bedrock geologic map of the delta mineral belt, tok mining district, alaska

128 Pages·2003·6.05 MB·English

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2
BEDROCK GEOLOGIC MAP OF THE DELTA MINERAL BELT,
TOK MINING DISTRICT, ALASKA
by
Samuel S. Dashevsky, Carl F. Schaefer, and Edward N. Hunter
Professional Report 122
Division of Geological & Geophysical Surveys
Fairbanks, Alaska
2003
i
STATE OF ALASKA
Frank H. Murkowski, Governor
DEPARTMENT OF NATURAL RESOURCES
Tom Irwin, Commissioner
DIVISION OF GEOLOGICAL & GEOPHYSICAL SURVEYS
Rodney A. Combellick, Acting Director
DGGS publications may be inspected at the following locations. Address mail orders to the Fairbanks
office.
Alaska Division of Geological University of Alaska Anchorage
& Geophysical Surveys Library
ATTN: Geologic Communications 3211 Providence Drive
794 University Avenue, Suite 200 Anchorage, Alaska 99508
Fairbanks, Alaska 99709-3645
Elmer E. Rasmuson Library Alaska Resource Library
University of Alaska Fairbanks 3150 C Street, Suite 100
Fairbanks, Alaska 99775-1005 Anchorage, Alaska 99503
Alaska State Library
State Office Building, 8th Floor
333 Willoughby Avenue
Juneau, Alaska 99811-0571
This publication was released by the Department of Natural Resources, Division of Geological & Geophysical Surveys,
produced at a cost of $16 per copy, and printed in Anchorage, Alaska. Publication of this report is required by Alaska Statute
41 “to determine the potential of Alaska land for production of metals, minerals, fuels, and geothermal resources; the location
and supplies of groundwater and construction materials; the potential geologic hazards to buildings, roads, bridges, and other
installations and structures; and shall conduct such other surveys and investigations as will advance knowledge of the geol-
ogy of Alaska.”
NOTE: Mention of any company or brand name does not constitute endorsement by any branch or employee of the State of
Alaska.
ii
CONTENTS
Introduction....................................................................................................................................................... 1
Previous studies................................................................................................................................................. 2
Exploration history............................................................................................................................................ 3
Regional geology............................................................................................................................................... 3
Structural geology.............................................................................................................................................. 5
Local geology.................................................................................................................................................... 7
Lithologic unit descriptions............................................................................................................................... 9
Macomb Belt.............................................................................................................................................. 9
Jarvis Belt................................................................................................................................................... 9
Tushtena Pass unit............................................................................................................................... 9
Lagoon unit ......................................................................................................................................... 11
Tiger unit............................................................................................................................................. 12
Drum unit ............................................................................................................................................ 12
Tok River unit...................................................................................................................................... 13
Hayes Glacier Belt...................................................................................................................................... 13
Intrusive rocks............................................................................................................................................ 14
Gabbroic intrusives ............................................................................................................................. 14
Granite................................................................................................................................................. 14
Alkalic mafic dikes and associated plutonic rocks ............................................................................. 14
Economic geology............................................................................................................................................. 14
Lagoon unit massive sulfide....................................................................................................................... 15
DW-LP Deposit................................................................................................................................... 18
PP2 Deposit......................................................................................................................................... 18
Drum unit massive sulfides........................................................................................................................ 18
DD Deposits........................................................................................................................................ 18
Lithochemistry and protolith determination...................................................................................................... 19
Methodology............................................................................................................................................... 19
Interpretation.............................................................................................................................................. 19
Protolith classification................................................................................................................................ 19
Sample screening methodology........................................................................................................... 20
Regional correlation.......................................................................................................................................... 20
