Cu Ni Au Mineralization

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    ORIGIN AND OCCURRENCE OFPLATINUM GROUP ELEMENTS, GOLD AND SILVER

    IN THE SOUTH FILSON CREEKCOPPER-NICKEL MINERAL DEPOSIT,

    LAKE COUNTY, MINNESOTA

    by

    Mary Jo P. Kuhns, Steven A. Hauck and Randal J. Barnes*

    March, 1990

    Technical ReportNRRI/GMIN-TR-89-15

    Funded by the Greater Minnesota Corporation

    Natural Resources Research Institute *Dept. Civil and Min. EngineeringUniversity of Minnesota, Duluth University of Minnesota5013 Miller Trunk Highway 500 Pillsbury Drive S.E.Duluth, Minnesota 55811 Minneapolis, Minnesota 55455

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    ABSTRACT

    The South Filson Creek Cu-Ni-PGE-Au-Ag mineral occurrence is located on the

    western margin of the Duluth Complex in Lake County, northeastern Minnesota. The

    occurrence of primary magmatic and late-stage, structurally controlled mineralization is

    located in the South Kawishiwi intrusion of the Duluth Complex, approximately 2200 feet

    above the basal contact. The primary host rock for the mineralization is a medium-grained

    augite troctolite. Petrographic studies indicate that there were at least two episodes of

    mineralization. Deposition of primary, coarse-grained, interstitial pyrrhotite, pentlandite, and

    chalcopyrite occurred in "cloud zones". Primary mineralization was followed by the

    introduction of hydrothermal fluids along fracture zones, as evidenced by the formation of

    hydrous minerals, sulfide replacement textures and geochemical signatures suggestive of

    remobilization. These late-stage fluids deposited secondary sulfides at redox boundaries

    created by the primary sulfides. The secondary assemblage includes chalcopyrite, bornite,

    chalcocite, digenite, covellite, violarite, sphalerite, mackinawite, valleriite, and the platinum

    group minerals, all which occur in extremely fine, discontinuous veinlets that are rarely

    recognizable in hand specimen. The veinlets were created by hydrofracturing of silicate

    minerals due to a volume increase initiated by serpentinization of olivine. These veinlets are

    always proximal to highly serpentinized fractures and are possibly associated with a proposed

    NE-trending fault zone along the south branch of Filson Creek.

    The copper-nickel ratio for the deposit is about 3:1. Platinum + palladium correlates

    with high copper and sulfur. Also, high inter-element correlation between Cu, Ni, Pd, Pt and

    Au suggests that secondary enrichment of these elements is local in extent and related to

    faulting and redox boundaries. Statistical analysis suggests, given the available data, that in-

    fill drilling could discover a significant quantity of mineralization.

    The alteration assemblage associated with the secondary mineralization is serpentine,

    biotite, stilpnomelane, iddingsite, chlorite, sericite, and clay minerals. The alteration is very

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    subtle and is best recognized in thin section. Both alteration and mineralized zones range in

    thickness from less than one foot to 90 feet.

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    TABLE OF CONTENTS

    ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

    LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

    LIST OF PLATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

    LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

    LIST OF APPENDICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii

    INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    REGIONAL GEOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    STRUCTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    GEOPHYSICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

    MINERALOGY AND TEXTURAL RELATIONSHIPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7ROCK FORMING MINERALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    Plagioclase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Olivine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Clinopyroxene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Orthopyroxene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Apatite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    OXIDES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    Ilmenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Magnetite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10SULFIDE MINERALIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    Pyrrhotite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Pentlandite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Chalcopyrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Cubanite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    SECONDARY SULFIDES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Chalcopyrite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Talnakhite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Bornite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Digenite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    Chalcocite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Covellite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Violarite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Mackinawite and Valleriite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Other Secondary Minerals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Sperrylite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    ALTERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17SERPENTINE/MICA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17ARGILLIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

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    HEMATITE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18GREENSCHIST ASSEMBLAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    GEOCHEMISTRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20ELEMENTAL RATIOS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20GEOCHEMICAL PLOTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22RARE-EARTH ELEMENT (REE) CHONDRITE PLOT . . . . . . . . . . . . . . . . . . . . . . . . 23

    PGE CHONDRITE PLOTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24CHEMICAL REACTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25SULFUR ISOTOPES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

    GEOSTATISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28DATA SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

    Drilling Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Summary Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Inter-Variable Correlations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Observations and Concerns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

    GROSS ECONOMIC AUXILIARY VARIABLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Creating the Auxiliary Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    Summary Statistics for the Auxiliary Variable . . . . . . . . . . . . . . . . . . . . . . . . . 32SPATIAL STATISTICS AND GEOLOGIC CONTINUITY . . . . . . . . . . . . . . . . . . . . . . 34Variogram Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Other Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

    GEOSTATISTICAL CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

    DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

    CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41BENEFITS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    APPENDIX A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

    APPENDIX B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

    APPENDIX C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . floppy diskette

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    LIST OF FIGURES

    Figure 1. Location of map of Cu-Ni mineral deposits in the Duluth Complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    Figure 2. Probable location of PGE-bearing fracture zones, South FilsonCreek Cu-Ni-precious metal prospect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    Figure 3. Rock classification chart for mafic and ultramafic rocks in theDuluth Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    Figure 4. Drill hole and cross-section location map, South Filson Creek . . . . . . . . . . . . . . 7

    Figure 5. Photomicrograph of secondary sulfide veinlets cross-cuttingprimary chalcopyrite, reflected light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    Figure 6. Photomicrograph of sperrylite grain replacing chalcopyrite in

    serpentine pocket, reflected light, crossed polars . . . . . . . . . . . . . . . . . . . . . . . 16

    Figure 7. Photomicrograph of biotite (light brown) and stilpnomelane (red-brown) rimming primary sulfide grain (black), plane polarizedlight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    Figure 8. Photomicrograph of titanite (sphene) crystal forming fromactinolite groundmass, crossed polars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    Figure 9. Paragenetic diagram of sulfide, oxide, and alteration mineralogy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

    Figure 10. Geochemical plots of South Filson Creek data: A. Percent Cuvs. percent Ni; B. Pt vs. Pd; C. Pd vs. Ir; D. Pt+Pd vs. Ru+Ir+Os. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    Figure 11. Geochemical plots of South Filson Creek data (cont.): A. Pt+Pdvs. log percent Cu; B. Pd vs. log percent Ni; C. Pt+Pd vs.Cu/(Cu+S); D. Pt+Pd vs. percent S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    Figure 12. REE chondrite plot of samples from the mineralized zone . . . . . . . . . . . . . . . . 24

    Figure 13. PGE chondrite plot. A. South Filson Creek samples; B.Comparison with other PGE occurrences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

    Figure 14. Schematic diagram of the relationship of serpentinized fracturezones to mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

    Figure 15. Location map of petrological samples collected at South FilsonCreek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

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    LIST OF PLATES

    Plate 1. South Filson Creek Cross-section A-A' . . . . . . . . . . . . . . . . . . . . . . . back pocket

    Plate 2. South Filson Creek Cross-section B-B' . . . . . . . . . . . . . . . . . . . . . . . back pocket

    Plate 3. South Filson Creek Cross-section C-C' . . . . . . . . . . . . . . . . . . . . . . back pocket

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    LIST OF TABLES

    Table 1A. Average Composition of Sperrylite, South Filson Creek . . . . . . . . . . . . . . . . . . 13

    Table 1B. Composition in Modal Percent for Sperrylite . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

    Table 2. Pt/Pt+Pd and Cu/Cu+Ni Ratios for Major PGE-Bearing Depositsand the South Filson Creek Deposit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    Table 3. High Grade PGE-Au-Ag and Cu-Ni Mineralization at South FilsonCreek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    Table 4. Summary Statistics for the South Filson Creek Data Set . . . . . . . . . . . . . . . . . 29

    Table 5. Estimated Inter-Variable Correlations for the South Filson CreekData Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

    Table 6. Mean & Median "Background" Metal Values for the Cloud ZoneSulfides at South Filson Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

    Table 7. Summary Statistics for the Gross Economic Variable with theSouth Filson Creek Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

    Table 8. Examples of Possible Hydrothermally-Related PGE Deposits . . . . . . . . . . . . . 40

    Table 9. Identification of Drill Holes Used in the Geostatistical Analysis . . . . . . . . . . . . 57

    Table 10. Samples Used to Estimate "Background" Metal Values inSulfides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

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    LIST OF APPENDICES

    Appendix A: Petrographic Descriptions of Samples Collected in Section 36,T. 62 N., R. 11 W. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

    Appendix B: Geostatistical Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

    Appendix C: Geochemical and Assay Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . floppy diskette

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    Figure 1. Location map of Cu-Ni mineral deposits in the Duluth Complex.

    INTRODUCTION

    The South Filson Creek prospect is located in SE 1/4, SW 1/4, Section 25, T. 62 N.,

    R. 11 W., in Lake County, Minnesota (Fig. 1). The prospect is accessed from the Spruce

    Road by a three-quarter mile south-trending dirt trail that becomes impassable approximately

    one-quarter mile north of the prospect. Previous copper-nickel exploration was done on the

    property by

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    Hanna Mining Company in the late 1960s. Twenty-four drill holes are located in the

    immediate

    vicinity of South Filson Creek (Appendix A). These holes outline sporadic, disseminated

    copper-nickel mineralization of low to moderate grade (up to 87.5 feet of 1.24% copper +

    nickel), the continuity and extent of which could not be established. American Copper &

    Nickel Company, Inc., a subsidiary of INCO, recently (1988) leased the property from the U.

    S. Forest Service.

    In 1987, in light of recent discoveries of platinum group element (PGE) values in other

    rocks of the Duluth Complex, the Natural Resources Research Institute analyzed the M. A.

    Hanna drill core (Hauck, 1988, unpubl. data) for precious metals based upon preliminary data

    and conclusions of Morton and Hauck (1987). When encouraging values (10s of feet with

    >1 ppm Pd) were returned, the current project and two others were submitted to the Greater

    Minnesota Corporation to study the occurrence and distribution of the PGE minerals in three

    Duluth Complex copper-nickel deposits (South Filson Creek, Water Hen, and Dunka Road).

    An understanding of the resultant model of this mineralized system could then be applied to

    other parts of the Duluth Complex. The aim of this project was to: 1) describe and model the

    occurrence of the South Filson Creek mineralization; 2) apply this knowledge to other deposits

    in the Duluth Complex; and 3) demonstrate the potential economic value of these data for

    precious metal mineral exploration in the Duluth Complex.

    ACKNOWLEDGEMENTS

    This study was funded by a grant from the Greater Minnesota Corporation, whose

    support is gratefully acknowledged. The M.A. Hanna Company graciously provided unlimited

    access to its core. We would also like to extend our thanks for the logistical support provided

    by the University of Minnesota Department of Geology and Geophysics, the Minnesota

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    Geological Survey, and the Mineral Resources Research Center. Dr. Penelope Morton was

    an invaluable resource for ore microscopy consultation.

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    REGIONAL GEOLOGY

    The Duluth Complex is a large mafic intrusion of Keweenawan age (1.1 Ga) in

    northeastern Minnesota. Rocks of the Duluth Complex are exposed in a large, arcuate belt

    that extends from Duluth, north toward Ely, and then northeast toward Hovland (Fig. 1). The

    Duluth Complex rocks are generally divided into an older anorthositic series and a younger

    troctolitic series. The anorthositic series rocks are all plagioclase cumulates, some of which

    are found as inclusions in the underlying troctolites. The troctolitic series is made up of

    several bodies (Foose and Weiblen, 1986; Weiblen and Morey, 1980; Severson, 1988;

    Severson and Hauck, 1990): 1) the Bald Eagle intrusion (an outer troctolite surrounding a

    core of olivine gabbro); 2) the South Kawishiwi intrusion (an augite troctolite unit below an

    upper troctolite unit); and 3) the Partridge River intrusion (augite troctolite and troctolite with

    subordinate amounts of olivine gabbro, anorthositic troctolite, and picrite). In the northern and

    western part of the Duluth Complex, large resources of copper-nickel have been identified at

    the base of the troctolitic series, just above the footwall contact with the country rocks.

    Additional potential copper-nickel resources have been located in "cloud zones", which are

    copper-rich areas of mineralization located 300 to 500 meters above the basal zone

    mineralization. Cloud zone mineralization has been described by several authors, including

    Ripley (1986) and Ervin (1987).

    The South Filson Creek deposit is one of the cloud zone mineralized zones. The

    deposit occurs within the troctolitic series rocks of the South Kawishiwi intrusion (SKI; Green,

    et al, 1966; Phinney 1969). Work by Foose and Weiblen (1986) characterizes the SKI as a

    plagioclase-olivine cumulate containing minor interstitial augite, oxides, and biotite. Modal

    layering is not common in the SKI and is absent at South Filson Creek. The mineralization at

    South Filson Creek occurs in an augite-troctolite subunit within a generally sulfide-free

    troctolite (Foose and Weiblen, 1986), at least 500 meters above the basal contact with the

    Archean (2.7 Ga) Giants Range Batholith (Fig. 1).

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    Figure 2. Probable location of PGE-bearing fracture zones, South Filson Creek Cu-Ni-precious metal prospect.

    STRUCTURE

    The large scale structure in the South Filson Creek area is poorly understood at this

    time and no major structural features have been mapped. However, fractures and shear

    zones in the core, joint measurements, and topographical lineaments suggest that there is a

    major northeast-trending, northwest-dipping fault (South Filson Creek Fault) along South

    Filson Creek (Fig. 2). Fracturing and alteration both increase toward the South Filson Creek.

    A major, steeply dipping structure is encountered near the bottom of drill holes K-21 and K-18

    (Plate 1). This structure consists of over 30 feet of highly sheared and brecciated augite

    troctolite.

    Numerous narrow, serpentine-filled fractures dominate the core in the mineralized

    zones. Fractures exhibit preferred orientations of 70, 55, and 20 degrees to the core angle.

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    These areas of serpentinization define two larger fracture zones (Fig. 2), one trending north

    and the other trending northwest. These mineralized fracture zones may be conjugate

    structures to the proposed South Filson Creek Fault.

    Microscopically, plagioclase grains show signs of strain, especially in drill holes near

    the creek. Plagioclase displays undulatory extinction, diffuse twin planes, and bent grains,

    especially when in close proximity to chloritic and serpentine fractures.

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    GEOPHYSICS

    The South Filson Creek area has a distinctive geophysical signature. The

    aeromagnetic map of the 7 1/2-minute Gabbro Lake SW quadrangle (Minnesota Geological

    Survey aeromagnetic map series) shows a well-defined, northeast-trending magnetic low

    centered on the South Filson Creek area. This low may be the result of a zone of "normal"

    troctolite sandwiched between troctolite zones containing high amounts of magnetite (V.

    Chandler, pers. comm., 1989), or could be a signature of a major northeast-trending structure

    with associated alteration and primary magnetite destruction.

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    Figure 3. Rock classification chart formafic and ultramafic rocks in the DuluthComplex. (after Phinney, 1972)

    MINERALOGY AND TEXTURAL RELATIONSHIPS

    A total of 341 thin and polished sections

    were studied for this project. Twenty-one drill

    holes were logged, thirteen of them in detail.

    Rock types were defined using the classification

    shown in Figure 3. The most abundant rock types

    were augite troctolite and anorthositic troctolite,

    with gabbro, gabbroic anorthosite, anorthositic

    gabbro, and anorthosite as subordinate

    lithologies. Coarse-grained, pegmatitic variations

    of these units were also present. Several

    samples of peridotite and feldspathic peridotite

    were also present in the deep drill hole K-1 (Fig. 4). Late stage, felsic dikes made up a very

    small portion of the section.

