Geochemistry and the challenge posed by exotic cover

Transcription

1 Geochemistry and the challenge posed by exotic cover David Cohen University of New South Wales and Association of Applied Geochemists Recent Practical Advances in Mineral Exploration Technologies September 11, 29

2 Ahmad Mokhtari Isfahan University of Technology, Isfehan RMRC Neil Rutherford Rutherford Mineral Resource Consultants, Sydney Simon Gatehouse Hellman and Schofield, Sydney David Kelley MMG Minerals and Metals Group, Denver Ravi Anand CRCLEME / CSIRO Exploration and Mining, Bentley William Coker BHP Billiton World Exploration, Vancouver Iain Dalrymple Actlabs, Perth

3 lusions

4 The problematic terrains for geochemical exploration Glacial deposits Thick gravel and scree deposits Thick alluvium or colluvium + deep weathering Aeolian deposits Volcanic ash?ice

5 Thompson Orogen Lachlan Fold Belt L. Mesozoic to Tertiary basins Significant mineral occurrences : Australian Minerals Atlas

6 Regolith Classification Weathered profiles typically have RESIDUAL and TRANSPORTED components of varying ages Several distinct phases of creation and destruction of secondary Fe, Ca, Si and Al minerals each with total or partial resetting of trace element geochemistry Systematic approach to identifying regolith materials and landforms Link evolution of regolith and its components to geochemical processes and sampling strategies Anand and Smith, 25 Transported Overburden Tertiary Qa Regolith developed on and in primary lithology Red-brn sandy clay Silic d sandy, silty + gravel colluv & alluv Pisolitic ferricrete Mega mottle palaeochannel clays Transported gravel Clays Base of sediments Lateritic residuum Mottled saprolite Saprolite Saprock Bedrock? unconformity mineralisation

7 Does dispersion occur through deep, transported cover? 1 m Unconformity Fe < 1 % > 2 Unconformity Cu < 15 (ppm) >2 Osborne Cu-Au deposit, Qld 4m Mesozoic Cover Dispersion mechanisms? Dispersion timing? Cu - Au Mineralisation Proterozoic Bedrock (data: Lawrance, 1999)

8 Does dispersion always occur? CSA Mine Lease, Cobar Recent Drainage m 15 Pb Palaeodrainage Line D 5m Chesney Fmn. siltstone Girilambone Gp 6 Zn Line C Line B Line A 3 Fe 1km C.S.A. Coarse silt + lag Transported silt + clay Residual regolith Line D Shen and Cohen, 25 U. Amphitheatre Gp. CSA Siltstone Chesney Fmn. (grwk) Chesney Fmn. (slts) Girilambone Gp.

9 Dispersion models vegetation effects McKinnons deposit, Cobar As.3 ppm 15 ppm Cypress pine needles Soil total 3 ppm non-mag Lag total magnetic Alluvial fill Pediment Mixed pediment / alluvium 25 m line 796N Saprolite 195 m Au mineralised structure 3 m Cohen et al, 1996

10 Dispersion models gases and aerosols Clay in droplet Deployment arrangement Collector assembly plastic cap (dust cover) backfill surface 5 μm NaCl + Na 2 CO 3 collector hole polystyrene -coated glass slide funnel 1 μm 5 cm Amorphous silica Rutherford et al., 25 2 μm

11 Osborne Cu-Au deposit, Qld ng cm -2 2 Cl Ore Zone K 1 5 Fe Plus a range of other transition and group I and group II elements Cu m

12 Osborne (Kulthor) Cu-Au deposit, Qld chalcopyrite pyrrhotite pyrite gypsum in soil regional soil background δ 34 S

13 Dispersion models dilatancy pumping Figure 5 - Model of Cameron et al Cameron et al., 24

14 Spence porphyry Cu deposit, Northern Chile 15 ppm Cu Surface regolith samples; weak selective extraction Vertical fracture with saline soil 2 ppm As.6 % Na Burial under 25m of Tertiary gravels 5 m Spence Deposit Eastern Fracture Zone

15 Dispersion models electrochemical Surface SP response Humus Varved clays Cathode: 15O 2 + 3H 2 O + 6e - 6OH - Ions chelated by humus Cation diffusion SP Cation conc. Cation conc. in humus Zone of increased cation conc. created by organic chelation of diffusive flux Govett, 1977 Smee, 1983 Till Country rock Cation migration Sulphide e - Anode: 4FeS 2 + 4H 2 O 4FeO.OH + 76H + + 8SO e - Cation migration Zone of increased cation conc. created by electrochemical migration Current lines Equipotential lines

16 Dispersion models electrochemical A horizon B horizon Zone of CaCO 3 Accumulation O 2 Zn Cd Oxidation Zone Low ph Carbonate Loss Zone of Metal Accumulation Water Table Upper limit of Carbonate Clay Zn 2+ + Cu 2+ + Fe 2+ Reduced Column Volcanic Rocks Sulphide Hamilton, 1998

17 Dispersion models electrochemical ph Ca, REE A horizon B horizon Zn Cd Oxidation Zone Low ph Carbonate Loss Water Table Clay Zn 2+ + Cu 2+ + Fe 2+ Reduced Column Volcanic Rocks Sulphide Hamilton, 1998

18 A basic premise Inherited metal content in regolith with overprint due to dispersion away from mineralisation OR other features [metals] A need to isolate this component or signal which may constitute <.1% of total signal Transported Cover Bedrock Min n

19 Selective Extractions Increasing age of mineral phase in regolith Less transitory metal contents Acid-base and redox reaction recycling Hydration - dehydration reaction cycling Soluble phases Adsorbed & Exch. Carbonates species Organics Mn-oxides & am. Fe-ox. Cryst. Fe-oxides Silicates Resistate minerals Integration of dispersion over time Progressive accumulation of metals Stabilisation and fixation of pattern Progressive degradation Reworking of metals

