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  1. Geochemical Exploration Techniques
  2. Geochemical Exploration? Searching geochemical anomaly – Searching geochemical anomaly – Elts disperse over large area around their source N l di t ib ti f th l t i th – Normal distribution of the elements in the geochemical media of interest Element distribution in d/t parts of the earth – Element distribution in d/t parts of the earth – Geochemical association of elements B h i f l t i th diff t – Behavior of elements in the different environments Sampling – Sampling – Preparation A l i – Analysis – Interpretation
  3. Basic Principles: a refresher Basic Principles: a refresher
  4. th he eart n of th osition compo mical Chem
  5. Surficial Environment Mi l Soluble Salts Mineral Breakdown Residual Minerals Oxides & Clays Sedimentation Diagenesis Uplift Erosion Sedimentary Rocks Igneous Rocks Sedimentary Ores Hydrothermal Ores Metamorphism Crystallization Magmatic Ores Mantle M lti Magma Metamorphic Ores Metamorphic Rocks Deep-Seated Environment Mantle Material Melting
  6. Geochemical Environments Deep Seated • magmatic- Surficial • weathering - g metamorphic • High P & T g sedimentation • low P & T • low free O2 • restricted fluid • free O2, CO2 H2O • free fluid movement movement most ores formed most ores destroyed
  7. The Dispersion Process Dispersion Transport Mobile phase Immobile phase Matrix Environment of mobilization Environment of deposition of mobilization
  8. Dispersion The process that takes an element and moves it from one environment to another, , generally decreasing the concentration by spreading it over a larger volume p g g Results in redistribution fractionation mixing Results in redistribution, fractionation, mixing and dilution
  9. Types of Dispersion 1. Clastic dispersion: solid particles moved physically p y y 2. Hydromorphic dispersion: solid particles dissolved in water & transported in dissolved in water & transported in chemical solution 3 Biogenic dispersion: solid particles moved 3. Biogenic dispersion: solid particles moved by biological activities
  10. Clastic dispersions
  11. Dispersion Dispersion Dispersion by wind by wind
  12. Subsurface Dispersion by lateral groundwater fan p y g flow- hydromorphic
  13. Biogenic Biogenic El t di l d b d t i Elements are dissolved by groundwater in contact with the ore vein; and are transported in solution in the direction of groundwater flow. Plant roots take up g p nutrients from the water solution
  14. Agents of Dispersion Mechanical • examples Chemical-biochemical • examples p – magma injection – glacial transport p – dissolution/deposition – melting/crystallization – alluvial sorting • results • results – fractionation common – mixing – fractionation not common – mixing – dilution common
  15. Types of Dispersion Deep-seated Surficial Primary Diffusion into Precipitation y wallrock at deposition p on seafloor (VMS) p ( ) Secondary Diffusion from ore during Weathering of ore deposit metamorphism
  16. 1. Primary dispersion • Distribution of elts in unweathered rks and minerals • The main media of survey are rocks • Not proved encouraging in mex b/c • Not proved encouraging in mex b/c • Rocks are not homogenous & hence bl f t ti problem of representativenes • High cost of analysis • Some deposits lack primary dispersion p
  17. 2 Secondary dispersion 2. Secondary dispersion • Dispersion of elements during weathering & diff i f lt f d i & diffusion of elts from ore during metamorphism • Occur during weathering & metamorphism of ore deposits • Good for MEX b/c • Dispersion covers larger/wider areas Dispersion covers larger/wider areas • Media of survey are soil, stream sediments water air plants & animals sediments, water, air, plants & animals
  18. Dispersion Halos Ore Ore
  19. Mobility Mobility • Characteristic of an element that allows it to Characteristic of an element that allows it to be selectively dispersed • Ease with which the element is dispersed l i h di i relative to the surrounding matrix