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    Apatite

    Apatite is present in trace amounts in several of the drill holes (K-6, K-16, K-25, and

    K-26). The distribution of apatite is highly variable, and the presence of apatite does not

    define a particular rock unit or horizon. Apatite is usually fine-grained (0.5 to 1 mm) and

    euhedral.

    OXIDES

    Ilmenite and magnetite are the two main oxide phases present at South Filson Creek.

    Ilmenite

    Ilmenite is the most abundant oxide and occurs as 70% coarse (1 to 2.5 mm), skeletal

    grains, 15 to 20% fine-grained, euhedral disseminations, and 5 to 10% in symplectitic texture

    with Opx. Ilmenite is rarely poikilitic, containing euhedral grains of Fe-chromite or spinel.

    Magnetite

    Magnetite is present as both skeletal grains (0.50 to 1 mm), as interstitial grains (0.1

    to 0.25 mm), and most dominantly as an alteration mineral in veinlets within reticulate olivine

    grains. Magnetite sometimes contains "exsolution-oxidation" lamellae of ilmenite.

    SULFIDE MINERALIZATION

    There are at least two generations of sulfides (primary and secondary) within the South

    Filson Creek deposit. Primary sulfides (3 to 5%) include coarse-grained, interstitial pyrrhotite,

    pentlandite, chalcopyrite, and cubanite. As in other cloud zone deposits, chalcopyrite is more

    abundant at South Filson Creek than pyrrhotite (Ervin, 1987). The sulfide distribution is

    controlled by: 1) texture (pegmatite zones); and 2) structure (the amount of extremely fine-

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    Cubanite occurs primarily as exsolution lamellae in primary chalcopyrite. Cubanite is

    also present as monomineralic blebs, usually finer-grained than the other primary phases.

    Modal percentage of cubanite is generally less than 3%, but can be as high as 25% in isolated

    samples from holes K-15 and K-21.

    SECONDARY SULFIDES

    Chalcopyrite

    Fine-grained chalcopyrite is the most abundant secondary sulfide in the South Filson

    Creek deposit. Secondary chalcopyrite composes approximately 60 to 80 modal percent of

    the secondary sulfides and it typically occurs as a replacement of primary pyrrhotite and

    pentlandite around grain margins and in extremely fine-grained veinlets that are widespread

    around fracture zones (Fig. 5). Fine-grained, anhedral disseminations of secondary

    chalcopyrite occur throughout the host augite troctolite in association with serpentine

    alteration.

    Talnakhite

    Talnakhite is a small but persistent phase within the secondary sulfides. It is difficult

    to distinguish from chalcopyrite in freshly polished samples, but tarnishes rapidly, sometimes

    within hours. Modal abundance for talnakhite is approximately 2 to 3 percent.

    Bornite

    Bornite is a characteristic mineral of the secondary suite, and is present in almost every

    sample that hosts secondary chalcopyrite veinlets. Bornite is the most abundant and visible

    replacement mineral of primary chalcopyrite and has a modal abundance of 1 to 7 percent.

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    Digenite

    Digenite frequently is present in those samples that have been partially replaced by

    bornite. The digenite is later than the bornite, and partially replaces it. Modal abundance of

    digenite is trace to 2 percent.

    Chalcocite

    Very fine-grained chalcocite is present in amounts of up to 3 percent of the total sulfide

    volume. Chalcocite replaces bornite and digenite as well as the primary copper sulfides, and

    is a minor constituent in the secondary veinlets.

    Covellite

    Covellite is observed replacing chalcocite in two instances. The covellite is very fine-

    grained and may be present in the secondary veinlets.

    Violarite

    Violarite is a common replacement mineral in pentlandite and chalcopyrite. The

    violarite is most abundant in drill holes containing the highest PGE values (K-21 and K-27).

    Mackinawite and Valleriite

    Fine-grained mackinawite frequently replaces pentlandite and valleriite replaces

    chalcopyrite. These minerals are very similar optically and are identified using an electron

    microprobe. Both mackinawite and valleriite replace primary sulfides along grain boundaries

    and discontinuities in the crystal structure.

    Other Secondary Minerals

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    Galena, millerite, and sphalerite are present as secondary phases in only minor

    amounts. The millerite forms secondary needles along preferred orientations in the

    pentlandite structure. The galena and sphalerite are present as inclusions in the secondary

    chalcopyrite.

    Sperrylite

    The platinum arsenide sperrylite, PtAs2 (containing 4.6% Au) was identified in several

    samples using the electron microprobe (Table 1). The sperrylite is a very late stage mineral

    replacing secondary chalcopyrite and is always in association with serpentinization (Fig. 6).

    Table 1A. Average Composition of Sperrylite, South Filson Creek

    AnalysisNumber Fe Co Ni Cu As Pt Au S Rh Pd Ag Total

    1. 0.30 0.18 0.06 0.46 41.46 45.95 4.64 0.79 0.78 0.49 0.00 95.10

    2. 0.48 0.00 0.15 0.58 41.33 49.04 5.07 1.02 0.91 0.33 0.81 99.74

    3. 0.41 0.09 0.04 0.64 41.20 43.25 3.81 1.06 0.84 0.39 0.38 92.10

    4. 0.45 0.07 0.00 0.32 40.86 44.90 4.91 0.85 0.64 0.30 0.79 94.10

    Ave. 0.41 0.09 0.06 0.50 41.21 45.78 4.60 0.93 0.79 0.37 0.49 95.26

    Table 1B. Composition in Modal Percent for Sperrylite

    AnalysisNumber Fe Co Ni Cu As Pt Au S Rh Pd Ag

    1. 0.58 0.34 0.11 0.81 64.15 27.26 2.66 2.78 0.81 0.46 0.00

    2. 0.89 0.00 0.22 1.00 61.56 28.00 2.79 3.46 0.89 0.33 0.78

    3. 0.81 0.11 0.00 1.17 64.28 25.87 2.22 3.86 0.93 0.35 0.35

    4. 0.35 0.11 0.00 0.58 64.19 27.09 2.82 3.06 0.70 0.23 0.82

    Ave. 0.84 0.17 0.11 0.89 63.29 26.93 2.66 3.33 0.87 0.39 0.51

    Note: The microprobe standards used were all pure metals, except for Fe and S in pyrite and As in indium arsenide and Cuin chalcopyrite.

    Although geochemical analyses suggest that palladium (Pd) is the major PGE in the

    deposit, no Pd minerals have been, as yet, identified. The most probable modes of

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    occurrence of the Pd minerals are: 1) in solid solution in the crystal lattices of primary or

    secondary sulfides or silicates; and/or 2) as discrete mineral phases replacing copper sulfides

    associated with serpentinization. Additional work with the electron microprobe is necessary

    to better identify these minerals.

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    Figure 5. Photomicrograph of secondary sulfide veinlets cross-cutting primary chalcopyrite,reflected light. (Cpy = chalcopyrite, Cub = cubanite, Pn = pentlandite, Vio = violarite)

    Figure 6. Photomicrograph of sperrylite grain replacing chalcopyrite in serpentine pocket,

    reflected light, crossed polars. (Sp = sperrylite, Cpy = chalcopyrite)

    Figure 7. Photomicrograph of biotite (light brown) and stilpnomelane (red-brown) rimmingprimary sulfide grain (black), plane polarized light. (Cpy = chalcopyrite)

    Figure 8. Photomicrograph of titanite (sphene) crystal forming from actinolite groundmass,crossed polars.

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    Photographic plates

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    ALTERATION

    Four alteration assemblages are present at the South Filson Creek deposit: 1) a

    serpentine/mica (possibly hydrothermal) assemblage; 2) a later argillic alteration; 3) a

    localized hematitic alteration; and 4) a greenschist facies assemblage.

