Geology of the Potter Mine Property,
Millstream Mines Limited,
Munro Township, Ontario
Card & Associates; Geosearch
86 Penfield Dr. Kanata, ON
In late August, 1996 the writer mapped the Potter Mine property of Millstream
Mines Limited, the Harrison Group of Companies, North Bay, