Chemical fingerprinting of Black Reef Conglomerate Gold at the East Rand A comparison with Witwatersrand and Lode Gold in South Africa

Transcription

1 8th Inkaba yeafrica Workshop Cape Town, South Africa 28 August 2 September 2011 Chemical fingerprinting of Black Reef Conglomerate Gold at the East Rand A comparison with Witwatersrand and Lode Gold in South Africa CDK Gauert 1, E Cook 2, M Schannor 3, L Hecht 4 1. Department of Geology, University of the Free State, South Africa, 2. Gold One Intern. Pty Ltd., Consolidated Modderfontein Operation, 3. Inst. für Geowissenschaften, Freie Universität Berlin, Germany, 4. Museum für Naturkunde Berlin, Humboldt Universität, Germany. Corresponding Author: CDK Gauert; Gauertcdk@ufs.ac.za

2 Background: KEYWORDS: Black Reef, Transvaal gold, Supergroup. chemical fingerprint, provenance The Palaeoproterozoic ( Ma) Black Reef Formation (BR) is a widespread thin small pebble unit at the base of the In the East Rand, Randfontein and Klerksdorp areas the unit contains considerable gold, less U, as well as minor associated PGE mineralization, Henry and Master (2008) identified shortcomings in our understanding of the genesis of Au mineralization, and the occurrence of Au, U and PGEs hosted by the Black Reef (BR), and their provenance. The source of gold in these reefs has long been a matter of speculation. West Rand Randfontein Vryburg Fm in Griqualand west Klerksdorp Cons Modder Figure 1. Locality map of the Black Reef sections under investigation in the Transvaal Supergroup of NE-South Africa. Boxes indicate areas of investigation being sampled (modified after Eriksson 28 August et al., 2 September 2006) From Eriksson 2011 in Johnson et al.,

3 Motivation as introduction Extensive sedimentological evidence, suggests a strictly detrital origin. The gold associated with the sulphides, such as pyrite, has been described as in situ hydrothermal deposition with the underlying Kimberley Reefs of the Central Rand Group proposed as the source (Hoffmann et al., 1977). However, mineralogical and geochemistry studies performed by Barton and Hallbauer (1996) on the pyrite grains of the Black Reef argue against this. Debated possible sources of BR gold: a) from reworked conglomeratic upper Witwatersrand reefs, b) from epithermal gold of granite-greenstone terrain hinterland, and c) from underlying pyrite-associated gold of Archaean VMS deposits. To solve the Au provenance question, geochemical fingerprinting of Au using electron microprobe (FE-EMPA), Synchrotron micro-xrf (SR-µ-XRF), time-of-flight secondary ion mass spectrometry (TOF-SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been carried out. 1 5 November

4 Mass spectral fingerprints for three adjacent gold mines (AARL, 1999) Gold fingerprinting It is a method for identifying native gold based on the impurities or trace elements it contains. Laser ablation inductively coupled plasma mass spectrometry (LA- ICP-MS) is used to fingerprint the sample (Au, pyrite) by mineralizing event and to a particular mine or bullion source. This technique has been used to lay claim to stolen or relocated gold. Even gold that has undergone salting can be identified as to its multiple sources. Multivariate discrimination diagram showing that the Black Reef detrital and concretionary pyrite are geochemically distinct both from each other and from the average Witwatersrand pyrite (Barton & Hallbauer, 1996)

5 Black Reef facies types at Consolidated Modderfontein, East Rand Schematic geological section through an eroded Black Reef channel at Consolidated Modderfontein Mine (modified after Barton et al., 1996)

6 A detailed investigation of the mineralogy of the mineralization in selected profiles of the arenitic to conglomeratic BR reveals: Mineralogy: Chromite, sphalerite, chalcopyrite, galena, uraninite, cassiterite, tourmaline, carbon, Pt-Ni-As minerals and Ni-Co-sulpharsenides such as cobaltite and gersdorffite next to abundant pyrite, as well as free gold. Late-stage hydrothermal fluids possibly overprinted BR rocks and remobilized Au grains into their present form, as evidenced by the associated alteration minerals such as chlorite, pyrophyllite and sericite. 1 5 November 28 August September

7 Slabs of BR pyrite leader - blanket facies 28 August 2 September

8 B-Reef Harmony - Tshepong mm 29 August 2 September

9 BSE-images of Gold particles Black Reef B-Reef Wits 7th Inkaba yeafrica Workshop - GFZ Potsdam, Germany 1 5 November

10 Sheba Gold Mine Barberton Au particles of MRC-1 ore body Au particles ZK-1 ore body 29 August 2 September

11 Analytical equipment JXA-8530F Field Emission Electron Probe Microanalyzer (EPMA), WD/ED Combined System for Ultra Micro Analysis Featuring a field emission (FE) electron gun, high vacuum system, ultra micro area analysis. Parameters: 30 kv, 100 na, 200 s count time on peak; 3nm secondary electron resolution. Element Ag Au Hg Cu conc (ppm) Element S Ti Pb Fe conc (ppm) Element Th Cd U Co conc (ppm) Element Sn Ni conc (ppm) Avg. detection limits as calculated by Jeol software compared to LA-ICP-MS (small table ) Element Ag Au Hg Cu conc (ppm) Element Pt Ti Pb Fe conc (ppm) Element Pd Cd Sb Co conc (ppm) Element Sn Ni Cr Mn conc (ppm) November

