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INTERPRETACION LITOGEOQUIMICA DE DATA MULTIELEMENTAL
- 1. Interpreting Multielement Geochemistry data ScottHalley July 2016 Mineralogical Patterns in Hydrothermal Systems. A seminar presented by;
- 2. ALS ME-MS61 4 aciddigest uses a combination of HCl (hydrochloric acid), HNO3 (nitric acid), HF (hydrofluoric acid) and HClO4 (perchloric acid). Because hydrofluoric acid dissolves silicate minerals, these digestions are often referredto as 'near-total digestions'. 48 elements by ICP-MS and ICP-AES analysis.
- 3. ALS ME-ICP61 4 aciddigest uses a combination of HCl (hydrochloric acid), HNO3 (nitric acid), HF (hydrofluoric acid) and HClO4 (perchloric acid). Because hydrofluoric acid dissolves silicate minerals, these digestions are often referredto as 'near-total digestions'. 33 elements by ICP-AES analysis.
- 4. Applied Lithogeochemistry 3 objectives •IdentifyRock types • Immobile trace elements • Sc, Ti, V, Zr, Hf, Nb, Th, La, Ce •Quantify Alteration • Major elements • Al, K, Na, Ca, Fe, Mg, (Ba, Rb, Cs, Sr) •Pathfinder patterns • As, Sb, W, Mo, Bi, Te, Tl, Ag, Au
- 5. Consider the importanceof ; •Digest Method •Assay Method •Detection Limit •Mineralogical Controls What is “background”?
- 6. Importance of DetectionLimits Outcropping Porphyry Cu deposit Blind Porphyry Cu deposits 300m below surface Bismuth assays by ICP-MS, detection limit 0.01ppm
- 7. Outcropping Porphyry Cu deposit BlindPorphyry Cu deposits 300m below surface Bismuth assays by ICP-AES, detection limit 5ppm Must use ICP-MS to get useful detection limits for most elements Importance of Detection Limits
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- 9. Accuracy versus Precision Comparisonof Scandium assays from 4 acid digest versus Li-borate fusion. Sc is hosted in Fe-silicates; easily dissolved in mixed acid. Li-borate analyses might have better ACCURACY, but the 4 acid digest provides better PRECISION. This is true for many of the immobile trace elements. 4 acid digest Lithium Borate Fusion This is a data set from a rhyolite-hosted VMS system;samples have been assayed twice; once with a lithium borate fusion AND once with a 4 acid digest.
- 10. Accuracy versus Precision Thisis an example of why precision is important. The chemical differences between units in this data is very subtle. With a fusion digest method, the results would have been more accurate, but the different units could not be distinguished. Sc versus Zr
- 11. 4 acid digest Aqua Regia digest Tungsten Comparisonof assays from 4 acid digest versus aqua regia digest. Is W insoluble as H2WO4 or is it in silicates? This is a data is from soil geochem over porphyry Cu project; sampleshave been assayedtwice; once with an aqua regia digest AND once with a 4 acid digest.
- 12. Mineralogical Controls •What arethe host minerals for each element? •Are these sparsely distributed accessory minerals, or ubiquitous, homogeneously distributed alteration minerals? •This has a big impact on assay “variance”. •Pathfinders hosted in homogenously distributed minerals allow a small sample size and a very broad sampling pattern; important when designing sampling programs.
- 13. Average crustal Abundance PathfinderChemistry
- 14. Zinc; blue<50ppm, red>500ppmAntimony; blue<1ppm, red>10ppm Hellyer, Tasmania Pathfinder chemistry from exploration holes surrounding the massive sulfide; Cu-Pb-Zn confined to small veinlets, but pyrite is pervasive. Trace elements from pyrite give a much more consistent near-miss signal!
