cu ni au mineralization
TRANSCRIPT
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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