Interpreting Multielement Geochemistry data

Transcription

1 Mineralogical Patterns in Hydrothermal Systems. A seminar presented by; Interpreting Multielement Geochemistry data Scott Halley July 2016

2 ALS ME-MS61 4 acid digest uses a combination of HCl(hydrochloric acid), HNO 3 (nitric acid), HF (hydrofluoric acid) and HClO 4 (perchloricacid). Because hydrofluoric acid dissolves silicate minerals, these digestions are often referred to as 'near-total digestions'. 48 elements by ICP-MS and ICP-AES analysis.

3 ALS ME-ICP61 4 acid digest uses a combination of HCl(hydrochloric acid), HNO 3 (nitric acid), HF (hydrofluoric acid) and HClO 4 (perchloricacid). Because hydrofluoric acid dissolves silicate minerals, these digestions are often referred to as 'near-total digestions'. 33 elements by ICP-AES analysis.

4 Applied Lithogeochemistry 3 objectives Identify Rock 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 importance of ; Digest Method Assay Method Detection Limit Mineralogical Controls What is background?

6 Importance of Detection Limits Bismuth assays by ICP-MS, detection limit 0.01ppm Blind Porphyry Cu deposits 300m below surface Outcropping Porphyry Cu deposit

7 Importance of Detection Limits Bismuth assays by ICP-AES, detection limit 5ppm Blind Porphyry Cu deposits 300m below surface Outcropping Porphyry Cu deposit Must use ICP-MS to get useful detection limits for most elements

8 Accuracy versus Precision

9 Accuracy versus Precision Comparison of 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. Lithium Borate Fusion 4 acid digest 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 This is 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. Scversus Zr

11 Tungsten Comparison of assays from 4 acid digest versus aqua regia digest. Is W insoluble as H 2 WO 4 or is it in silicates? Aqua Regia digest 4 acid digest This is a data is from soil geochemover porphyry Cu project; samples have been assayed twice; once with an aqua regiadigest AND once with a 4 acid digest.

12 Mineralogical Controls What are the 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 Pathfinder Chemistry Average crustal Abundance

14 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! Zinc; blue<50ppm, red>500ppm Antimony; blue<1ppm, red>10ppm

15 Pyrite is a host for a wide variety of pathfinder elements. This soil geochemsurvey 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. Hangingwall stratigraphy Projected position of massive sulfide VMS horizon Footwall stratigraphy VMS stringer zone. Cu is in chalcopyrite; restricted to sparse veinlets. 1km Sbis in the lattice of the pyrite. Maps the footprint of the stringer zone.

16 What is the background? For some elements, the background is strongly lithologydependent. 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 pathfinder elements 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; Immobile trace 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; Major elements to map Alteration. Major elements track changes in the abundance of rock-forming minerals and alteration minerals.

20 Lithogeochemistry Workflow; Rock types 1. xyplots Scvs Cr, Mg, Al, Zr(to pick ultramafic rocks) 2. xyplots Scvs Ti, Th, V, Zr, Nb, P 3. xyplots Tivs Sc, Th, V, Zr, Nb, P 4. Check Scvs Cr, Al, La, Ce 5. Plot Scvs V to check for magnetite fractionation 6. Plot Zrvs Hfto 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 Xyplot of Scvs 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 Scvs 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, colouredby 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 Mineral Composition Cations Ratio Illite KAl 3 Si 3 O 10 (OH) 2 K/Al 1/3 Orthoclase KAlSi 3 O 8 K/Al 1/1 Albite NaAlSi 3 O 8 Na/Al 1/1 Montmorillonite (Na,Ca) 0.33 (Al,Mg) 2 (Si 4 O 10 )(OH) 2 nh 2 O Kaolinite Al 2 Si 2 O 5 (OH) 4 Pyrophyllite Al 2 Si 4 O 10 (OH) 2 Dickite Al 2 Si 2 O 5 (OH) 4 Calcite CaCO 3 Dolomite CaMg(CO 3 ) 2 Biotite K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 K/Al 1/1 Chlorite (Fe,Mg) 5 Al 2 Si 3 O 10 (OH) 8 (Fe+Mg)/Al 5/2