Acknowledgments............................................................................................................................................. 26
Bibliography...................................................................................................................................................... 26
Appendix I—Chronology of exploration.......................................................................................................... 31
Appendix II—Igneous protolith discrimination plots....................................................................................... 33
Appendix III—Tables of mineral deposits, prospects, and occurrences and tables of
geochemical data with sample descriptions ............................................................................................... 37
Table 1. Mineral deposits, prospects, and occurrences (p. 39–43)
Table 2. Inferred resource of massive-sulfide deposits (p. 45)
Table 3. Sample rock types and location coordinates (p. 47–71)
Table 4. Fire assay gold and inductively coupled plasma (ICP) analyses (p. 73–89)
Table 5. Whole-rock major oxide XRF analyses (p. 91–106)
Table 6. Trace element x-ray fluorescence analyses (p. 107–122)
FIGURES
Figure 1. Location map of Delta mineral belt area in the eastern Alaska Range........................................ 2
2. Regional bedrock geology of the eastern Alaska Range showing the principal terranes,
major schist belts, and intrusions of the Yukon–Tanana terrane ................................................. 4
iii
3. Simplified bedrock geology of the Delta mineral belt showing schist unit
subdivisions of the Jarvis belt and locations of mineral resources.............................................. 6
4. Tectonic classification of metaplutonic and metavolcanic rocks from the
Jarvis schist belt........................................................................................................................... 7
5. Longitudinal schematic section, looking northeast, showing the DW-LP and PP2
massive-sulfide horizons.............................................................................................................. 16
6. Three-dimensional rendering of modeled DW-LP and PP2 massive-sulfide sheets
intersected by drilling .................................................................................................................. 16
7. Protolith classification model used to segregate igneous samples from sedimentary
or highly altered samples of the Delta mineral belt..................................................................... 21
8. Classification of greenschist-facies metavolcanic rocks from the Jarvis schist belt................... 21
9. Classification of greenschist-facies metavolcanic rocks from the Tushtena Pass unit
(Jarvis schist belt)........................................................................................................................ 22
10. Classification of greenschist-facies metavolcanic rocks from the Lagoon unit
(Jarvis schist belt)........................................................................................................................ 22
11. Classification of greenschist-facies metavolcanic rocks from the Tiger unit
(Jarvis schist belt)........................................................................................................................ 23
12. Classification of greenschist-facies metavolcanic rocks from the Drum unit
(Jarvis schist belt)........................................................................................................................ 23
13. Classification of greenschist-facies metavolcanic rocks from the Tok River unit
(Jarvis schist belt)........................................................................................................................ 24
14. Classification of greenschist-facies metavolcanic rocks from the Quad marker
(Jarvis schist belt)........................................................................................................................ 24
PHOTOS
Photo 1. Folded limestone from the Delta mineral belt showing plastic deformation and
axial planar cleavage.................................................................................................................... 5
2. DW East massive-sulfide outcrop exhibiting an extensive footwall chloritic alteration
assemblage with chalcopyrite stringer-type mineralization......................................................... 8
3. Oblique aerial view (to the southeast) of the mostly blind and till-covered DW-LP, and
PP2 massive sulfide systems........................................................................................................ 17
4. Surface expression and iron seeps of the PP2 massive-sulfide system....................................... 18
TABLE
Table 1. Lithologic units and correlation in the Delta mineral belt........................................................... 10
SHEETS
(in envelope)
Sheet 1. Bedrock geologic map of the Delta mineral belt, Tok mining district, Alaska
2. Isotopic age and geochemical sample location map of the Delta mineral belt,
Tok mining district, Alaska
iv
BEDROCK GEOLOGIC MAP OF THE DELTA MINERAL BELT,
TOK MINING DISTRICT, ALASKA
by
Samuel S. Dashevsky,1 Carl F. Schaefer,1 and Edward N. Hunter1
INTRODUCTION
The Bedrock Geologic Map of the Delta Mineral Belt Alaska Range. This paper divides the complex geology of
focuses on a segment of a paleo-volcanic arc/basin system the area into seven mappable metamorphic units that
in east-central Alaska where base- and precious-metal-rich identify and follow the time–stratigraphic horizons along
massive-sulfide deposits formed and are preserved. This which massive sulfides were deposited on (and below) the
volcanic arc, or series of arcs, was active off the western Mississippian–Devonian seafloor.
margin of ancestral North America, during the Middle Exploration mapping and drilling by early operators in
Devonian to Early Mississippian. This major tectonic the region created a framework and foundation from
feature is represented today by the remnants of geologic which the current interpretation and understanding of the
belts distributed from southern British Columbia to central geology has evolved. Principal among these prior
Alaska. operators was Resource Associates of Alaska Incorporated
Substantial geologic debate and uncertainty have (RAA), a Fairbanks-based group of consulting geologists
surrounded the Delta mineral belt pertaining to the style of who were responsible for the discovery, early delineation,
mineralization; the location, correlation, and number of and naming of massive-sulfide deposits and stratigraphic
mineralized horizons; the juxtaposition of mineralized units in the Delta mineral belt.