    SERPENTINE/MICA

    The most widespread alteration assemblage at South Filson Creek is the

    serpentine/mica alteration. This alteration assemblage is early because it was overprinted by

    later alteration assemblages described below. The principal minerals consist of primarily

    hydrous mineral phases such as medium- to coarse-grained biotite and stilpnomelane

    (unrelated to pegmatite units), serpentine, iddingsite, chlorite, and sericite. The

    serpentine/mica alteration at South Filson Creek had not been recognized prior to this study

    because of its subtle nature. Such subtle alteration may indicate a fluid that was somewhat

    compatible with the magmatic minerals (Barnes, 1979). Sericitization of the plagioclase is

    seen macroscopically only as a slight greenish cast to the feldspars. Microscopically, sericite

    is abundant in the cores of the feldspar grains. Fracture zones have also been highly

    serpentinized, and in some areas, the rock is totally converted to serpentine and iddingsite.

    Partial serpentinization of the olivines is common, but reticulation of the olivine grains is more

    intense in the vicinity of serpentinized fractures. M. A. Hanna Company drill logs describe

    these grains as `black olivines' due to their high serpentine content. Chlorite-filled fractures

    are also commonly associated with the serpentinization. The chlorite fractures appear to

    cross-cut both the serpentine fractures and the secondary biotite grains and are interpreted

    to be younger.

    The predominant mode of occurrence of biotite is as rims around primary sulfides.

    Biotite is commonly joined by stilpnomelane (Fig. 7), which is identified petrographically by its

    distinctive color, red-brown to golden yellow pleochroism, and pseudo-uniaxial interference

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    figure. Stilpnomelane is distinguished in core by its orange-pink to reddish color, compared

    to the brown or black biotite. Stilpnomelane is a product of hydrothermal alteration (Kerr,

    1959) and is close in structure to hydrobiotite (Zoltai and Stout, 1984). Fleischer, et al(1984)

    indicate that the composition of the stilpnomelane (determined by optical properties) is

    K(Fe,Mg,Al)10Si12O30(OH)12. Both biotite and stilpnomelane are syn-to post-serpentinization

    and pre-chloritization. Biotite layers are often bent, indicating post-crystallization deformation.

    ARGILLIC

    Argillization, breakdown of the primary minerals into clay minerals, is pervasive around

    the mineralized zones, causing the rock to have a "frosted" appearance when compared to

    unmineralized core. Feldspars are white or bleached due to breakdown into clays. This type

    of alteration can affect from 5% to as much as 70% of the rock locally. In the highly altered

    areas associated with fracture zones, the more intensely altered plagioclase grains are a

    pronounced white color. Under the microscope, this type of alteration causes the plagioclase

    twin planes to appear more diffuse. Argillic alteration overprints both serpentine and chloritic

    alteration, and is a later stage, lower temperature alteration product.

    HEMATITE

    Hematite alteration is present in drill holes K-18 and K-29 associated with syenite dikes

    which intruded along zones of weakness created by serpentinized fractures. The hematite

    replaces all iron-bearing silicate phases and is locally pervasive. This alteration type

    overprints the serpentine/mica alteration. The timing of the hematization with respect to the

    greenschist assemblage is not as definitive.

    GREENSCHIST ASSEMBLAGE

    A lower greenschist facies assemblage present in the eastern portion of the study area

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    Figure 9. Paragenetic diagram of sulfide, oxide, and alterationmineralogy.

    is especially prevalent in drill hole K-5, but is also present in drill holes K-20 and K-26. This

    assemblage consists of two varieties of chlorite (optically identified as prochlorite and

    penninite), actinolite, prehnite, rutile, calcite, and titanite (sphene). Titanite crystals in drill hole

    K-26 occur in a groundmass of actinolite. These crystals were formed from the groundmass

    as seen by some unusual textures. The crystals appear to penetrate the groundmass, with

    one end of the crystal still ragged (Fig. 8). The more Mg-rich chlorite replaces Cpx, and the

    Mg-poor variety is present in association with relict grains of plagioclase. This pervasive

    alteration is present in drill holes to the east of the mineralization and overprints the

    serpentine/mica alteration near South Filson Creek. This alteration may have affected a large

    area of the Duluth Complex as regional metamorphism, or be a result of deuteric alteration.

    Additional regional mapping is needed to determine the extent of this alteration.

    A paragenetic sequence diagram (Fig. 9) illustrates the relationship between the

    mineralization events and the alteration assemblages.

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    GEOCHEMISTRY

    The geochemical signature of the South Filson Creek sulfides is consistent with the

    mineralogy and textural relationships. Geochemical data are provided on floppy disk in

    Appendix B. Platinum, palladium, silver, and gold content was analyzed for all previously

    analyzed M.A. Hanna Company pulps at the M. A. Hanna Company laboratory in Nashwauk,

    Minnesota. In addition, seven mineralized drill core samples were submitted to Bondar-Clegg

    in Vancouver, B.C. These samples were analyzed for whole rock elements plus 47 rare-earth

    elements (REE), trace elements, and base and precious metals. In addition, a total PGE scan

    (platinum+palladium+iridium+osmium+rhodium+ruthenium) was conducted. These data are

    discussed under the PGE chondrite plot section below.

    The seven whole rock samples were collected from mineralized intervals in drill holes

    K-16, K-17, K-21, and K-27 with >0.71% Cu (0.72 - 1.35%). The whole rock, trace element

    and REE analyses show very little variability between the samples. Unmineralized rocks have

    not been analyzed for comparison with mineralized rocks. The largest variability in the

    mineralized zone is exhibited by the Cu, Ni, S, and precious metal content.

    The highest precious metal values occur in drill holes K-21 and K-27 (>2 ppm Pt+Pd).

    These drill holes also have the highest Os, Ir, Ru, Rh values. Silver values are highest in drill

    hole K-6 (8.4 ppm). Elevated Pt+Pd+Au values (>250 ppb) also occur sporadically in 3.5 to

    7 ft. zones in drill holes (K-11, K-12, K-20).

    ELEMENTAL RATIOS

    The Pd/Ir ratios at South Filson Creek range from 54 to 93, and average 77. Typical

    ratios from magmatic deposits are less than 10, and hydrothermal ratios are greater than 100

    (Paterson, et al, 1982). Values are high from hydrothermally-related ores because the Ir is

    not easily mobilized under hydrothermal conditions. This is illustrated at Kambalda, Australia

    where magmatic sulfide Pd/Ir ratios range from 0.35 to 1.5, and the hydrothermal vein sulfide

    ratios have values of 436 to 877 (Stumpfl, 1986). The hybrid ratios at South Filson Creek

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    Figure 10. A. Percent Cu vs. percent Ni; B. Pt vs. Pd; C. Pd vs. Ir; D.Pt+Pd vs. Ru+Ir+Os.

    equilibrium with a nickel-rich host rock at magmatic temperatures (Table 2). Values again

    bracket both magmatic and hydrothermal ranges.

    GEOCHEMICAL PLOTS

    The geochemical plots (Figs. 10 and 11) illustrate the relationships between the

    sulfides of the primary and secondary mineralization events. Copper and nickel are highly

    correlated, as has been predicted by previous observations (Morton and Hauck, 1987; Fig.

    10A), with the slope indicating the relative abundance of copper over nickel at approximately

    3 to 1. The individual platinum group elements correlate well with each other (Fig. 10B, C, D).

    The approximate ratio of palladium to platinum is 2 to 1.

    When comparing the PGE content to copper-nickel distribution, Pt+Pd show a higher

    correlation to copper than to nickel (Fig. 11A,B). This suggests that

    introduction/remobilization of the PGE minerals occurred with secondary copper enrichment,

    which agrees with the findings of Morton and Hauck (1989). The relationship between the

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    Figure 11. A. Pt+Pd vs. log percent Cu; B. Pd vs. log percent Ni; C.Pt+Pd vs. Cu/(Cu+S); D. Pt+Pd vs. percent S.

    PGEs and sulfur is not as straightforward (Fig. 11C,D). The Pt+Pd versus Cu/Cu+S plot (Fig.

    11C) shows high PGEs associated with a consistent Cu/Cu+S value of approximately 0.4.

    These high PGE values may be associated with the secondary chalcopyrite. In Figure 11D,

    Pt+Pd is plotted directly against S and demonstrates a relationship between high S (about

    1%) and high Pt+Pd during the proposed secondary enrichment event. This same

    relationship exists between Pt+Pd and high copper (Fig. 11A).

