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Geochemistry of Mafic Layered Intrusions Do s and Don’t s

James D. Miller Precambrian Research Center Department of Geological Sciences University of Minnesota Duluth. Geochemistry of Mafic Layered Intrusions Do s and Don’t s. Workshop on Nickel -Copper-Platinum Group Element Mineralization  Thunder Bay, Ontario January 14, 2011.

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Geochemistry of Mafic Layered Intrusions Do s and Don’t s

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  1. James D. Miller Precambrian Research Center Department of Geological Sciences University of Minnesota Duluth Geochemistry of Mafic Layered Intrusions Dos and Don’ts Workshop on Nickel -Copper-Platinum Group Element Mineralization  Thunder Bay, Ontario January 14, 2011

  2. Geochemical Analyses for Exploration The Problem with Cumulates Major Element Chemistry Trace Element Chemistry Mineral Chemistry Assay Data for Cu-Ni-PGE Mineralized Intrusions Outline

  3. PURPOSE OF GEOCHEMICAL ANALYSES OF MLI ROCKS IN EXPLORATION (IN ORDER OF IMPORT) ESTABLISH GRADE OF ORE DEPOSIT EVALUATE THE POTENTIAL FOR MINERALIZATION EVALUATE THE COMPOSITION OF THE PARENTAL MAGMA (SOURCE OF METALS) AND POSSIBLE CONTAMINANTS (COMMONLY THE SOURCE OF S) EVALUATE THE CRYSTALLIZATION AND DIFFERENTIATION HISTORY OF THE MAGMA Geochemical Analyses for Exploration

  4. Geochemical Analyses for Exploration ICP-MS/AES Part. digestion $20-25/smpl XRF+ICP-MS Full digestion >$60/smpl 2009 Acme Analytical Lab Brochure Fire Assay $20-30/smpl No Si

  5. The Problem with Cumulates The Classic View of Cumulate Rocks is that they are Mixtures of Primocrysts and a Liquid Component • Primocrysts are: • Enriched in high-T solid solution components (Mg in mafic phases, Ca in plagioclase) • Enriched in compatible trace elements (e.g. Ni in Ol, Sr in Pl) • Liquid component is: • Enriched in low-T solid solution components (Fe in mafic phases, Na,K in plagioclase) • Enriched in incompatible minor and trace elements

  6. The Problem with Cumulates • The concentration (X) of any element (a) in a cumulate rock (WR) is dependent on: • The relative proportions of primocrysts (PC) and the liquid component (LC) • The compositions of those components • Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC)

  7. The Problem with Cumulates What parts of this mass balance can we know? Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC) Modes of Primocrysts? Problem is that cumulus phases continue to crystallize post-cumulus rims Plagioclase – possible if zoning preserved, but painstaking Olivine and pyroxene –not precisely, zoning lost due to subsolidus re-equilibration Oxide - ????

  8. The Problem with Cumulates What parts of this mass balance can we know? Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC) Compositions of Primocrysts? Problematic because most primocrysts are solid solutions phases Plagioclase – possible, but cumulus cores can be very complexly zoned Olivine and Pyroxene – Ease of re-equilibration leads to “trapped liquid shift” Oxides – easily re-equilibrated and oxy-exsolved

  9. The Problem with Cumulates What parts of this mass balance can we know? Xa(WR) = %PC1* Xa(PC1) + %PC2* Xa(PC2) + ..... + %LC* Xa(LC) Compositions of the “Trapped” Liquid Component.... and might this be representative of the Parent Magma? Problem is ... the amount of liquid in the cumulate changes over time due to compaction driven by bouyancy and crystal accumulation, and .... the composition of the liquid in the cumulate changes over time due to fractional crystallization of the intercumulus liquid. From Tegner et al., 2009

  10. The Problem with Cumulates Rare Solution to Estimating the Parent Magma of an MLI – Chill Zone Tamarack Intrusion, Minnesota ~1cm Basal Chill Zone – 30% Ol phenocrysts in a fine gabbroic groundmass (Goldner, UMD MS thesis, in progress)