felsic volcanic rocks with mafic igneous bodies; the age This publication draws on the results of new geologic
and geologic setting of deposits relative to those in other mapping, lithochemistry, airborne geophysics, and core
mineralized districts; and the ultimate potential to host drilling carried out by and under the direction of the
economically mineable mineral deposits. In addressing authors on behalf of American Copper & Nickel Company
these questions, approximately $20 million have been Incorporated (ACNC) and Grayd Resource Corporation
expended by private industry (1976–2001) exploring for (Grayd) of Vancouver, British Columbia, during the years
and evaluating base-metal and gold deposits in the Delta 1994–1999. Assembly of this map and report forced a re-
mineral belt. examination and synthesis of our prior work, and the
Exploration work has included detailed- and recon- interpretations and conclusions contained herein are not
naissance-scale geologic mapping; chemical analysis of necessarily endorsed by ACNC and Grayd.
approximately 30,000 rock, soil, drill core, stream At the time of this writing, all data and geologic
sediment, and pan concentrate samples; ground and materials produced by ACNC, RAA, and Grayd in their
airborne geophysical surveys; and about 35 miles of core exploration of the Delta mineral belt are the sole property
drilling in more than 186 holes. That work resulted in the of Grayd. Publication of this map is the result of a
discovery of more than 40 massive-sulfide occurrences collaborative effort between Grayd, Northern Associates
and more than a dozen gold prospects, and created a library Incorporated (NAI), and Alaska Division of Geological &
of proprietary geologic data and information. A synopsis Geophysical Surveys (DGGS) under State of Alaska
of those data is assembled in this report for presentation at contract #10-00-082.
the scale 1:63,360 (1 inch = 1 mile). The following The map area spans approximately 400 square miles,
sections summarize the regional, local, and economic covering portions of the Mount Hayes A-1, A-2, B-1 and
geology; discuss lithochemistry and protolith B-2, and Tanacross A-6 and B-6 quadrangles in east-
interpretations; present results from recent 40Ar/39Ar and central Alaska (fig. 1). Geologic data upon which the map
U–Pb dating; and provide major oxide and trace element is built are concentrated within a corridor roughly 20 miles
chemical analyses for 827 rock samples collected from long and 10 miles wide that follows a northwest–southeast
across the map area. axis through the central map area. Information beyond this
This report and the accompanying map present an end- corridor is of a reconnaissance nature, and is provided as
of-the-20th-century progress report on the understanding context only to aid display of the more detailed informa-
of the geology related to the formation and distribution of tion developed during industry-sponsored mineral
volcanic-related massive-sulfide deposits in the eastern exploration. An inset map on the margins of sheet 1
1Northern Associates, Inc., 1831 Musk Ox Trail, Fairbanks, Alaska 99709
2 Professional Report 122
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Figure 1. Location of Delta mineral belt map area in the eastern Alaska Range.
displays the primary contributors and sources of correlations based on these field interpretations.
geologic data utilized for specific regions covered by the Without benefit of sufficient petrography or whole rock
map. chemical analyses to accurately guide those
interpretations, the resulting map was inadequate as a
PREVIOUS STUDIES guide to exploration. A subsequent regional synthesis by
This is the first published map that displays the Duke (in Newkirk et al., 1985) delineated metamorphic
metamorphic rock units and mineralization in the Delta and tectonic units, and formed the basis for the most
mineral belt at a scale suitable for use in the field. As a recently published map of the Delta mineral belt
basis for our initial fieldwork, we relied upon numerous deposits, which appears as a 1:800,000-scale figure in
unpublished works by geologists working on behalf of Lange et al. (1993). Duke’s 1985 map was the founda-
RAA, Anaconda Copper Company, and ACNC. tion for the current map, but the work described in this
Principal among these are the 1:30,000-scale report resulted in substantial changes in stratigraphic
reconnaissance traverse maps made by RAA geological order, ages, and contacts of the major units; and
teams in 1976–1978 (Blakestad et al., 1976; Muntzert et structural and stratigraphic relations of some sulfide
al., 1977; Blakestad et al., 1978). These initial traverse deposits.
