Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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Guide to the Port Coldwell alkalic complex
INTRODUCTION:
The Port Coldwell Alkali Massif (Fig. 1) is located within
an Archean volcanic—sedimentary belt extending along the North
shore of Lake Superior near Marathon. Previous work on the
complex consists of reconnaissence work by Kerr (1910), detailed
mapping by Tuominen in 1958—1959 (O.D.M. Prelim. Map P 114) and
by Puskas in 1960 (O.D.M. Prelim. Map P 114, revised). The
western contact of the complex wag mapped by Walker (1956); the
easter-n contact by Thomson (1931) and Milne (1964).
The present study was largely carried out by the writer
and associates while employed by the Ontario Department of Mines.
The author wishes to express his thanks to Professor Henri Loubat
of Lakehead University and Clarence Kustra, Ontario Department
of Mines, Resident Geologist, for their constant interest and
co—operationh S. Spivak drafted the diagrams.
FIELD AND GENETIC
RELATIONSHIPS
The Port Coldwell Alkali Massif (Fig. 1) lies within the
eugeosynclinal portion of an Archean volcanosedimentary belt,
approximately 18 miles wide and extending westward from White
Lake, along the north shore of Lake Superior.
The volcanosediments have been tightly folded in a N 70°E
direction; less important, more northerly trending, structures
may be attributed to cross—folding.
The Archean rocks have been successively intruded by sill—
like bodies of basic and ultrabasic composition, granitoids,
dikes of diabasic composition, and lastly by the Port Coldwell
Alkali Massif.
The Alkali Massif is a lopolith (Puskas, 1964; Corbett,
1968) circular in plan and approximately 580 sq. kilometers in
area. The Massif is considered to typify the so—called (Benson)
Laccomorphic class of emplacements.
The rocks of the massif can be divided into two groups
called here the Main Group and the Secondary Group. However,
in common with many intrusions of this type the long crystal—
lisation history has resulted in numerous complex and sometimes
confusing cross cutting relationships.
The Main Group is composed of gabbros, the oldest and
more-peripherally located rock—type Map unit 2), and laurvikites
(Map unit 3). Both the gabbros and laurvikites can exhibit
rhythmic layering which dips inward at moderate angles. The