- 15. Pyrite is ahost for a wide variety of pathfinder elements. This soil geochem survey gives no indication of the proximity of a VMS deposit, but it very clearly maps a pathfinder element signature in the footwall pyrite! If just Cu-Pb-Zn had been analysed, this target would have been missed. VMS stringer zone. Cu is in chalcopyrite; restricted to sparse veinlets. Sb is in the lattice of the pyrite. Maps the footprint of the stringer zone. Projected position of massive sulfide VMS horizon Hangingwall stratigraphy Footwall stratigraphy 1km
- 16. What is thebackground? • For some elements, the background is strongly lithology- dependent. There are ways to deal with this. • For example, in unaltered rocks, Cu, Zn, Mn, V are highly correlated with Sc. • Cu, Zn, Mn, V are much more mobile than Sc. • Use the Sc values to “normalise” the other elements.
- 17. Pathfinder Ranges Plot pathfinderelements as a factor of average crustal abundance levels for each element. A coherent footprint (multi-point anomaly) of >10 x average crustal abundance is a significant anomaly!
- 18. ALS ME-MS61; Immobiletrace elements to map Lithology. From the ME-MS61 package, the following elements are considered to be the most immobile. Use these to classify rock compositions. Note that the immobile trace elements have an ionic charge of +3 or +4.
- 19. ALS ME-MS61; Majorelements to map Alteration. Major elements track changes in the abundance of rock-forming minerals and alteration minerals.
- 20. Lithogeochemistry Workflow; Rocktypes 1. xY plots Sc vs Cr, Mg, Al, Zr (to pick ultramafic rocks) 2. xY plots Sc vs Ti, Th, V, Zr, Nb, P 3. xY plots Ti vs Sc, Th, V, Zr, Nb, P 4. Check Sc vs Cr, Al, La, Ce 5. Plot Sc vs V to check for magnetite fractionation 6. Plot Zr vs Hf to check for zircon fractionation Lithogeochemistry Workflow; Alteration 1. K/Al (molar) vs Na/Al (molar) to pick sericite and advanced argillic alteration 2. Ca-K-Na ternary plot to pick hydrothermal feldspars 3. Al-K-Mg ternary plot to pick Mg metasomatism 4. Fe vs S to pick sulfidation 5. Cu-Fe-S ternary to pick Cu-sulfide mineralogy 6. Ca vs Mg to pick carbonate mineralogy
- 21. Lithogeochemistry Workflow; Alteration2 1.Plot Xy plot of Sc vs Cu, Zn, Mn, Co, Ni, In to look for evidence of enrichment or depletion of divalent transition element metals 2. Plot K vs Tl (thallium) to look for low temp Tl-bearing pyrite 3. Plot K vs Cs to look for low temp, disordered illite 4. Plot K vs Ba to look for evidence of barite 5. Plot Sc vs V to look for evidence of organic carbon (in black rocks) or extreme oxidation (in red rocks) 6. Plot V vs Mo, U, Cr, As to look for evidence of organic carbon Lithogeochemistry Workflow; Pathfinders 1. Split cumulative frequency plots, coloured by mineralogy to look fro correlations between pathfinders and alteration; Au, Cu, Mo, Sn, W, Se, Bi, Te, As, Sb, Tl, etc
- 22. Common Hydrothermal minerals MineralComposition Cations Ratio Illite KAl3Si3O10(OH)2 K/Al 1/3 Orthoclase KAlSi3O8 K/Al 1/1 Albite NaAlSi3O8 Na/Al 1/1 Montmorillonite (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2·nH2O Kaolinite Al2Si2O5(OH)4 Pyrophyllite Al2Si4O10(OH)2 Dickite Al2Si2O5(OH)4 Calcite CaCO3 Dolomite CaMg(CO3)2 Biotite K(Mg,Fe)3AlSi3O10(OH)2 K/Al 1/1 Chlorite (Fe,Mg)5Al2Si3O10(OH)8 (Fe+Mg)/Al 5/2
- 24. Useful Conversion Factors Alx 1.889 = Al2O3 Na x 1.348 = Na2O Ba x 1.699 = BaSO4 Nb x 1.431 = Nb2O5 Ba x 1.117 = BaO P x 2.291 = P2O5 Be x 2.775 = BeO Rb x 1.094 = Rb2O C x 3.663 = CO2 Si x 2.139 = SiO2 Ca x 1.399 = CaO Sn x 1.270 = SnO2 Ca x 2.497 = CaCO3 Sr x 1.183 = SrO Cr x 1.462 = Cr2O3 Ta x 1.221 = Ta2O5 F x 2.055 = CaF2 Th x 1.138 = ThO2 Fe x 1.286 = FeO Ti x 1.668 = TiO2 Fe x 1.430 = Fe2O3 U x 1.179 = U3O8 K x 1.205 = K2O V x 1.785 = V2O5 Mg x 1.658 = MgO W x 1.261 =WO3 Mg x 3.468 = MgCO3 Y x 1.270 = Y2O3 Mn x 1.291 = MnO Zr x 1.351 = ZrO2
- 25. Examples of recommendedscatterplots; selecting Ultramafic rocks Start by plotting Sc versusCr, Mg, Al and Zr. It is not always reliable picking ultramafic rocks from the Mg content since Mg can be mobile during alteration and it is easily stripped during weathering. Ultramafic rocks have >1000ppm Cr. They will have a low Al content (since they have abundant olivine, but low plagioclase, and they will have low Zr. We are looking for those samples that meet all of these criteria.