  11. Major Element Chemistry Tamarack Parent Magma Calculation PELE Crystallization Models (Boudreau, 2005) Equilibrium Crystallization QFM buffer, P=1 Kb Fractional Crystallization QFM buffer, P=1 Kb (Goldner, UMD MS thesis, in progress)

  12. Major Element Chemistry DO NOT PLOT CUMULATES ON PLOTS INTENDED FOR MAGMA COMPOSITIONS Primocrystic Plagioclase fractionation Liquid Component

  13. Major Element Chemistry Troctolite Cumulate (PO) Fo77 olivine, An76 plagioclase, mg#50 liquid Dunite Cumulate (O) Fo84 olivine, mg#60 liquid plactioclase + olivine orthocumulate orthocumulate mesocumulate mesocumulate adcumulate adcumulate porphyritic porphyritic olivine liquid liquid

  14. Major ELement Chemistry Major and Minor Element chemistry can serve as approximate proxies for abundances of primocryst phases e.g. Al – Plagioclase Mg – Augite + Olivine Ti – Ilmenite and Ti-magnetite From Joslin (2004)

  15. Major ELement Chemistry Major and Minor Element chemistry (that includes accurate SiO2) can be used to calculate CIPW Norms. Helpful for metamorphosed / altered cumulates (assuming a closed system) Helpful for calculating average An content (Ca/(Ca+Na+K)) of complexly zoned plagioclase

  16. Trace ELement Chemistry Like major elements, the absolute concentration of a trace element in a cumulate rock is typically dependent on the relative proportions AND compositions of the primocrysts and the liquid component. Liquid Cpx? Primocrysts

  17. Trace ELement Chemistry Compatibility – degree to which an element prefers to partition into the solid over the liquid phase . Kd(i)1– Mineral-Liquid Partition Coefficient for element iin mineral 1 Kd(i)1= C(i)mineral1/ C(i)liquid (C(i) - concentration of element i in wt. %) Kd(i)1> 1 – Compatible, Kd(i)1< 1 – Incompatible D(i)– Bulk Rock Partition Coefficient for element i D(i) = x1Kd(i)1+ x2Kd(i)2+ x3Kd(i)3+ .... (x1 – proportion of mineral 1)

  18. Trace ELement Chemistry Bulk Rock Partition Coefficient of Ce,Yb, and Ni for Crystallization of: 1) Troctolite (70% Pl, 30% Ol) D(Ce) = xPlKd(Ce)Pl+ xOlKd(Ce)Ol = .7*.103 + .3*.007 = 0.092 D(Yb) = xPlKd(Yb)Pl+ xOlKd(Yb)Ol = .7*.07 + .3*.065 = 0.069 D(Ni) = xPlKd(Ni)Pl+ xOlKd(Ni)Ol = .7*.01 + .3*25= 7.5 2) Olivine Gabbro (63% Pl, 12% Ol, 25% Cpx) D(Ce) = xPlKd(Ce)Pl+ xOlKd(Ce)Ol+ xCpxKd(Ce)Cpx = .63*.103 + .12*.007 + .25*.09 = 0.088 D(Yb) = xPlKd(Yb)Pl+ xOlKd(Yb)Ol+ xCpxKd(Yb)Cpx = .63*.07 + .12*.065 + .25*.09 = 0.074 D(Ni) = xPlKd(Ni)Pl+ xOlKd(Ni)Ol + xCpxKd(Ni)Cpx = .63*.01 + .12*25 + .25*8 = 5 Compatible Incompatible From Rollinson (1993)

  19. Trace ELement Chemistry Rayleigh Distillation: CL/Co = F(D-1) CL/Co Troctolite CL/Co Olivine Gabbro F (fraction of liquid remaining) F (fraction of liquid remaining) Conclusions: Fractional crystallization of mafic magmas gradually increases the concentrations of similarly incompatible elements, but has a minimal effect on their ratios; and strongly decreases the concentrations of compatible elements