20 Selective Extractions Soluble phases Adsorbed & Exch. species Carbonates Mn-oxides & am. Fe-ox. Cryst. Fe-oxides Silicates Resistate minerals Organics Water (Qld Beer) MMI Guinness Ammonium acetate Na-pyro / H 2 O 2t Acetate + HOAc Enzyme Leach / H 2 O 2 Weakly acidified NH 2 OH EDTA / H + Regoleach Strongly acidified NH 2 OH HCl Aqua regia Mixed acids (Canadian wine) HF / fusion (Cohen et al, 21; based on Gray, 1999)

21 Mandamah, NSW NSW 5 m MACD 312 MHACD 228 Transported Regolith Mandamah 3 m Lower Saprolite Ser-chl-mt-alb alt n Ser-py alt n g/t Au.37 % Cu + Mo Andesite

22 ph cm 25 5 Organic matter accumulation Ba accum. Higher leachable Fe and Al Incr conc of some trace elements e.g. Cu, Ni and Zn in 75 1 Carbonate formation dol, gyp and Na 2 CO 3 Increase in gypsum Fe-enriched mottling zone Na, S accum. NaCl + SO 4 2- Increasing Cs U Be Th in all Accumulation of Cu, U & REE 2

23 3 cm depth, ph 5 K-acetate 1, ppm Northern block Central block Southern block 1, Ca Mg 1 1 Ce 1.1 Mo Cu.1 mineralisation mineralisation

24 ph % 1.8m ph Ca.5 Ca 14 ppm Na 7 Na S CEC 24 ppm cmolc/kg S CEC 3 % Cu Mineralization 15m mE

25 Cu by NH 4.OAc ppm Depth below surface (m) 1.8m Cu by NH 2 OH 6 4 ppm 2 Cu by aqua regia 2 ppm % Cu Mineralization 15m mE

26 24 2 EM38 vertical coil mode EM38 horizontal coil mode 16 EC a (ms/m) % Ca.5 28 cmolc/kg 1.8m Ca CEC CEC 3 % Cu Mineralization 15m mE

27 Dispersion model for Mandamah? archaeo-electrochemical Initial weathering phase (period of elevated rainfall) Leaching of elements 3 m Movement of Fe 2+ and hydrolysis generating large amounts of H + In-situ Weathering of mineralization and H + production due to oxid n of sulfide minerals Ore body Oxid n of mineralization and production of H + and ore-related ions Host rock

28 Prograde Phase (still high rainfall) Pedogenic carbonate General loss of pedogenic carbonate or limited formation 5 m Migration of H + into transported regolith and clay alteration Transported 3 m In-situ Ore body Host rock

29 Retrograde Phase (Post onset of aridity) Pedogenic carbonate Recent migration of mobile alkalis (e.g. Na). Zone of (palaeo) clay modification with low CEC + Non-carbonate alkalinity Transported Addition of H + to surface ceases or diminishes In-situ Ore body Host rock

30 Analysis Objective detecting samples whose geochemistry appears anomalous Traditional approach satellite spotting Multivariate distance Probability function

31 Reality for data very complex multivariate signals Mandamah Aqua regia Al Fe Sample from above mineralised sites Sample from above non-mineralised sites Weak hydroxylamine Y Sm transformed and standardised

32 Chimborazo porphyry Cu, Chile 1.2 ppm. 1. ppm..8 Ave. 25m 23 Cu As gravel Enzyme Leach Aqua Regia 8 Supergene enrichment Libra zone 7-point moving Virgo zone As average As variance Porphyrytic andesite Monzonite porphyry m

33 lusions RED scheme (Anand et al, 1998)

34 lusions Summary of various forms of interactions of deeply buried mineral deposits with hydrosphere and biosphere Decreasing Eh 2 m Biological accumulation GW Table Higher ph Vapor or groundwater dispersion Reduced column Cu 2+ Sulphides Low ph Cyclical dilatancy pumping along structural zone Microbial Redox Transported cover In-situ regolith Bedrock Higher ph

35 lusions Seeing Geochemical Dispersion Haloes OBJECTIVE: Separate processes that have affected the elemental and mineralogical composition of regolith (both gain and loss) METHODS: Isolate signals that can be related to the effects or presence of mineralisation through: PHYSICAL (the extraction) and/or NUMERICAL (data processing) methods

36 lusions not (yet) the silver bullet for thick transported or deeply weathered cover, but has prompted: research into fundamental processes of geochemical dispersion development of new approaches to definition and detection of geochemical patterns related to the effects of underlying mineralisation The bests tool for detecting deeply buried, sulphide mineralisation, may yet prove to be a simple ph meter and a bottle of dilute HCl Future Research Directions Developments in media and analysis, based on process modelling Application of isotopes to refine understanding of dispersion mechanisms and regolith evolution models Abandoning baselines, backgrounds and anomalies in favour of new data processing methods that recognise geochemical signals related to processes of interest

37 General Ponsonby leads the Scots Greys against the Polish lancers at Waterloo Field Marshal Haig leads his troops nowhere in particular Emperor Napoleon leads his remaining troops back from Moscow Advance! Dispersion models Analytical improvements War of Attrition Selective extractions Exploring in deep transported cover modelling Retreat! Old thinking on defining geochem anomalies

38 People First year (excludes engineering servicing) 2 Phys Geog 1 1 Geology 15 Students Upper years 4 Mineralogy 3 Ore deposits Geology hons 2 4 Students