- 26. Examples of recommendedscatterplots; mafic rocks xY plots Sc vs Ti, Th, V, Zr, Nb, P; This is an example that shows sequential lava flows (eruptions) from a fractionating magma chamber. The ultramafics were picked from the previous Sc-Cr plot. Point density contour overlays were used here to highlight the compositional clusters in the data. The mafic rocks typically have 30 to 50ppm Sc.
- 27. Examples of recommendedscatterplots; mafic rocks xY plots Sc vs Ti, Th, V, Zr, Nb, P; This is the same plot as the previous slide with the point density contoursremoved. The red arrows show fractionation pathways. Ti and V are fractionation indicators for titanomagnetite. V and Sc substitute for Fe in silicate minerals. However,V can substitute into oxides, but Sc does not. When these melts start to crystallize significant amounts of titanomagnetite, the Ti and V contents increase, until the melt becomes depleted in V (pink group). All the time, the HFSE elements (eg Zr, Nb, Th, REE) are increasing during fractionation.
- 28. Examples of recommendedscatterplots; mafic rocks xY plots Ti vs Sc, Th, V, Zr, Nb, P; Plotting Sc first is useful to pick broad compositional groups eg, mafic, intermediate, felsic. Plotting Ti first is useful for picking fractionation sequences.
- 29. Examples of recommendedscatterplots. If we plot Ti (which is hosted in early crystalizing Fe-Ti oxides) against Zr, Hf, Th, La, Ce, etc (which are hosted in late crystallizing zircons and phosphates) then we will see an array of linear trends that project back towards the origin. Mafic rocks will plot with a steep slope; felsic rocks with a shallow slope.
- 30. Examples of recommendedscatterplots. Plot Ti versus Nb. There are a couple of scattered andesitic populations in purple and mauve. Note the pale blue group; these are sample that are quartz-rich; Quartz-rich to the extent that all the other components are diluted! Mantle melts plot with a high Ti/Nb ratio. Crustal melts and fractionated magmas plot with a low Ti/Nb ratio.
- 31. Examples of recommendedscatterplots; magnetite fractionation Plot Sc versus V. Most magmas have a Sc to V ratio of around 1:7. Both Sc and V substitute for Fe in amphibole and pyroxene, and they tend to have very linear correlations. However, V can substitute into oxides, but Sc does not. There is a very high partition coefficient of V into titanomagnetite. Calc-alkaline rocksbegin fractional crystallization of magnetite early in the cooling history. As the melts fractionate, V is depleted. These rocks show a very clear sequence of magnetite fractional crystallization, indicated by the array of arrows.
- 32. Examples of recommendedscatterplots, Zircon Fractionation Hf and Zr always plot with a near perfect straight line correlation; Hf can only substitute into the lattice of zircon crystals. However, as zircon crystallizes, the melt very gradually evolves to higher Hf/Zr ratios. Zircons tend not to nucleate as new crystals, rather they just form overgrowingrims, so the final Hf/Zr ratio remains constant. However, where there is fractional crystallization of zircons, early formed zircons are left behind in a restite phase, and the separated melt has a lower zircon content but a higher Hf/Zr ratio.
