Field Guide to the Volcanology, Structure, Alteration, and Mineralization of Archean Greenstone ...

212 Pages·2008·42.73 MB·English

by  George J. Hudak

Field Guide to the Volcanology, Structure,
Alteration, and Mineralization of Archean
Greenstone Belts in the Vicinities of Sturgeon
Lake and Rainy River, Ontario and Lake
Vermilion, Minnesota

October 5 – 12, 2008

Field Trip Guidebook

1,2 2,3
George Huda , Ron Morton , and
2,3
Dean Peterson

1
Geology Department, University of Wisconsin Oshkosh, Oshkosh, WI
54901
2
Precambrian Research Center, Natural Resources Research Institute,
5013 Miller Trunk Highway, Duluth, MN 55811
3
Department of Geological Science, University of Minnesota – Duluth, 229
Heller Hall, 1114 Kirby Drive, Duluth, MN 558112
Field Guide to the Volcanology, Structure, Alteration, and Mineralization of
Archean Greenstone Belts in the Vicinities of Sturgeon Lake and rainy River,
Ontario and Lake Vermilion, Minnesota

Field Trip Guidebook, October 5 – 12, 2008

Table of Contents

Table of Contents.................................................................................................................i

Contact Information for Field Trip Leaders and the Precambrian Research Center.....................ii

Stratigraphy, Physical Volcanology, Hydrothermal Alteration, and Massive Sulphide
Deposits of the Neoarchean Sturgeon Lake Caldera Complex……………………………………1

Appendix 1 (Hudak et al., 2003; AGU Monograph 140)……………………………..…………60

Appendix 2 (Morton et al., 1991; Economic Geology)………………………………………….76

Notes on Rainy River Resources Limited Richardson Township Gold Explorations…………...87

Architecture of an Archean Greenstone Belt: Stratigraphy, Structure, and Mineralization…....104

Volcanic Stratigraphy, Hydrothermal Alteration, and VMS Potential of the Lower Ely
Greenstone, Fivemile Lake to Sixmile Lake Area…………………………………………..….137

Economic Geology of Archean Gold Occurrences in the Vermilion District Northeast
of Soudan, Minnesota…………………………………………………………………………..182

i
Contact Information for the Short Course Field Trip Leaders

Dr. George J. Hudak, P. G.
Associate Professor of Geology / Associate Director, Precambrian Research Center
Department of Geology University of Wisconsin Oshkosh
800 Algoma Blvd. Oshkosh, WI 54901
Phone:[phone]
FAX: [phone]
e-mail: [email protected]

Dr. Ron Morton
Professor of Geology
Department of Geological Sciences
University of Minnesota – Duluth
229 Heller Hall
1114 Kirby Drive
Duluth, MN 558112
Phone: [phone]
FAX: [phone]
e-mail: [email protected]

Dr. Dean Peterson
Senior Vice President-Exploration
Duluth Metals Limited
Co-Director, Precambrian Research Center
Natural Resources Research Institute
013 Miller Trunk Highway
Duluth, MN 55811
Phone: [phone]
FAX: [phone]
e-mail: [email protected]

Precambrian Research Center
Natural Resources Research Institute
5013 Miller Trunk Highway
Duluth, MN 55811
Dr. Jim Miller, Co-Director
Phone: [phone]
FAX: [phone]
e-mail: [email protected]
ii
Stratigraphy, Physical Volcanology, Hydrothermal Alteration,
and Massive Sulphide Deposits of the Neoarchean Sturgeon
Lake Caldera Complex

By

George J. Hudak1and Ron Morton2

with contributions from

James M. Franklin3, Jamieson Walker4, and Peter K. Jongewaard5

October, 2008

1. Department of Geology, University of Wisconsin Oshkosh, 800 Algoma Blvd., Oshkosh,
WI USA 54901
2. Department of Geology, University of Minnesota – Duluth, Duluth, MN USA 55812
3. Franklin Geosciences, Ltd., 24 Commanche Drive, Nepean, ON K2E 6E9
4. Jamieson Geological Inc., 69 Ginger Lily Terrace, Henderson, NV USA 89074

5. United Taconite, P. O. Box 180, Eveleth, MN, USA 55734
1
Table of Contents

Table of Contents................................................................................................................2

Introduction .........................................................................................................................4

Regional Geology.................................................................................................................7

Stratigraphy and Physical Volcanology of the Sturgeon Lake Caldera Complex ..............10

Hydrothermal Alteration within the Sturgeon Lake Caldera Complex................................25

Summary of the Geological History of the Sturgeon Lake Caldera Complex....................31

Field Trip Stop Descriptions..............................................................................................37
F-Group and Darkwater Regions ..............................................................................37
Mattabi Region (Area 16).........................................................................................43
Lyon Lake and Sturgeon Lake Mine Regions (Area 17 and Area 23).......................50