    RARE-EARTH ELEMENT (REE) CHONDRITE PLOT

    The rare-earth element (REE) abundance diagram from South Filson Creek (Fig. 12)

    is a typical normalized pattern (Henderson, 1982), showing light REE enrichment. The

    positive europium anomaly is associated with the plagioclase in the troctolitic rocks. The

    preferential uptake of europium results from the existence of both Eu2+ and Eu3+ oxidation

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    Figure 12. REE chondrite plot of samplesfrom the mineralized zone.

    states in the magma; the other REE ions are

    usually present only in the 3+ state. A significant

    amount of plagioclase involved in fractional

    crystallization will cause the accumulated

    solids to have a positive Eu anomaly, and

    the residual liquids will have a negative one

    (Henderson, 1982). Note also the

    homogeneity of the seven samples from four

    different drill holes in the mineralized zone.

    PGE CHONDRITE PLOTS

    The PGE-Au-Ag-Cu-Ni values for high grade intersections in four drill holes are listed

    in Table 3. Figure 13A illustrates the average chondrite-normalized PGE+Au concentrations

    in the sulfide fraction (after Naldrett and Duke, 1980) for the values in Table 3.

    Table 3. High Grade PGE-Au-Ag and Cu-Ni Mineralization at South Filson Creek

    Drill Hole Footage Cu Ni Pt Pd Ir Os Rh Ru Au Ag

    K-16 75-85 1.13 0.35 340 930 10

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    (3) 7.5Mg2SiO4 + 6H2O + O2 = 3[Mg3Si2O5(OH)4] + 2Fe3O4 + 1.5SiO2(Olivine) (Serpentine) (Magnetite) (Quartz)

    Excess silica from reactions 1, 2, and 3 can be available for the actinolite (4),

    secondary biotite (5), and stilpnomelane (6).

    (4) 0.96[Ca(Mg,Fe)Si2O6] + O2 + 2.12SiO2 + 0.677H2O =(Augite) (Quartz)

    0.097[Mg3Si2O5(OH)4] + 0.064Fe3O4 + 0.484[Ca2Fe5Si8O22(OH)2](Serpentine) (Magnetite) (Actinolite)

    (5) 1.2[Mg3Si2O5(OH)4] + 0.8Fe3O4 + 1.6SiO2 + CaAlSi3O8 + 2K+ =

    (Serpentine) (Magnetite) (Quartz) (Plagioclase)

    0.4H2O + 0.4O2 + 2[K(Fe,Mg)2AlSi3O10(OH)2] + Ca2+

    (Biotite)

    Stilpnomelane is similar in structure to hydrobiotite (Zoltai and Stout, 1984). It

    commonly alters to biotite in greenschist-facies muscovite-bearing rocks by a number of

    different reactions described by Brown (1971, 1975). Many lines of evidence outlined by

    Brown (1971) indicate that brown stilpnomelane develops by alteration. The brown color of

    the mineral can also indicate formation at low PO2 (Brown, 1967). The rocks at South Filson

    Creek, however, are muscovite-free and textures demonstrate stilpnomelane is at least coeval

    with or later than biotite. A reaction (6) is a possible method for stilpnomelane formation from

    biotite hydration. Chemical formulas are for minerals of average composition as precise

    compositions are not available for the South Filson Creek area at this time.

    (6) 3.75[K(Fe,Mg)2]AlSi3O10(OH)2 + 27.75SiO2 + 2Fe3O4 =(Biotite) (Quartz) (Magnetite)

    7.5Al(OH)3 + 3.75[K(Fe,Mg)2AlSi4O10(OH)2] + 1.5H2O + 3[Mg3Si2O5(OH)4]

    (Stilpnomelane) (Serpentine)

    A reaction (7) describing the formation of titanite (sphene) during greenschist facies

    metamorphism was developed by Hunt and Kerrick (1977) and modified by P. Morton (pers.

    comm., 1990):

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    GEOSTATISTICS

    DATA SUMMARY

    Drilling Statistics

    The South Filson Creek data set is comprised of 13,929 feet of drilling in 21 cores

    holes. The shortest hole is 40 feet, the longest hole is 3420 feet, and the average hole length

    is 663 feet.

    Of the 21 core holes, 14 are recorded as vertical. The 7 reported angle holes dipped

    between 43 and 85 degrees. All reported borehole orientations are based on collar surveys:

    the is no record of "down-the-hole" surveys on any of the 21 cores.

    The 21 core holes include 451 assays. 44 of these 451 assays were duplicates; thus

    there were 407 unique assay intervals. While the individual assay lengths vary from 1 to 25

    feet, the vast majority of the assays are supported by 10 feet of core. The total assayed

    length is 3,496 feet; thus, the average assay length is 8.6 feet.

    The "standard assay length" is 10 feet, but the beginning and ending points for

    assaying, were ultimately determined by visual inspection of the core. Visually barren lengths

    of core were not assayed. On the other hand, numerous exceptional core segments were

    assayed more than once.

    Summary Statistics

    Table 4 presents a suite of summary statistics for the seven assayed elements

    included in this analysis: Cu, Ni, Pd, Pt, Au, Ag, and Co. These statistics are based upon all

    407 assays. Duplicate assays were averaged. Furthermore, these statistics include length

    weighting, so a ten foot assay interval is given ten times as much weight as a one foot assay

    interval.

    The reported measures of skewness, the relatively high coefficients of variation, and

    the initial graphical data analyses, indicate that the distributions of all seven elements are

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    asymmetric, with long positive tails. As is common practice in the statistical analysis of base

    and precious metals, a logarithmic transformation was applied. However, a lognormal

    distribution did not offer a particularly good model for the data.

    Cu Ni Pd Pt Au Ag Co(%) (%) (ppb) (ppb) (ppb) (ppm) (%)

    N used 407 407 230 230 230 229 321N missing 0 0 177 177 177 178 86 Assay Feet 3495 3495 1620 1620 1620 1613 2478

    Mean 0.362 0.136 235.5 113.9 64.8 2.39 0.011Variance 0.092 0.012 121310.1 24509.0 25956.0 2.60 0.00*Std. Dev. 0.303 0.107 348.3 156.6 161.1 1.61 0.005Coef. Var. 83.682 78.743 147.9 137.4 248.8 67.37 42.951Skewness 0.905 1.644 2.5 2.1 7.8 0.71 0.921Kurtosis 3.043 11.316 11.6 8.3 78.9 2.96 9.831

    Minimum 0.010 0.010 2 5 1 0 0.000

    25th %tile 0.100 0.060 21 10 3 1 0.008Median 0.290 0.100 49 33 17 2 0.01075th %tile 0.550 0.203 388 178 69 3 0.014Maximum 1.340 1.330 2206 907 1765 8 0.057

    Table 4. Summary Statistics for the South Filson Creek Data Set.These statistics are based upon all available assays - duplicateassays were averaged, and a length weighting was used.* denotes variance

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    representative of the bulk of the data.

    Table 6 is an estimate of the "background" metal values in sulfide zones outside of the

    mineralized zones that contain secondary mineralization (see Appendix B - Table 10 for assay

    values). These values are not truly representative of the original magmatic values because

    trace secondary mineralization, e.g., bornite, is identified in some samples by petrographic

    methods. Even though the statistics in Table 4 include the samples used in this estimate, a

    comparison of the means and medians can roughly estimate the enrichment that occurred

    during the secondary mineralization event (see Table 6).