- 33. Crustal melts Mantlemelts Fractional Crystallization of zircons Project X Regional Lithogeochemistry, Fractionation plots Usually these trends are easily recognised with resorting to ratios or log plots, but here is an example where it works really well; log Hf/Zr versus log Ti/Nb
- 34. Crustal melts Mantlemelts Fractional Crystallization of magnetite Project X Regional Lithogeochemistry, Fractionation plots Usually these trends are easily recognised with resorting to ratios or log plots, but here is an example where it works really well; log Sc/V versus log Ti/Nb
- 35. Alteration Classification With a4 acid digest method, the changes in whole rock chemistry due to hydrothermal alteration reactions can be investigated. Consider a rock that is totally sericitized. The mineralogy of the rock might be muscovite-quartz-carbonate- pyrite. All of the K and Al in that rock will be within sericite. Muscovite has a composition of KAl3Si3O10(OH)2.Thereforethe ratio of K:Al in the sericitized rock is 1:3. Similarly, a totally K feldspar (KAlSi3O8) altered rock will have a K:Al ratio of 1:1. In the same way, albitisation can also be tracked. Albite is NaAlSi3O8: Na:Al =1:1. Relatively unaltered dacite Albite Illite Alkali Feldspar Adularia Chlorite K/Al versus Na/Al molar ratio plot
- 36. 3 6 Anorthite Albite Ca Na K Alteration Classification Ca:K:Na ternaryplot with point density contour overlay; useful for mapping hydrothermal feldspar compositions. Sodic-Calcic Alteration (Oligoclase) Albite Alteration Potassic Alteration Kspar or Muscovite Least-altered andesite
- 37. Alteration Classification Al-K-Mg ternaryplot to pick Mg metasomatism.
- 38. Alteration Classification Al-K-Mg ternaryplot to pick Mg metasomatism.
- 39. Alteration Classification; Extentof sulfidation Plot Fe versus S. On this plot, the pyrite line shows Fe to S ratios that match the stoichiometry of pyrite, ie, Fe:S (molar) = 1:2 In the vast majority of hydrothermal systems, pyrite precipitation is just a sulfidation process; the reaction just utilizes Fe that is already present in the rock. The trend of increasing Fe and S up the pyrite line maps samples that contain additional pyrite in veins. Points that plot of the Sulfur-rich side of the pyrite line contain sulfates. sulfidation
- 40. Alteration Classification Plot Caversus Mg to map carbonate mineralogy. For example, this plot shows a limestone (40 wt% Ca) being partially replaced by dolomite.
- 41. Sulfide Mineralogy Plot Cu-Fe-Sternaryto map sulfide mineralogy.
- 42. Alteration Classification Plot Xyplot of Sc vs Cu, Zn, Mn, Co, Ni, In to look for evidence of enrichment or depletion of divalent transition element metals. This plot shows that samples with relatively acid alteration mineralogy have depleted levels of Zn, Mn, Ni and Co relative to less-altered samples of the same lithology. This also provides the basis for identifying samples with enrichment of these metals relative to background.
- 43. Alteration Classification Plot Kvs Tl (thallium) to look for low temp Tl-bearing pyrite. Thallium is a most unusual element as it can reside in either silicates of sulfides. In the vast majority of cases, Tl substitutes for K in silicate minerals. This producesa linear trend on a K vs Tl plot. At low temperatures, Tl can substitute into the lattice of pyrite. These points plot on the Tl-rich side of the silicate trend. Thallium in sulfides
- 44. Organic Carbon Signature PlotSc vs V to look for evidence of organic carbon (in black rocks) or extreme oxidation (in red rocks). When all the V and Sc are hosted in silicate minerals, the V:Sc ratio is typically around 7:1 (along the trend of the arrow) The orange and dark green points are mildly enriched in Vanadium; the red points are highly enriched.
- 45. Organic Carbon Signature PlotV vs Mo, U, Cr, As to look for evidence of organic carbon. The Vanadium-enriched samples from the previous figure show a correlation with elevated Mo, U, (Bi, As, Sb). The V-Mo-U signature in particular in a classic organometallic signature.