References........................................................................................................................56

List of Tables and Figures

Tables
Table 1. Grade and tonnage statistics for VMS in the SLCC...............................................4
Table 2. Summary of geochronology in the south Sturgeon Lake region............................8

Figures
Figure 1. Geology of the South Sturgeon Lake Region.......................................................5
Figure 2. Detailed Geology, Western area, Sturgeon Lake Caldera Complex..................12
Figure 3. Detailed Geology, Central area, Sturgeon Lake Caldera Complex....................13
Figure 4. Detailed Geology, Eastern area, Sturgeon Lake Caldera Complex...................14
Figure 5. Geological Sections Across the Sturgeon Lake Caldera Complex.....................15
Figure 6. Lithogeochemical Classification of Strata, South Sturgeon Lake Region...........16
Figure 7. Pre-caldera Strata Associated with the SLCC....................................................17
Figure 8. Jackpot Lake and High Level Lake Succession Strata, SLCC...........................18
Figure 9. Bell River and Tailings Lake Succession Strata, SLCC .....................................19
Figure 10. Mattabi Succession Strata, SLCC....................................................................20
Figure 11. Lower- and Middle-L Succession Strata, SLCC...............................................21
Figure 12. Upper L and Bell River Lake Succession Strata, SLCC...................................22
Figure 13. No Name Lake Succession Strata, SLCC........................................................23
Figure 14. Lyon Creek Succession Strata, SLCC .............................................................24
Figure 15. Hydrothermal Alteration Mineral Assemblages, SLCC.....................................27
Figure 16. Hydrothermal Alteration in the Vicinity of the F-Group VMS Deposit ...............29
Figure 17. Hydrothermal Alteration in the Vicinity of the Mattabi VMS Deposit.................29
Figure 18. Hydrothermal Alteration in the Area-17 and Area 23 Regions..........................30
2
Figure 19. Eruption Volume – Caldera Diameter Relationships........................................32
Figure 20. Map of F-Group Area Field Trip Stops.............................................................39
Figure 21. Map of Mattabi Area Field Trip Stops...............................................................45
Figure 22. Geological Sketch Map, Field Trip Stop M-1....................................................46
Figure 23. Map of Field Trip Stops, Areas 17 and 23........................................................53

3
Introduction
Detailed field, petrographic, and lithogeochemical studies performed since the
mid-1980’s have indicated that the south Sturgeon Lake region of northwestern Ontario
is underlain by an extremely well-preserved, though partially eroded, Neoarchean
subaerial to submarine volcanic caldera complex (Figure 1). This caldera complex, the
Sturgeon Lake Caldera Complex (SLCC), hosted six massive sulphide orebodies which
produced nearly 20 million tons of VMS ore during mining operations between 1972 and
1991 (Table 1), as well as numerous sub-economic massive and semi-massive sulphide
showings.

Caldera 106 Cu Zn Pb Ag Ag
Deposit Sequence Tons % % % (g/t) (o/t)
F-Group ECS 0.34 0.64 9.51 0.64 60.4 2.13
Mattabi ECS/LCS 12.55 0.74 8.28 0.85 104.0 3.67
Sturgeon Lake LCS 2.07 2.55 9.17 1.21 164.2 5.79
Creek Zone/Sub-Creek Zone* LCS 0.91 1.66 8.80 0.76 141.5 4.99
Lyon Lake* LCS 3.95 1.24 6.53 0.63 141.5 4.99
Total/Average 19.82 1.06 8.50 0.91 119.7 3.85

* VMS deposits interpreted to have originally formed as part of Sturgeon Lake deposit

Table 1. Grade and tonnage statistics from the VMS ore deposits of the south Sturgeon Lake
region (after Franklin, 1996)

The combination of well-preserved volcanic and hydrothermal alteration textures,
the variable 55° to 90° dip of a north-facing, essentially homoclinal volcanic sequence,
and more than 600,000m of diamond drilling over an apparent 4,500 meter stratigraphic
interval enables geologists the opportunity to examine the lithological, lithogeochemical,
and metallogenic evolution of the SLCC. This includes initial subaerial to shallow
subaqueous pre-caldera volcanism (Pre-caldera Sequence, PCS), early subaerial to
submarine explosive silicic volcanism (Early Caldera Sequence, ECS) which led to the
development a multi-cyclic, piecemeal caldera complex up to 25km in strike length, and
later intracaldera submarine mafic to felsic, dominantly effusive submarine volcanism
and clastic and chemical sedimentation (Late Caldera Sequence, LCS). Post-volcanic
structural deformation has not only led to the apparent transposition of non-caldera