maps and a few other field maps (at a variety of scales) Studies of the Delta massive sulfides, including ore
provide field descriptions of the metamorphic rock and gangue mineralogy, mineral textures, and lead- and
types, and form a valuable record. Many subsequent sulfur-isotope analyses are presented in Frank (1979),
geologic maps present field calls of protolith only, Culp (1982), and Lange et al. (1993) and are not repeated
without surviving notation of field lithology. Duke and in this report. Studies of the geology, mineral resource
Nauman (1982) produced the first regional geologic assessment, and mineral occurrences and deposits of the
map of the area, which was an attempt to map protolith Mount Hayes Quadrangle were published by Nokleberg
of individual formations and construct regional and others (Nokleberg and Aleinikoff, 1985; Nokleberg
Bedrock geologic map of the Delta mineral belt, Tok mining district, Alaska 3
et al., 1986, 1990, 1991, 1992a, 1992b). Similar studies REGIONAL GEOLOGY
of the Tanacross Quadrangle were published by Cobb The Delta mineral belt is located within the eastern
(1972), Cobb and Eberlein (1980), and Singer et al. Alaska Range along the southern margin of the pre-
(1976). Other published studies pertaining to the Delta Pennsylvanian age Yukon–Tanana terrane (YTT; Jones
mineral belt include Nauman et al. (1980), Duke et al. et al., 1987). The YTT underlies a large portion of east-
(1984), Lange et al. (1987, 1990), Nauman (1984), central Alaska and much of central and western Yukon.
Nokleberg and Aleinikoff (1985), Nokleberg and Lange This assemblage of polydeformed metamorphic rocks
(1985), and Newkirk and Eastoe (1986). Masters theses lies between the autochthonous North American
completed in the Delta mineral belt include Anderson continental margin to the northeast and allochthonous
(1982), Culp (1982), Foley (1984), and Frank (1979). terranes to the southwest (Jones et al., 1987). The YTT
The geochemical and sample location information in east-central Alaska is separated from Wrangellia
included in tables 2 through 5 in Appendix III was terrane to the southwest by the major dextral Denali fault
released previously through DGGS as Raw Data File (Nokleberg et al., 1992b).
2001-2. The portion of the Yukon–Tanana terrane covered by
this map is divided into three metamorphic belts, herein
EXPLORATION HISTORY named the Macomb, Jarvis, and Hayes Glacier belts,
There is no historical record of successful which strike northwest–southeast and dip to the
prospecting for alluvial gold deposits in the map area. southwest (fig. 2). The Macomb belt in the northeastern
Minor production of antimony from a quartz–stibnite part of the map contains metagranitic plutons in a
vein system on Stibnite Creek principally occurred prior ductilely deformed gneiss–schist assemblage. The Jarvis
to World War II (Ebbley and Wright, 1948), but belt crosses through the central study area and contains
continued intermittently until 1973. interlayered, submarine, schistose, felsic-dominated
The massive-sulfide deposits of the Delta mineral metavolcanic rocks and metasedimentary rocks. The
belt were discovered in 1976 by geologists and Hayes Glacier belt in the southwestern part of the map
prospectors working for RAA under contract to Cook is dominated by phyllitic schists and mylonite derived
Inlet Region Incorporated (CIRI), an Alaska Native from sedimentary protoliths, and felsic and mafic
corporation. Extensive prospecting, geologic mapping, volcanic protoliths (Duke, 1985). These belts are
ground geophysics, and core drilling of deposits in the considered to be of Mississippian–Devonian age
Delta mineral belt were carried out by RAA from 1976 (Nokleberg et al., 1992a; J. Aleinikoff, written com-
through 1987 on behalf of CIRI and their joint venture mun., 2002) based on U–Pb ages for metarhyolite
partners. Phelps Dodge conducted a short program of interlayers. The belt boundaries are nearly equivalent to
drilling and ground geophysics in 1990, but this work the boundaries of the Macomb, Jarvis Creek Glacier,
did not significantly advance exploration of the district. and Hayes Glacier subterrane of Nokleberg et al.
In 1994, ACNC commenced exploration in the belt and (1992b) differing primarily at the Jarvis–Hayes Glacier
implemented a comprehensive program of geological contact and along internal member contacts.
and geochemical surveys, airborne and ground The three belts are part of a metamorphosed
geophysical surveys, and diamond-core drilling. assemblage of clastic and igneous rocks interpreted by
ACNC’s work expanded the tonnage of the known Lange et al. (1993) and Nokleberg et al. (1992b) to
deposits, resulted in the discovery of several high-grade represent progressively higher structural portions of a
base metal occurrences, and considerably improved the volcanic arc. The interpreted subvolcanic portion of the
understanding of the geologic context for the massive- arc is represented by the Macomb belt, which is
sulfide deposits. structurally juxtaposed against the Jarvis belt. However,
From 1976 through 2001, a total of approximately field relations mapped by RAA indicate that the Jarvis–
$20 million was spent exploring for base metal and gold Hayes Glacier boundary is a transitional facies change
deposits in the Delta mineral belt. As a result of this between two equivalent time–stratigraphic units
work, 49 massive-sulfide occurrences and 14 gold (Nauman et al., 1982) on the flanks of the volcanic arc,
prospects are documented in the Delta mineral belt. not a structural contact as described by Nokleberg et al.