  20. Trace ELement Chemistry Fractional crystallization of olivine from a komatiitic melt Since incompatible elements are 2-3 orders of magnitude greater in abundance than primocrysts, the REE pattern of cumulates (especially orthocumulates and adcumulates) will approximate that of their parental magmas and the magma source Fractional crystallization increases the REE abundance, but has a neglible effect on the REE pattern From Rollinson (1993) From Jirsa and Miller (2006)

  21. Trace element chemistry Proposed Tamarack Parent Magma Trace Element Composition compared to Early MCR Volcanics Sample/Primitive Mantle (Goldner, UMD MS thesis, in progress)

  22. Trace ELement Chemistry Spidergrams Tectonic Discrimination Diagrams rock/chondrite rock/chondrite From Bedard (2001) Increasing compatibility

  23. Mineral Chemistry Stratigraphic variations in the compositions of solid-solution cumulus minerals generally reflect the progressive differentiation of the parental magma and the occurrence of recharge event... but not exactly. From Miller (2004)

  24. Mineral Chemistry PLAGIOCLASE Zoning is preserved and records a history of cumulus and postcumulus crystallization Strategy 1: Compare only An of cumulus cores... Problem: Cores are commonly complexly zoned Strategy 1: Calculate An from CIPW norm Problem: Integrates cumulus and postcumulus components White (2009)

  25. Mineral Chemistry OLIVINE AND PYROXENE Zoning is NOT preserved and thus integrates cumulus and postcumulus compositions Overcoming the Trapped Liquid Shift Solution: Compare only like types of cumulates (ortho, meso, ad) Problem: Evaluating the type of cumulate is qualitative

  26. Mineral Chemistry Evaluating the Trapped Liquid Shift POcf cumulate TLS Gradual increase in incompatible elements; can assume nearly constant over limited stratigraphic thickness Magma Recharge From Miller (2006)

  27. Mineral Chemistry Evaluating the Trapped Liquid Shift POcf cumulate Postcumulus mineral abundance are general proxies for amount of trapped liquid TLS From Miller (2006)

  28. Mineral Chemistry Evaluating the Trapped Liquid Shift POcf cumulate Assuming that well foliated cumulates have lower porosity – i.e. lower volume of trapped liquid TLS From Meurer & Boudreau (1997) From Miller (2006)

  29. Mineral Chemistry Mineral chemistry also allows the estimation of the magma composition in equilibrum with that mineral KD = (XFeOol/XFeOliq)*(XMgOliq/XMgOol) = 0.3 (Roedder and Emslie, 1970) which translates in determining the mg# of the liquid as: mg#liq = 100 / (3.333(FeO/MgO)ol + 1) This assumes no trapped liquid shift. Therefore, one should apply this only to adcumulates A procedure for calculating the equilibrium distribution of trace elements among the minerals of cumulate rocks, and the concentration of trace elements in the coexisting liquids I Jean H. Bedard Chemical Geology 118 ( 1994 ) 143-153

  30. Meters above Cu-Au break Assay data Variation in the Cu/Pd is one of the best monitors of sulfide saturation in magmatic systems, but need high precision Pd analyses (<2 ppb DT) Sonju Lake Intrusion Precious Metals Zone (PMZ) Greenwood Lake Intrusion From Miller (2004) From Joslin (2004)

  31. Assay data Dsulf/sil~104-108 Pd is several orders of magnitude more compatible in sulfide melt relative to Cu Dsulf/sil~102

  32. Assay data R = Xsil/Xsulf after Barnes and others, 1987

  33. Assay data Cu/Pd Pd(ppb) From Jirsa & Miller (2006) From Joslin (2004)

  34. Assay data Quadrant w/ No Potential Quadrant w/ Potential at Depth Indicates duration since initial saturation Quadrant w/ Best Potential Quadrant w/ Ore Grade From Jirsa & Miller (2006)

  35. Assay data From Jirsa & Miller (2006)

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