Mystery Lake,Manitoba

Mystery Lake, located approximately 12 km northeast of the city of
Thompson in the northern part of the Thompson Nickel Belt (TNB), is
underlain by variably deformed members of the Ospwagan Group
comprising siliciclastic and chemical metasedimentary rocks and mafic
and ultramafic volcanic and plutonic rocks. The geology of the Mystery
Lake area was investigated as a contribution to the multiyear,
multidisciplinary CAMIRO investigation of the TNB (Peck, GS-3, this volume).
In Mystery Lake, the Ospwagan Group is bounded to the
northwest by the Paleoproterozoic Mystery Lake granodioritic pluton
(1.836 Ga, U-Pb monazite age; Syme et al., 1993) and to the southwest
by Archean polymetamorphic ortho- and paragneisses that are interpreted
as retrograded parts of the Pikwitonei granulite domain (Weber, 1990). At
Mystery Lake, heterolithic gabbro intrudes and partially assimilates a
sequence of Ospwagan Group felsic detrital metasedimentary rocks that
include quartzite, arkose, and wacke, interpreted as parts of the Setting
Lake formation. Pillowed, massive and brecciated basalt flows and
interlayered, spinifex-textured ultramafic volcanic rocks are interpreted to
be coeval and possibly cosanguineous.
Partially sulphidized oxide facies iron formation is associated with
felsic and pelitic sediments and intruded by gabbro. The sulphidization of
this iron formation may have resulted from
metasomatism caused by the thermal aureole of the gabbroic intrusion.

INTRODUCTION
Ultramafic rocks in the Mystery Lake area were first reported in 1972
(Coats et al., 1972). Mapping of the Mystery Lake area by R.F.J. Scoates
(Weber and Scoates, 1976) determined that the ultramafic rocks occur in
conjunction with metasedimentary and mafic metavolcanic rocks. The
stratigraphy of the Ospwagan Group in the Thompson Nickel Belt
acquired a special significance subsequent to the discovery of the nickel
deposits in the belt and the recognition that most of the mineralized
ultramafic bodies occur in the Pipe Formation of the Ospwagan Group.
Mystery Lake is underlain by both the Ospwagan Group and a
deformed occurrence of ultramafic intrusive rocks (Weber and Scoates,
1976) that contains nickel-copper sulphide deposits. A two-week