4
Figure 1. Location map (inset) and regional geological map of the south Sturgeon Lake region (modified after Trowell, 1983; Morton et al.,
1991; Morton et al., 1999; Galley et al., 2000).
5
associated strata (Lyon Lake Fault Sequence, LLFS) up-section from the caldera-
associated volcanic sequence, but has locally led to remobilization of massive sulphide
mineralization along a major fault zone to form smaller, but locally economic, massive
sulphide deposits (e.g. the Lyon Lake, Creek Zone, and Sub-Creek Zone deposits).
The stratigraphic evolution in the SLCC represents one of the few examples of
Neoarchean calderas which illustrate the modern “caldera cycle” (Smith and Bailey,
1968; Hudak et al., 2003: Mueller et al., 2004).
This field trip has several purposes: 1) to illustrate the textures, lithologies , and
geological structures resulting from various volcanological and sedimentological
processes associated with various stages of caldera complex development; 2) to
illustrate the hydrothermal alteration mineral assemblages within the caldera complex,
and their spatial relationships to volcanogenic massive sulphide (VMS) mineralization;
and 3) to develop a model by which the exploration geologist can use physical
volcanology, hydrothermal alteration mineral assemblages, and structural geology,
along with other lithogeochemical and geophysical data, to effectively explore for VMS
deposits in the Sturgeon Lake camp. The present guidebook represents and updated
version of the field trip to the Sturgeon Lake region which took place during the
Geological Association of Canada – Mineralogical Association of Canada meeting
during May, 1996 (Morton et al., 1996).
In addition to this guidebook, we have also included reproductions of two
important papers which describe the physical volcanology and mineralization present
within the SLCC. These papers provide detailed overviews of the regional geology as
well as the literature related to subaerial and submarine volcanic processes and the
genesis of VMS mineralization. Morton et al. (1991) presents an early interpretation of
the stratigraphic sequence within the SLCC, and discusses the interrelationships
between volcanological and mineralizing processes that formed the Mattabi VMS
orebody. This classic paper is the first to discuss the development of a submarine
caldera complex in the south Sturgeon Lake region, and how processes associated with
caldera development led to conditions favourable for massive sulphide mineralization.
Hudak et al. (2003) is a detailed discussion of submarine explosive silicic volcanological
processes within the SLCC, and how these processes dictated the stratigraphic
6
intervals, as well as the intracaldera locations, where economic VMS mineralization
occurs within the SLCC. In addition, Hudak et al. (2003) includes the most up-to-date
stratigraphic nomenclature and interpretations of the volcanological and ore-forming
processes associated with the genesis of this exceptionally well-preserved Neoarchean
subaerial to submarine caldera complex. In essence, Hudak et al. (2003) indicate that
larger VMS deposits correlate with larger explosive eruption events within the SLCC.

Regional Geology
The SLCC is located within the South Sturgeon Sequence of the Savant Lake-
Sturgeon Lake Greenstone belt that occurs in the Wabigoon Greenstone Belt of the
Superior Province (Figure 1; Sanborn-Barrie and Skulski, 1999). The SLCC is up to 25
km in strike length, and contains an approximately 3000 meter stratigraphic thickness of
north-facing, vertical to moderately north-dipping (55°) intracaldera fill (Morton et al.,
1991; Morton et al., 1999; Hudak et al., in press) composed of greenschist to locally
amphibolite facies (Trowell, 1974, 1983; Groves et al., 1988) metavolcanic,
metasedimentary, and meta-intrusive rocks of Neoarchean age (Table 2). The eastern
margin of the SLCC has been interpreted by Morton et al. (1991, 1999) to be the Lac
David Fault. The western margin of the SLCC is poorly constrained by limited outcrop
and diamond drilling, but based on the western limit of intracaldera strata, numerous
dikes, and extensive hydrothermal alteration, is believed to be located beneath
Biedelman Bay at the West Sturgeon Fault (Figure 1). The caldera complex and
associated ore deposits formed within an evolved, continental margin oceanic arc
(Sanborn-Barrie et al., 2001; Galley, 2002) which contained magmas derived from back-
arc basalts (Galley, 2003) that were not contaminated to any large extent by older 3.0
Ga Wabigoon Province continental crust (Bernier et al., 1999; Bernier et al., in review).
Two coeval intrusive complexes intrude into the pre-caldera supracrustal volcanic
strata in the Sturgeon Lake region, and have been extensively studied by Galley et al.
(2000), Galley (2002), and Galley (2003). The Pike Lake Layered Complex (PLLC)
consists of massive to crudely layered ferrogabbro, melanogabbro, and gabbro that,
combined, are up to 10km in strike length and up to 2500m thick. The PLLC is intruded
along its eastern margin by the Beidelman Bay Intrusive Complex (BBIC), which is
7