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1-Geochemical-basic.pdf
- 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
- 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
- 13. Subsurface Dispersion by lateral groundwater fan p y g flow- hydromorphic
- 14. 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
- 15. 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
- 16. 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
- 17. 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
- 18. 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
- 20. 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
- 21. 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
- 22. 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
- 23. 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
- 24. 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
- 25. 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
- 26. • stability of solids • aqueous speciation • Co-precipitation and adsorption
- 27. 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 (%)
- 28. 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
- 29. 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
- 30. 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
- 31. 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
- 32. 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
- 33. 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