12 Chemistry of BR, Wits and BGS Gold FE-EMPA MfN Berlin: Au, Ag Gold provinces show distinct Silver content of <1% to ~14% BR >> B-Reef (Wits) > Barb.- GSB / Approach: Show chemical variations of Gold using Au as dependant element in a Harker diagram Ranges of Gold content within provinces variable from >80 to 98.5 % BR << B-Reef (Wits) </= Barb-GSB 29 August 2 September

13 Chemistry of BR, Wits and BGS Gold FE-EMPA MfN Berlin: Hg, Cu Gold provinces show distinct Mercury content of on avg. <0.3% to ~2.2%, max. 7.2% B-Reef (Wits) > BR >> Barb-GSB Copper content high in Wits-Reef and Barb-GSB from % BR << B-Reef (Wits) </= Barb-GSB 29 August 2 September

14 Ternary Gold classification 7th Inkaba yeafrica Workshop - GFZ Potsdam, Germany 1 5 November

15 Chemistry of BR, Wits and BGS Gold FE-EMPA MfN Berlin: S, Fe Gold provinces show distinct iron content of <0.02 % to ~0.98% (max. 5.4%, pyrite inclusion?) BR >> B-Reef (Wits) >/= Barb-GSB S contents range from on avg. 1.4 to 0.008% (max. ~18% Pyr incl.) BR >> B-Reef (Wits) >/= Barb-GSB Strong Fe-S correlation (pyrite) 29 August 2 September

16 Chemistry of BR, Wits and BGS Gold FE-EMPA MfN Berlin: Th, U (Th variability) Gold provinces show distinct U content of <0.09 % to ~0.8% (max. 7.1%, uraninite inclusion?) BR > Barb-GSB > B-Reef (Wits) Th contents range from on avg. 0.3 to 0.022% (max. ~3.6% uraninite inclusion); BR >/= Barb-GSB >> B-Reef (Wits) High Th variability 29 August 2 September 2011? 16

17 Chemistry of BR, Wits and BGS Gold FE-EMPA MfN Berlin: Co, Fe vs. Ag Avg. Ag content of gold provinces very distinct: BR: 11.1%, B-Reef (Wits): 7.2% and Barb-GSB: % Co contents range from on avg to 0.005% (max. ~0.33%); BR >/= B-Reef (Wits) > Barb-GSB 29 August 2 September

18 Principle component analysis Taking into consideration: Au, Ag, Hg, Cu, S, Fe, Th, Co 7th Inkaba yeafrica Workshop - GFZ Potsdam, Germany 1 5 November

19 Cluster analysis of all EMP Au data Taking into consideration: Au, Ag, Hg, Cu, S, Fe, Th, Co B R G o l d BR Gold BR Gol d Wits B- Ree f Gol d BGB-ZK Gold 1 5 November 2010 BGB-MRC Gold B R G o l d 19

20 Conclusions and summary Gold Province Black Reef - East Rand (Cons. Modder) Free State Witwatersrand Gold fields Barberton Greenstone Belt (Sheba Mine) High conc., major to minor element Low conc., (u) trace element Remarks Ag (max. 17.5%), Hg, Cu, Th, Co, Ni Supergene enrichment U (avg. 0.8, max. 7.1%), Fe (EMP, SR-µ-XRF, LA- ICP_MS) Cr, As, Pb, Pt, Pd, Sn, Sb, Ta (EMP, SR-µ-XRF,LA- ICP_MS) Uraninite inclusions, PGEs Pyrite, Ni-Co-sulphars. association Ag (avg. 7.1 %),Hg, Cu Co, Ni, Pb, Ti Uraninite inclusions Fe Pd, Ru, Se-Te, U? (EMP, µ-xrf, LA- ICP_MS) (EMP, µ-xrf, LA-ICP_MS) Ag ( %), (ZK), Cu U, Th, Co, Hg Arsenopyrite, (chalco-) pyrite association Fe (MRC) (EMP, LA-ICP_MS) (EMP, LA-ICP_MS) 29 August 2 September