    Approx. Enrichment FactorNo. of

    Mean* Median* Samples Mean Median

    Copper (wt. %) 0.099 0.090 126 4 3Nickel (wt. %) 0.058 0.060 126 2 2Cobalt (wt. %) 0.011 0.010 116 0 0Sulfur (wt. %) 0.195 0.190 122 4** -Palladium (ppb) 20 24 126 12 2Platinum (ppb) 13 14 126 9 2

    Gold (ppb) 4 4 126 16 4Silver (ppm) 1.3 1.5 126 2 2

    *Log transformed values**Mean S = 0.72% on n = 361

    Table 6. Mean and Median "Background" Metal Values for CloudZone Sulfides at South Filson Creek

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    GROSS ECONOMIC AUXILIARY VARIABLE

    Creating the Auxiliary Variable

    Due to the time constraints on this analysis, a detailed study of each individual element

    was not possible. Rather, an auxiliary variable combining the seven elements (excluding S)

    into one gross economic indicator is generated and investigated. This new variable is the sum

    of the seven metal grades times their respective market prices. The result is expressed in

    dollars per ton. This does not incorporate any reduction in value due to metallurgical recov-

    ery, and this new variable does not include the cost associated with the extraction, processing,

    or sales of a product. Nonetheless, a market price weighting of the individual elements offers

    a simple, easy to understand, means of data analysis.

    The market prices used were taken from the Engineering and Mining Journal (1989):

    Cu $1.33 / lbNi $5.95 / lbAg $5.16 / tr ozAu $362.00 / tr ozPt $485.00 / tr ozPd $136.00 / tr ozCo $8.15 / lb

    The resulting auxiliary variable is defined by the following equation:

    $/ton = 26.6 (% Cu) + 119.0 (% Ni) + 0.1505 (ppm Ag) + 0.01056 (ppb Au) + 0.01415 (ppb Pt) + 0.003967 (ppb Pd) + 163.0 (% Co)

    Summary Statistics for the Auxiliary Variable

    Table 7 shows a suite of summary statistics for auxiliary variable. The largest gross

    economic value is supported by only 2 feet of core. The second largest gross economic value

    is 101.15 ($/T). These are the only two assay intervals with gross economic value greater

    than 100 ($/T).

    Using the prices detailed previously, there are no ore reserves in the volume of rock

    under study in this report. There are but two assay interval (supported by a total of 12 feet

    of core) with a gross value in excess of $100 per ton (assuming 100% recovery of all seven

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    Complex by a late-stage fluid, although operating under higher temperatures than a typical

    hydrothermal fluid.

    Hydrothermal fluids as a source of additional sulfur or metals cannot be adequately

    evaluated from the available data. However, the high inter-element correlation suggests the

    metals are derived locally and concentrated by faulting/fracturing and redox boundaries.

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    BENEFITS

    1. Documents alteration assemblage associated with late-stage hydrothermalprecious metal enrichment/introduction.

    2. Documents the structural controls of the secondary mineralizing event.

    3. Demonstrates that significant intervals, both laterally and vertically, of elevatedPt+Pd+Au+Ag mineralization can occur in the copper-nickel deposits of theDuluth Complex.

    4. The combination of the above provides the first data necessary to develop Cu-Ni-Pd-Pt-Au-Ag exploration targets.

    5. Statistical analysis of the available data suggests that in-fill drilling coulddiscover a significant quantity of ore.

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    REFERENCES

    Barnes, H.L., 1979, Geochemistry of Hydrothermal Ore Deposits: John Wiley and Sons, NewYork, 798 pp.

    Beaudoin, G., and Laurent, R., 1989, PGE geochemistry of the Blue Lake Cu-Ni-PGE

    massive sulfide deposit [abs.]: Geol. Assoc. of Can., Progs. with Abs., v. 14, pp. A47.

    Brown, E.H., 1967, The greenschist facies in part of eastern Otago, New Zealand: Contr.Mineral. and Petrol., v. 14, pp. 259-292.

    -------, 1971, Phase relations of biotite and stilpnomelane in the greenschist facies: Contrib.Mineral. and Petrol., v. 31, pp. 275-299.

    ------, 1975, A petrogenetic grid for reactions producing biotite and other Al-Fe-Mg silicates inthe greenschist facies: Jour. Petrology, v. 16, part 2, pp. 258-271.

    Butler, B.K., 1989, Hydrogen isotope study of the Babbitt Cu-Ni deposit, Duluth Complex,

    Minnesota: Unpubl. M.S. thesis, Indiana University, 180 pp.

    Cooper, R.W., 1978, Lineament and structural analysis of the Duluth Complex, Hoyt Lakes-Kawishiwi area, northeastern Minnesota: Unpubl. Ph.D. dissertation, University ofMinnesota, 280 pp.

    Cooper, R.W., Weiblen, P.W., and Morey, G.B., 1981, Topographic and aeromagneticlineaments and their relationship to bedrock geology in a glaciated Precambrianterrane, northeastern Minnesota: in O'Leary, D.W., and Earle, J.L., (eds.), Proceedingsof the Third International Conference on Basement Tectonics, Denver, Colorado, pp.137-148.

    Dillon-Leitch, H.C.H., Watkinson, D.H., and Coats, C.J.A., 1986, Distribution of platinum groupelements in the Donaldson West deposit, Cape Smith Belt, Quebec: Econ. Geol., v.81, pp. 1147-1158.

    Economou, M.I., 1986, Platinum group elements (PGE) in chromite and sulfide ores within theultramafic zone of some Greek ophiolite complexes: in Gallagher, M.J., Ixwe, R.A.,Neary, C.R., and Prichard, H.M.,(eds.), the Metallogeny of Basic and Ultrabasic Rocks;The Institution of Mining and Metallurgy, London, pp. 441-452.

    Engineering and Mining Journal, 1989, v. 190, no. 9, pp. 21.

    Ervin, S.M., 1987, The relationship between the cloud zone and the basal zone Cu-Ni

    sulfides, and the significance of mafic pegmatites, Minnamax property, DuluthComplex, Minnesota: Unpubl. M.S. thesis, University of Minnesota, Duluth, 137 pp.

    Fleischer, M., Wilcox, R.E., and Matzko, J.J., 1984, Microscopic determination of thenonopaque minerals: U.S. Geological Survey Bull. 1627, 453 pp.

    Foose, M.P., and Cooper, R.W., 1981, Faulting and fracturing in part of the Duluth Complex,northeastern Minnesota: Can. Jour. Earth Sci., v. 18, pp. 810-814.

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    Morton, P., and Hauck, S.A., 1987, PGE, Au, and Ag contents of Cu-Ni sulfides found at thebase of the Duluth Complex, northeastern Minnesota: Natural Resources ResearchInstitute, Technical Report, NRRI/GMIN-TR-87-04, 67 pp.

    ------, 1989, Precious metals in the copper-nickel deposits of the Duluth Complex (abs.):Minn. Geol. Surv., Inf. Circ., pp. 47-48.

    Mountain, B.W., and Wood, S.A., 1988, Chemical controls on the solubility, transport, anddeposition of platinum and palladium in hydrothermal solutions: a thermodynamicapproach: Econ. Geol., v. 83, pp. 492-510.

    Naldrett, A.J., 1981, Nickel sulfide deposits: classification, composition, and genesis: Econ.Geol. 75th Anniv. Vol., pp. 628-685.

    Naldrett, A.J., 1982, Platinum group metals in Ontario: Ontario Geol. Surv., Open File Rept.5380, 7 pp.

    Naldrett, A.J., and Duke, J.M., 1980, Platinum metals in magmatic sulfide ores: Science, v.208, pp. 1417-1424.

    Pasteris, J.D., 1984, Further interpretation of the Cu-Fe-Ni sulfide mineralization in the DuluthComplex, northeastern Minnesota: Canadian Mineralogist, v. 22, pp. 39-53.

    Patterson, G.S., and Watkinson, D.H., 1984a, The geology of the Thierry Cu-Ni mine,northwestern Ontario: Canadian Mineralogist, v. 22, pp. 3-11.

    ------, 1984b, Metamorphism and supergene alteration of Cu-Ni sulfides, Thierry mine,northwestern Ontario: Canadian Mineralogist, v. 22, pp. 13-21.