Locations of mineral deposits, prospects, and (1992b).
occurrences are shown as numbered locations on sheet An abrupt decrease in thermal metamorphic grade is
1, and are summarized in table 1 of Appendix III. A apparent moving westward from the epidote–
chronological summary of modern exploration within amphibolite facies Macomb belt rocks across the Elting
the map area is provided in Appendix I. The proprietary Creek fault to the mid–lower greenschist-facies Jarvis
reports and data from this period currently are held by and Hayes belts. Within the Jarvis belt, metamorphic
Grayd and NAI in Vancouver and Fairbanks. grade decreases to the southwest. This metamorphic
4 Professional Report 122
Figure 2. Regional bedrock geology of the eastern Alaska Range showing the principal terranes, major schist belts,
and intrusions of the Yukon–Tanana terrane (modified after Nokleberg et al., 1992).
Bedrock geologic map of the Delta mineral belt, Tok mining district, Alaska 5
gradient was described by Nokleberg et al. (1986) as a suite of metavolcanics. The Jarvis belt has been traced
retrogressive metamorphism. beyond the extent of this map area from the Glenn
The results of 40Ar/39Ar analyses indicate two Highway northwest to the West Fork of the Robertson
Cretaceous thermal events in metavolcanic rocks River (fig. 2), where it is displaced by left-lateral faulting
associated with massive-sulfide deposits in the Jarvis (Nauman et al., 1982). The offset northern extension of
belt. An Early Cretaceous (117 Ma) thermal event is the central Jarvis belt occupies the highlands of the
indicated by complete re-setting of the argon content of Alaska Range, and is largely covered by alpine glaciers.
the rocks, with a subsequent episode of argon loss in The southeastern projection is obscured by vegetated
Late Cretaceous (80–65 Ma; P. Layer, written commun., hills and valley alluvium.
2001). The earlier event corresponds to the timing of
crustal extension described by Hansen and Dusel-Bacon STRUCTURAL GEOLOGY
(1998) farther north in the Yukon–Tanana terrane, while Three main styles of regional deformation are
the later event is coincident with igneous intrusive observed in the map area. One or several compressive
activity in the region (Foley, 1984). episode(s) imparted cleavage (S ) sub-parallel to
1
The member lithologies within all three belts are bedding, and caused thickening of the stratigraphy by
highly deformed, but within the central (Jarvis) belt, regional thrusting. Foster et al. (1994) suggest that this
where we have the most detailed information, these occurred in the Mesozoic (Jurassic?) coincident with
appear to remain generally upright and internally arc-continent collision. A major thrust fault in the south-
coherent on the large scale. A partial repetition of central part of the map area (fig. 3, sheet 1) places
stratigraphy by thrusting is inferred within the Jarvis Devonian over Mississippian rocks.
belt, but no large-scale overturned units or nappe The second major type of deformation resulted from
structures are documented within the map area. extension, which imposed a flatter, axial planar cleavage
Massive-sulfide occurrences associated with (S ) and was accompanied by large-scale, north–
2
volcanic units are present in both the Jarvis and Hayes northwest-trending normal faulting. This deformation
Glacier belts. In terms of size and grade, the most thinned the stratigraphy locally, and also developed
significant sulfide occurrences are hosted by a suite of local-scale folding of the S cleavage into a series of
1
interbedded greenschist-facies Devonian metavolcanic mostly southwest-verging, tight to isoclinal fold sets (F )
2
and metasedimentary rocks in the Jarvis belt. The dubbed “cascades” by previous workers. The more
central, and most mineralized, part of the Jarvis belt is ductile lithologies are tectonically thickened in the hinge
dominated by metavolcanic rocks flanked by thick zones of F recumbent folds, and possibly by stacking
2
metasedimentary sequences. The larger massive-sulfide along basal glide planes (photo 1). These effects resulted
deposits are associated with the felsic to intermediate from one or more episodes, possibly during the Early
Photo 1. Folded limestone from the Delta mineral belt showing plastic deformation
and axial planar cleavage. Unidentified geologist; photo from RAA 1977 Annual
Report.