Bulyanhulu: Anomalous gold mineralisation in the Archaean of Tanzania Claire Chamberlain, Jamie Wilkinson, Richard Herrington, Ettienne du Plessis
Atypical Archaean gold deposits Groves et al., 2003
Regional geological setting After Borg & Shackleton, 1997
Sukumaland geology
Bulyanhulu geology 85ºNE
Bulyanhulu geology 85ºNE Y Reef 1 vein X Pyrite Zone Co-volcanic porphyry intrusions Y X Mafics Felsics Mafics
Main-stage veining FW: Basalt HW: Pyrite Zone Quartz stringers 1 m
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
Syn-volcanic pyrite chemistry 1 28 Pyrite Zone: No anomalous Cu or Au (or Pb-Zn) δ 34 S: 0.09 12.05 per mil 70 0.5 S and Fe (atomic %) 60 50 40 30 20 10 S Fe As Au Ag Zn Co Cu 0.4 0.3 0.2 0.1 Trace elements (atomic %) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 0
Timing of metamorphism and veining 1 cm 1 cm 1 cm Basalts Rhyodacites Qtz-eye porphyries
Gold related to late quartz
Location of gold Au Qtz Au Cpy Qtz Sph Cpy Py Au 0.1 mm 0.1 mm
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
Quartz vein/fluid stages White, mottled, preserved early quartz: No sulphide association Dark and intensely deformed late quartz: Hosts sulphides + Au
Early (pre-ore) quartz vein fluids δd -79 Saline 420 C
Ore-stage quartz vein fluids δd -79 Saline 420 C δd -50 Aqueocarbonic 380 C Magmatic ( porphyry ) fluids overprinted by aqueo-carbonic?
Bulyanhulu geology 85ºNE Y Reef 1 vein X Pyrite Zone Co-volcanic porphyry intrusions Y X Mafics Felsics Mafics
Basalts Least altered Altered 1 mm
Basaltic alteration traverse 1.8 1.6 Major Elements Ratioed to Al 2 O 3 LOI Fe2O3 MgO CaO Na2O K2O P2O5 TiO2 SiO2 5.0 4.5 Major oxides (wt %) 1.4 1.2 1.0 0.8 0.6 4.0 3.5 3.0 2.5 SiO 2 (wt %) 0.4 2.0 0.2 1.5 0.0 0.3 0.5 1.0 2.0 3.0 4.0 5.0 6.0 8.0 10.0 Metres away from Reef 1 1.0
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 59.54 73.03 86.51 100.00 Correlation coefficients + + - Au Cu Zn Ag Fe 2 O 3 MnO LOI CaO TiO 2 Zr Y
Rhyodacites Least altered Altered 1 mm
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
Principal component scores 14 20 PCS 1 PCS 3 10 6 2-2 -6 12 8 4 0-4 -8-12 -16 PCS 2 0-5 -10-4 -2 0 2 4-4 -2 0 2 4 Probability PCS 1 Probability PCS 2 PCS 4-4 Felsics Gabbros Mafics Pyrite Zone QFPs -8-4 -2 0 2 4-4 -2 0 2 4 Probability PCS 3 Probability PCS 4 15 10 5 8 4 0
Principal component scores PCS 1 14 20 10 15 Mineralised 10 6 Felsics 5 Barren 2 Mafics 0-2 -5 Rock type -6-10 Mineralisation -4-2 0 2 4-4 -2 0 2 4 Probability PCS 1 Probability PCS 2 PCS 2 PCS 3 12 8 4 0-4 -8-12 -16 Least altered Most altered PCS 4-4 Felsics Gabbros Alteration Mafics Pyrite Zone QFPs -8-4 -2 0 2 4-4 -2 0 2 4 Probability PCS 3 Probability PCS 4 8 4 0 Fractionation or alteration? Primitive? Fractionated?
PCL: all rock units PCL 2 0.7 0.6 Au Ag 0.5 Zn 0.4 0.3 0.2 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 -0.2-0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 PCL 1 PLC 1 (rock type) vs PCL 2 (mineralisation)
PCL 1 vs PCL 2 PCL 2 0.7 0.6 Au Ag 0.5 Zn 0.4 0.3 0.2 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 -0.2-0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 PCL 1 PCL 2 0.7 0.6 0.5 0.4 Ore 0.3 0.2 Felsics 0.1 Mafics 0.0-0.1-0.2-0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 PCL 1 PLC 1 (rock type) vs PCL 2 (mineralisation)
PCL 1 vs PCL 3 PCL 3 0.6 0.5 0.4 0.3 0.2 0.1 0.0-0.1-0.2-0.3-0.4 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 -0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 Cr Fe 2 O 3 LOI PCL 3 0.6 0.5 0.4 0.3 0.2 0.1 0.0-0.1-0.2-0.3-0.4 Rock Immobiles Alteration PCL 1 PCL 1 Ore Alteration Rock Immobiles -0.4-0.3-0.2-0.1 0.0 0.1 0.2 0.3 0.4 PLC 1 (rock type) vs PCL 3 (alteration)
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
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 300-450 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
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
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?