    Paterson, H.L., Donaldson, M.J., Smith, R.N., Lenard, M.F., Gresham, J.J., Boyack, D.J., andKeays, R.R., 1982, Nickeliferous sediments and sediment-associated nickel ores at

    Kambalda, Western Australia:in

    Buchanan, D.L., and Jones, M.J., (eds.), Sulfidedeposits in mafic and ultramafic rocks; Proc. of IGCP Projects 161 and 91, Third NickelSulfide Field Conf., Perth, Western Aust., May 23-25, pp. 81-94.

    Phinney, W.C., 1969, The Duluth Complex in the Gabbro Lake quadrangle, Minnesota:Minnesota Geological Survey, Report of Investigations RI-9, 20 pp.

    Phinney, W.C., 1972, Duluth Complex, history and nomenclature: in Sims, P.K., and Morey,G.B., eds., Geology of Minnesota: A Centennial Volume, Minn. Geol. Survey, pp. 333-334.

    Rao, B.V., and Ripley, E.M., 1983, Petrochemical studies of the Dunka Road Cu-Ni deposit,Duluth Complex, Minnesota: Econ. Geol., v. 78, pp. 1222-1238.

    Ripley, E.M., 1986, Application of stable isotope studies to problems of magmatic sulfide oregenesis, with special reference to the Duluth Complex, Minnesota: in Freidrich, G.H.,Genkin, A.D., Naldrett, A.J., Ridge, J.D., Sillitoe, R.H., and Vokes, F.M., (eds.),Geology and Metallogeny of Copper Deposits, Springer-Verlag, Berlin, pp. 25-42.

    Rowell, W.F., and Edgar, A.D., 1986, Platinum group element mineralization in ahydrothermal Cu-Ni sulfide occurrence, Rathbun Lake, northeastern Ontario: Econ.Geol., v. 81, pp. 1272-1277.

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    Weiblen, P.W., and Morey, G.B., 1976, Textural and compositional characteristics of sulfideores from the basal contact zone of the South Kawishiwi intrusion, Duluth Complex,northeastern Minnesota: Mining Symposium, 37th Annual American Inst. of Mining andMetall. Engineers; Minnesota Section, 49th Annual Meeting, Duluth, 1976,Proceedings, pp. 22-1 to 22-24.

    Weiblen, P.W., and Morey, G.B., 1980, A summary of the stratigraphy, petrology, and

    structure of the Duluth Complex: American Jour. Sci., v. 280A, pp. 88-133.

    Wood, S.A., and Mountain, B.W., 1989, The hydrothermal transport of platinum andpalladium: thermodynamic constraints revisited [abs.]: Geol. Assoc. Can., Prog. withAbs., v.14, pp. A79.

    Zoltai, T., and Stout, J.H., 1984, Mineralogy Concepts and Principles: Burgess, Minneapolis,505 pp.

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    During July, 1989, eight outcrop samples were taken in the South Filson Creek area,

    on a Newmont Exploration lease (NW 1/4 Section 36, T. 62 N., R. 11 W., Lake County,

    Minnesota). Sample locations can be found on the attached map (Fig. 15). This property is

    located just to the south of the area drilled for Cu-Ni by the M. A. Hanna Company in the late

    1960s. The samples were taken at each lithology change encountered on a traverse along

    South Filson Creek. Rock types range from anorthositic troctolite to gabbro. All samples

    show microfracturing due to cataclasis. Samples NM36-D and NM36-F contained the highest

    sulfide content. Microprobe work would be necessary to determine if several minute, highly

    reflective grains in sample D are PGE minerals. Samples NM36-B, C, and E contain small

    quantities of secondary sulfides (but no primary sulfides) associated with serpentinization.

    Sample NM36-A

    This sample is an augite troctolite (AGT) to olivine gabbro (OG) and contains 60-65%

    plagioclase (andesine) and 10-12% olivines mesocumulate grains. Intercumulate minerals

    include 10% orthopyroxene (hypersthene), 5-7% clinopyroxene (augite), and 2-3% oxides.

    Late-stage replacement minerals are biotite (2-3%), serpentine (trace [tr]-1%) and chlorite (tr).

    The olivines are reticulate and partially serpentinized. Olivine also forms glomerocrysts that

    are rimmed by a thin layer of hypersthene or poikilitic augite. Biotite rims the skeletal oxide

    grains.

    No polished section was available for NM36-A.

    Sample NM36-B

    This sample is an augite troctolite typical of the South Filson Creek area. Andesine

    plagioclase (55-60%) occurs with a mesocumulate texture, and olivine (10-15%) is meso- to

    intercumulate, as well as highly reticulate and serpentinized. Augite (5-7%), hypersthene (3-

    5%), and oxides (3-6%) are also intercumulate. Biotite (4-5%), stilpnomelane (tr-1%),

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    This sample has the highest sulfide content, even though the sulfides comprise only

    10-15 modal percent of the opaque minerals. Chalcopyrite is always associated with

    serpentine pockets and veinlets. Primary sulfides are coarse-grained chalcopyrite (3-4%),

    pentlandite (1-2%), and cubanite (1%), and secondary sulfides are fine-grained chalcopyrite

    (5-7%) and bornite (2%). Several very small, highly reflective grains in serpentine may be

    PGE minerals. However, polishing compound is also highly reflective and has been known

    to wedge into the softer serpentine during finishing. These grains appear much too coarse

    to be polishing compound, however, more detailed analysis is required for a definitive

    composition.

    Sample NM36-E

    This sample is an anorthositic troctolite composed of 70-75% plagioclase (andesine-

    labradorite), and 10-12% olivine mesocumulates, 4% oxides, 3-4% augite, and 1-2%

    hypersthene, and traces of primary sulfides as intercumulate minerals. Replacement

    mineralogy includes biotite (1-2%), serpentine (tr-1%), and secondary sulfides (tr). As in the

    other samples, this sample is microfractured and the olivines are reticulate and serpentinized.

    The polished section contains 70-75 modal percent oxides and 25-30 modal percent

    sulfides. Primary sulfides are coarse-grained pyrrhotite, pentlandite, and chalcopyrite.

    Secondary fine-grained chalcopyrite and bornite replacement textures are rare.

    Sample NM36-F

    This rock is a troctolite and petrographically distinct from sample E directly across

    South Filson Creek. This lithologic difference may be due to a structural discontinuity across

    the creek. Primary mineralogy includes mesocumulate plagioclase (55-65%) and olivine (20-

    25%), with intercumulate, poikilitic augite (5-7%) and hypersthene (tr-1%). The olivines exhibit

    "raindrop texture", typical of other parts of the Duluth Complex but rarely documented in the

    South Filson Creek area. Replacement minerals are biotite (1-2%), serpentine (tr-1%), clay

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    minerals (tr-1%), chlorite (tr), and stilpnomelane (tr). This sample contains more clay

    alteration than the others, and plagioclase twin planes are often diffuse, probably due to the

    alteration effects. Stilpnomelane is a very late stage mineral, replacing biotite.

    This sample does not contain an appreciable amount of oxides, compared to the other

    samples in this series, therefore the modal percentages of the sulfides appear quite high. The

    actual volume of the sulfides is about the same as in the other samples. This sample contains

    primary pyrrhotite (25-30%), coarse-grained chalcopyrite (10-12%), and cubanite (10%), with

    fine-grained secondary chalcopyrite (8-10%) and digenite (3%). Secondary mineralization is

    associated with the serpentinized areas.

    Sample NM36-G

    This rock is the typical augite troctolite of the South Filson Creek area and contains 60-

    65% plagioclase (oligoclase-andesine) and 10-12% olivine mesocumulates. The

    intercumulate fraction is composed of hypersthene (5-7%), augite (3-5%), and oxides (3%).

    Replacement minerals include biotite (2-3%), serpentine (1-2%), and traces of iddingsite,

    stilpnomelane, and clay minerals. The sample is microfractured and olivines are moderately

    serpentinized.

    Reflected light work reveals that 90-95 modal percent of the opaque minerals are

    oxides. Primary pentlandite (3-5 modal percent) and chalcopyrite (2-3 modal percent) are the

    common sulfides. Bornite (tr) replaces chalcopyrite locally.

