Cpy Bi

14 Quartz vein/fluid stages White, mottled, preserved early quartz: No sulphide association Dark and intensely deformed late quartz: Hosts sulphides + Au

15 Early (pre-ore) quartz vein fluids δd -79 Saline 420 C

16 Ore-stage quartz vein fluids δd -79 Saline 420 C δd -50 Aqueocarbonic 380 C Magmatic ( porphyry ) fluids overprinted by aqueo-carbonic?

17 Bulyanhulu geology 85ºNE Y Reef 1 vein X Pyrite Zone Co-volcanic porphyry intrusions Y X Mafics Felsics Mafics

18 Basalts Least altered Altered 1 mm

19 Basaltic alteration traverse Major Elements Ratioed to Al 2 O 3 LOI Fe2O3 MgO CaO Na2O K2O P2O5 TiO2 SiO Major oxides (wt %) SiO 2 (wt %) Metres away from Reef 1 1.0

20 Variation in basalts Clusters Au Ag Cu Fe2 O 3 MnO LOI CaO K2 O Ba P2 O 5 TiO2 Zr Y Sr SiO2 Na2 O Al2 O 3 MgO Cr Zn Correlation coefficients Au Cu Zn Ag Fe 2 O 3 MnO LOI CaO TiO 2 Zr Y

21 Rhyodacites Least altered Altered 1 mm

22 Rhyodacites Qtz-fld porphyries Cluster analysis Au, Ag K 2 O, Ba, Fe 2 O 3, LOI, CaO, Cu, Zn Au, Cu K 2 O, Ba, Fe 2 O 3, LOI, CaO, Ag, Zn, TiO 2 Al 2 O 3, TiO 2, Zr, SiO 2 Al 2 O 3, Zr, SiO 2 Correlation coefficients + Au, Ag, Cu + Au, Cu + K 2 O, Ba, Cu, LOI, CaO, Fe 2 O 3 + K 2 O, Ba, LOI, CaO, Fe 2 O 3, MnO, Zn - Zr, Al 2 O 3, TiO 2, P 2 O 5, SiO 2 - Zr, Al 2 O 3, SiO 2, P 2 O 5

23 Principal component scores PCS 1 PCS PCS Probability PCS 1 Probability PCS 2 PCS 4-4 Felsics Gabbros Mafics Pyrite Zone QFPs Probability PCS 3 Probability PCS

24 Principal component scores PCS Mineralised 10 6 Felsics 5 Barren 2 Mafics Rock type Mineralisation Probability PCS 1 Probability PCS 2 PCS 2 PCS Least altered Most altered PCS 4-4 Felsics Gabbros Alteration Mafics Pyrite Zone QFPs Probability PCS 3 Probability PCS Fractionation or alteration? Primitive? Fractionated?

25 PCL: all rock units PCL Au Ag 0.5 Zn Ba MnO K 2 O 0.1 SiO 2 P 2 O 5 Cu LOI 0.0 Sr Fe 2 O 3 Zr TiO 2 CaO -0.1 Na 2 O Al 2 O 3 Cr Y MgO PCL 1 PLC 1 (rock type) vs PCL 2 (mineralisation)

26 PCL 1 vs PCL 2 PCL Au Ag 0.5 Zn Ba MnO K 2 O 0.1 SiO 2 P 2 O 5 Cu LOI 0.0 Sr Fe 2 O 3 Zr TiO 2 CaO -0.1 Na 2 O Al 2 O 3 Cr Y MgO PCL 1 PCL Ore Felsics 0.1 Mafics PCL 1 PLC 1 (rock type) vs PCL 2 (mineralisation)

27 PCL 1 vs PCL 3 PCL Na 2 O Al2 O 3 SiO 2 Zr Ba Sr K 2 O Au P 2 O 5 Zn Cu Ag TiO 2 MnO MgO Y CaO Cr Fe 2 O 3 LOI PCL Rock Immobiles Alteration PCL 1 PCL 1 Ore Alteration Rock Immobiles PLC 1 (rock type) vs PCL 3 (alteration)

28 Bulyanhulu: Conclusion Early saline brines, anomalous Cu and the Cu-Au association is suggestive of a magmatic ( magmatic )-related enrichment event (somewhere), that has been overprinted by lode-gold mineralization δd -79 Saline 420 C δd -50 Aqueo-carbonic 400 C

29 Lode-gold model - consistent features Terrane-scale structural control Mineralised sub-parallel structures in different host rocks Low-moderate salinity CO 2 -CH 4 -bearing fluids, homogenisation at C, with evidence of phase separation during vein formation Narrow zone of largely symmetrical alteration consistent with fluid introduction into wallrocks from shear zones Gold distributed over extensive strike and dip

30 Lode-gold model - inconsistent features Magmatic ( porphyry )-like saline fluids Overprinting orogenic aqueo-carbonic fluids Cu-enriched ore suite (+ Bi-Te, Mo) Independent statistical association of ore-suite metals: Au+Cu and Au+Ag

31 Lode-gold model - inconsistent features Magmatic ( porphyry )-like saline fluids Overprinting orogenic aqueo-carbonic fluids Cu-enriched ore suite (+ Bi-Te, Mo) Independent statistical association of ore-suite metals: Au+Cu and Au+Ag Implications - exploration in the Archaean Are there juicy (precursor) granitoids? Do multi-stage systems provide higher grade and/or tonnage deposits?