  20. Controls on Mobility • Mechanical properties of medium/elt – viscosity of the fluid viscosity of the fluid – size, shape and density of the grains • Chemical differentiation • Chemical differentiation – relative stability in immobile solids l ti t bilit i th bil fl id h – relative stability in the mobile fluid phase
  21. Simple Dissolution + ⇒ = + 4 2 4 a a SO Pb PbSO = + = 4 4 2 PbSO SO Pb a a a K + = 2 4 PbSO K m = = + + 4 4 2 2 SO SO Pb Pb m m γ γ
  22. Selecting the Reaction i l i Simple reaction = + + ⇒ ) ( 3 2 ) ( ) ( 3 aq aq solid CO Zn ZnCO ) ( ) ( ) ( q q More realistic − + + + ⇒ + ) ( 3 2 ) ( ) ( ) ( 3 aq aq aq solid HCO Zn H ZnCO Trace Component Trace Component + + + ⇒ + 2 ) ( ) ( 3 2 ) ( ) ( 3 aq calcite aq calcite Zn CaCO Ca ZnCO
  23. Deep-seated Dispersion Mechanisms • Melt-solid equilibrium Melt solid equilibrium – size, charge, coordination, bonding Melt fl id eq ilibri m • Melt-fluid equilibrium – complexing important • Solid-fluid equilibrium
  24. Surficial Dispersion Surficial Dispersion Dominated by aqueous fluid transport • ionic potential (IP)=I.charge/I.radius – IP<3 form ionic solutions & dispersion p dependent on pH; e.g alkalis, Ca, Ba, Sr, Fe2+, Mn2+, Cu, Zn – IP b/n 3 &10 less soluble & solubility influenced by pH; e.g. Mg, Al, Fe3+, Mn4+ – IP>10 form soluble anion complexes; e.g. S, P, N, S, As
  25. • stability of solids • aqueous speciation • Co-precipitation and adsorption
  26. Coefficient of Mobility Empirically determined water drainage C R M K ⋅ = 100 rock total C R ⋅ M = elt concentration in water (mg/L) M elt concentration in water (mg/L) R= total mineral residue in the water (%) C= element concentration in the rock (%)
  27. Mobility of The Elements 1 1A 2 2A 3 3B 4 4B 5 5B 6 6B 7 7B 8 6 9 8B 10 8 11 1B 12 2B 13 3A 14 4A 15 5A 16 6A 17 7A 18 8A H He H He Li Be B C N O F Ne Na Mg Transition Metals Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Fr Ra Lanthanides (Rare Earth Elements) Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu y Th Pa U Actinides K < 0 1 immobile 0 1 1 slight 1 10 moderate >10 high K < 0.1 immobile 0.1 – 1 slight 1 – 10 moderate >10 high Oxidizing pH < 4 Fe(II ) reducing uncertain? Radioactive acidic oxidizing or reducing
  28. Indicator Elements The elements measured in order to detect an ore body y • Often the element of economic interest • The ore element may not be suitable • The ore element may not be suitable – difficult to measure – immobile – difficult to interpret
  29. Pathfinders • Elements that are associated with the ore elements in the target ore t pe and are sed elements in the target ore type and are used because of a more favorable geochemistry t bilit – greater mobility – clearer patterns (greater contrast) – easier to detect analytically easier to detect analytically • There is a direct correlation with the ore element of interest element of interest • Examples include As for Au; Ni,Fe,Mg for C V TiPt f F Pt C A A f C Ni Cr; V,TiPt for Fe; Pt,Co,As,Au for Cu-Ni etc
  30. As as Pathfinder for Au As as a pathfinder for Au 1200 600 800 1000 1200 pm As 0 200 400 600 pp Au 0 -600 -400 -200 0 200 400 Traverse position p
  31. As as Pathfinder for Au As as a pathfinder for Au log scale 1000 10 100 1000 pm As 0 1 1 10 pp Au 0.1 -600 -400 -200 0 200 400 Traverse position p
  32. Element G roup A ssociations Element Associations G roup A ssociations General A ssociations K -Rb Ca-Sr A l-G a Si-G e Zr-H f REE-La-Y PGM Igneous R ocks F l i Si K N Felsic Si-K-Na Alkaline A l-Na-Zr-Ti-N b-Ta-F-P- REE M afic Fe-M g-Ti-V Ultram afic M g-Fe-Cr-N i-Co Som e pegm atites Li-Be-B-Rb-Cs-REE-Nb-Ta- U -Th Som e Skarns M o-Sn-W Som e Skarns M o-Sn-W Potash Feldspars K -Ba-Pb Other potash m inerals K -N a-Rb-Cs-Tl Ferrom agnesian M in Fe-M g-M n-Cu-Zn-Co-N i Sedim entary R ocks Fe-oxides Fe-As-Co-N i-Se M n-oxides M n-A s-Ba-Co-M o-Ni-V -Zn Ph h it P A M Pb F U Phosphorite P-A g-M o-Pb-F-U Black Shales A l-A g-A s-A u-Bi-Cd-M o-Ni- Pb-Sb-V -Zn