21 Conclusions and summary EMP analyses show a fineness of BR gold between 825 and 882, as well as elevated Hg contents, minor conc. of Fe, S, Co and Ni, and detectable copper values (max.450 ppm). Synchrotron micro-xrf spectra of BR gold reveal the presence of Cr, Ni and Ta at higher concentrations than in Witwatersrand gold, whereas Wits gold contains more Cu confirming the EMP results. In favour of a reworked Witwatersrand gold origin of the BR gold argues the very similar heavy mineral content. Against a Witwatersrand origin argues the frequency of concretionary pyrite with a unradiogenic Pb isotope composition (Barton&Hallbauer), the less abundantly occurring Ni-Co-Fesulpharsenides in the Witwatersrand reefs, however its frequent appearance in BR. The gold in the BR has a lower fineness, lower Hg and Cu content compared to Witwatersrand gold, whereas its Fe, S, Ti and Ni concentrations are higher. Greenstone Gold has lowest Ag, Hg, and Th, S, Fe conc. of the Au prov., however high Cu. It is important to understand the genesis and provenance of Au, U and PGE mineralization in the Black Reef because it has exploration significance, are underlying Wits and/or greenstone terrains necessary for the BR to potentially host an orebody? 29 August 2 September

22 THANK YOU FOR YOUR ATTENTION QUESTIONS? Acknowledgements: Michael Brauns - Curt-Engelhorn Engelhorn-Centre Mannheim Evan Cook - Gold One Pty Ltd. Dept. of Geology, University of the Free State David Batchelor, Rolf Simon, ANKA, KIT

23 Conclusions and summary Black Reef and Witwatersrand gold can be most effectively distinguished by true fineness, by Hg and Cu concentrations, possibly also by U, Fe, S, and Co trace element content (~ 400 analyses). Sources of BR gold at Consolidated Modderfontein: 1. reworked conglomeratic upper Witwatersrand reefs, 2. epithermal gold of granite-greenstone terrains, 3. from underlying Archaean gold-bearing VMS deposits. EMP analyses show a fineness of BR Gold between 825 and 882, as well as elevated Hg contents, minor concentrations of Fe, S, Co and Ni, whereas detectable copper values are low (max. 450 ppm). Synchrotron micro-xrf spectra of BR Gold reveal the presence of Cr, Ni and Ta at higher concentrations than in Witwatersrand Gold, whereas Wits gold contains more Cu confirming the EMP results. In favour of a reworked Witwatersrand gold origin of the BR gold argues the very similar heavy mineral content. Against a Witwatersrand origin argues the frequency of concretionary pyrite with a unradiogenic Pb isotope composition, the less frequently occurring Ni-Co-Fe-sulpharsenides in the Witwatersrand reefs, however its frequent appearance in BR. The gold in the BR at Consolidated Modderfontein has a lower fineness, lower Hg and Cu content compared to Witwatersrand gold, whereas its Fe,S,Ti and Ni concentrations are higher. Our understanding of the genesis of Au, U and PGE mineralization hosted by the Black Reef (BR), and its provenance is important because it has exploration significance. The fertile versus barren BR discussion has to answer the question if BR has to be underlain by Wits and/or greenstone terrains to potentially host an orebody. 29 August 2 September

24 Remobilisation strong at B-Reef (carbon-linked) Make sure comparable particles Enlarge database Compare with EMP with ICP-MS and SRµXRf data

25 Synchrotron micro-xrf spectra (30 kev): Black Reef vs B-Reef Gold. K- and L-lines at 30 kev, Si(Li) detector, beam µm 2 S Kα Ag Lα Ti Kα V, Cr Kα Fe Kα Fe Kß Cu Kα, Ta? Zn Kα, ReLα Au Lα Au Lß? Pb Lα Pb Lß1,2 Au Lγ Zr,Nb Kα? Witwatersrand Ru Kα? Pd Kα? Ag Kα Cd Kα? Ag Kß 5 10 Energy (kev)

26 Black Reef pyrite leader sections µ B- Reef sections

27 Ternary Gold classification 7th Inkaba yeafrica Workshop - GFZ Potsdam, Germany 1 5 November

28 LA-ICP_MS Au data Gold Black Reef Wits B Reef sample (n=5) (n=3) line (n=5) (n=3) (n=7) line Table 1: Spot LA-ICP-MS analyses and line scans of BR Gold (Cons. Modder) and of Witwatersrand Gold (B- Reef). Ti d.l Cr Mn d.l. 28 d.l Fe (%) Co Ni Cu Zn d.l. 36 d.l d.l. 20 As Se d.l. 25 d.l d.l. 15 Ru d.l d.l Rh d.l. d.l. d.l d.l. 0.0 Pd Ag(%) Sn d.l. 2.8 d.l Sb d.l. 41 d.l Te d.l Pt d.l. d.l. d.l Au(%) Pb d.l Bi d.l. 11 d.l Hg(%) n.d. n.d. n.d >0.1 > November