    Sample NM36-H

    No thin section was available for this sample. Sulfides comprise 5 modal percent of

    the opaque minerals. Cubanite (1%) is the only primary sulfide present. Very fine-grained

    secondary chalcopyrite (3-4 modal percent) and mackinawite or valleriite (tr-1 modal percent)

    are associated with serpentine pockets.

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    The presence of olivine-rich rocks, a pervasive fracture system, and late stage fluids

    (evidenced by the hydrous secondary minerals, and secondary sulfides) indicate that the

    mineralizing conditions in the section 36 area were similar to those just to the north. If this

    system should intersect primary cloud zone sulfides laterally or at depth, the sulfur-rich wall

    rocks may have provided a redox boundary. This, in turn, could have promoted deposition

    of secondary minerals, including the PGE's. Based on the information provided from these

    samples, the section 36 area has potential for the occurrence of PGE mineralization.

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    Figure 15

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    DATA SOURCES

    The sources of the quantitative data used in this analysis are the spreadsheet files

    FILCHEM.WK1 and KSERIES.WK1 (Appendix C).

    The spreadsheet file KSERIES.WK1 contains the geometric information for 30 core

    holes. Each hole is a separate record. Each record includes seven (7) fields:

    Drill Hole NumberUTM NorthingUTM EastingCollar ElevationTotal DepthDip of hole at the collar

    Azimuth of hole at the collar

    ! HOLE ID - The hole identifiers are a "K" followed by a number between 1 and29.

    ! COLLAR NORTHING - The collar northings are UTM coordinates.

    ! COLLAR EASTING - The collar eastings are UTM coordinates.

    ! COLLAR ELEVATION - The collar elevations are expressed as distance above

    mean sea level.

    ! HOLE DIP AT THE COLLAR - The dip of the holes at the collars are expressedin degrees from horizontal. Of the 21 holes included in the analysis, 14 arevertical (dip = 90 ). The reported dip of the 7 non-vertical holes vary from 43to 83 .

    ! DIP DIRECTION AT THE COLLAR - The dip directions at the collar areexpressed as bearings.

    The spreadsheet file FILCHEM.WK1 (Appendix C) contains 451 assay records from

    21 separate drill holes (Table 9; 9 of the holes included in KSERIES.WK1 did not appear in

    FILCHEM.WK1 because there were no assays). Each record is comprised of the 53 fields:

    a hole identification, three geometric variables, two sample data fields, one rock type field, 40

    elemental assay fields, and seven derived variables:

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    DRILL HOLE # FM (ft) TO (ft) INTERVAL (ft)SAMPLE # SAMPLE LAB ROCK TYPE

    Cu (wt%) Ni (wt%) Co (wt%) Co (ppm) S (wt%) Pd (ppb)Pt (ppb) Au (ppb) Ag (ppm) Rh (ppb) As (ppm) Sb (ppm)Bi (ppm) Se (ppm) Te (ppm) Pb (ppm) Zn (ppm) TFe (wt%)

    FeO (wt%) Cr (wt%) Ti (wt%) V (wt%) Al (wt%) Ca (wt%)Mg (wt%) Na (wt%) K (wt%) C (wt%) F (ppm) S (ppm)B (ppm) Mo (ppm) W (ppm) P (ppm) Cd (ppm) Ba (ppm)Mn (wt%) Be (ppm) Sr (ppm)

    Cu/Ni Cu/(Cu+Ni) Pt/(Pt+Pd) Cu/S Ni/SPt+Pd/(100*S)Ag/S

    The vast majority of these individual fields were empty for most records. As such, the analysis

    presented in this report included the seven most prevalent elements and economically

    important elements:

    Cu (%) Ni (%) Pd (ppb) Pt (ppb) Au (ppb) Ag (ppb) Co (%)

    This spreadsheet file contains all of the assay data used in the analysis presented in this

    report.

    Table 9. Identification of Drill Holes Used in the Geostatistical Analysis

    COLLAR AZI.DDH UTM UTM ELEVATION TD /DIPNO. NORTHING EASTING (FT.) (FT.) (DEG) Azimuth------------------------------------------------------K-1 599161 5297635 1490 2645 90 0K-2 592612 5293617 1450 2240 90 0K-3 602662 5298606 1500 40 43 330K-3A 602662 5298606 1500 284 43 330K-4 595162 5296040 1500 1697 85 39K-5 599822 5297027 1500 307 45 0K-6 599259 5297012 1495 300 45 0K-7 602730 5298702 1500 300 90 0K-8 593079 5293252 1430 3420 90 0K-11 599819 5297457 1495 362 45 0K-12 599836 5297431 1495 214 90 0K-13 599697 5297400 1505 100 90 0K-15 599195 5297153 1505 100 90 0K-16 599254 5297034 1500 265 45 328K-17 599229 5297110 1490 189 90 0K-18 599316 5297067 1490 200 90 0K-20 599258 5297982 1490 250 90 0K-21 599315 5297108 1490 150 90 0K-25 599474 5297326 1500 166 90 0K-26 599690 5297457 1510 186 90 0K-27 599264 5297108 1490 200 90 0K-29 599256 5297217 1490 494 45 135

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    Table 10. (con't.)

    DDH FROM TO INT. CU NI CO S PD PT AU AG

    FT. FT. FT. WT.% WT.% WT.% WT.% PPB PPB PPB PPMK-12 70 74 4 0.01 0.14 0.01 0.06 19 5 0.5 0.1K-12 74 75 1 0.14 0.08 0.01 0.38 55 33 10 2.4K-12 75 79 4 0.03 0.02 0.01 0.14 8 5 0.5 1.2K-12 114 119 5 0.35 0.12 0.01 0.62 39 18 11 3.1K-12 122.5 126.5 4 0.05 0.05 0.01 0.18 16 14 0.5 0.6K-12 126.5 134 7.5 0.03 0.06 0.01 0.10 18 5 0.5 1.7K-12 190 202.5 12.5 0.05 0.02 0.01 0.13 25 5 1 1.6K-12 209 214 5 0.03 0.04

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    Table 10. (con't.)

    DDH FROM TO INT. CU NI CO S PD PT AU AGFT. FT. FT. WT.% WT.% WT.% WT.% PPB PPB PPB PPM

    K-25 16 25 9 0.12 0.05 0.013 1.09 14 14 0.5 0.9K-25 25 35 10 0.55 0.14 0.018 0.60 37 5 30 0.6K-25 35 45 10 0.60 0.15 0.018 0.51 45 58 30 0.8K-25 45 51 6 0.44 0.13 0.016 0.39 10 50 131 0.5K-25 51 63 12 0.55 0.14 0.015 0.49 83 16 42 0.3K-25 63 72.5 9.5 0.47 0.14 0.012 0.51 63 5 35 0.1K-25 72.5 82 9.5 0.38 0.10 0.007 0.31 27 33 6 4.0K-25 82 94 12 0.53 0.14 0.012 0.74 14 9 2 1.3K-25 94 100 6 0.03 0.03 0.012 0.04 51 14 0.5 1.4K-25 100 110 10 0.26 0.09 0.017 0.36 29 20 3 3.0K-25 110 117 7 0.52 0.15 0.020 0.71 24 18 9 4.2

    K-25 117 122.5 5.5 0.07 0.04 0.009 0.12 19 5 0.5 1.5K-26 8 10 2 0.24 0.09 0.008 0.48 29 5 8 3.3K-26 10 20 10 0.31 0.12 0.016 0.77 43 14 12 3.7K-26 20 30 10 0.23 0.10 0.014 0.53 35 5 6 2.7K-26 30 40 10 0.26 0.10 0.014 0.62 43 44 10 1.8K-26 40 48.5 8.5 0.16 0.06 0.008 0.42 39 5 4 2.3K-26 48.5 55 6.5 0.28 0.06 0.005 2.77 23 22 4 2.7K-26 55 65 10 0.18 0.05 0.005 1.79 10 5 2 2.2K-26 65 75 10 0.23 0.08 0.008 0.71 33 36 8 2.7K-26 75 85 10 0.33 0.11 0.010 0.68 73 21 22