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24 Useful Conversion Factors Al x = Al2O3 Ba x 1.699= BaSO4 Ba x = BaO Be x =BeO C x = CO2 Ca x = CaO Ca x = CaCO3 Cr x = Cr2O3 F x = CaF2 Fe x = FeO Fe x = Fe2O3 K x = K2O Mg x = MgO Mg x = MgCO3 Mnx = MnO Na x = Na2O Nbx = Nb2O5 P x = P2O5 Rbx = Rb2O Si x = SiO2 Sn x = SnO2 Srx = SrO Ta x = Ta2O5 Thx = ThO2 Tix = TiO2 U x = U3O8 V x = V2O5 W x =WO3 Y x = Y2O3 Zrx = ZrO2

25 Examples of recommended scatterplots; selecting Ultramafic rocks Start by plotting Scversus Cr, 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 recommended scatterplots; mafic rocks xyplots Scvs 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 recommended scatterplots; mafic rocks xyplots Scvs Ti, Th, V, Zr, Nb, P; This is the same plot as the previous slide with the point density contours removed. The red arrows show fractionation pathways. Tiand V are fractionation indicators for titanomagnetite. V and Scsubstitute for Fe in silicate minerals. However, V can substitute into oxides, but Scdoes not. When these melts start to crystallize significant amounts of titanomagnetite, the Tiand 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 recommended scatterplots; mafic rocks xyplots Tivs Sc, Th, V, Zr, Nb, P; Plotting Scfirst is useful to pick broad compositional groups eg, mafic, intermediate, felsic. Plotting Tifirst is useful for picking fractionation sequences.

29 Examples of recommended scatterplots. If we plot Ti(which is hosted in early crystalizing Fe-Tioxides) 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 recommended scatterplots. 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 recommended scatterplots; magnetite fractionation Plot Scversus V. Most magmas have a Scto V ratio of around 1:7. Both Scand V substitute for Fe in amphibole and pyroxene, and they tend to have very linear correlations. However, V can substitute into oxides, but Scdoes not. There is a very high partition coefficient of V into titanomagnetite. Calc-alkaline rocks begin 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 recommended scatterplots, Zircon Fractionation Hfand Zralways plot with a near perfect straight line correlation; Hfcan only substitute into the lattice of zircon crystals. However, as zircon crystallizes, the melt very gradually evolves to higher Hf/Zrratios. Zircons tend not to nucleate as new crystals, rather they just form overgrowing rims, so the final Hf/Zrratio remains constant. However, where there is fractional crystallization of zircons, early formed zircons are left behind in a restitephase, and the separated melt has a lower zircon content but a higher Hf/Zrratio.

33 Project X Regional Lithogeochemistry, Fractionation plots Usually these trends are easily recognisedwith resorting to ratios or log plots, but here is an example where it works really well; log Hf/Zr versus log Ti/Nb Fractional Crystallization of zircons Crustal melts Mantle melts

34 Project X Regional Lithogeochemistry, Fractionation plots Usually these trends are easily recognisedwith resorting to ratios or log plots, but here is an example where it works really well; log Sc/V versus log Ti/Nb Fractional Crystallization of magnetite Crustal melts Mantle melts

35 Alteration Classification With a 4 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-carbonatepyrite. All of the K and Al in that rock will be within sericite. Muscovite has a composition of KAl 3 Si 3 O 10 (OH) 2. Therefore the ratio of K:Al in the sericitized rock is 1:3. Similarly, a totally K feldspar (KAlSi 3 O 8 ) altered rock will have a K:Al ratio of 1:1. In the same way, albitisation can also be tracked. Albite is NaAlSi 3 O 8 : Na:Al=1:1. K/Al versus Na/Al molar ratio plot Adularia Alkali Feldspar Relatively unaltered dacite Illite Chlorite Albite

36 Alteration Classification Ca:K:Na ternary plot with point density contour overlay; useful for mapping hydrothermal feldspar compositions. Ca Anorthite Least-altered andesite Potassic Alteration Kspar or Muscovite Sodic-Calcic Alteration (Oligoclase) Albite Alteration K Na Albite 3 6

37 Alteration Classification Al-K-Mg ternary plot to pick Mg metasomatism.

38 Alteration Classification Al-K-Mg ternary plot to pick Mg metasomatism.

39 Alteration Classification; Extent of 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 Ca versus 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-S ternary to map sulfide mineralogy.

42 Alteration Classification Plot Xyplot of Scvs 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 K vs 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 produces a linear trendona 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 Plot Scvs V to look for evidence of organic carbon (in black rocks) or extreme oxidation (in red rocks). When all the V and Scare 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 Plot V 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.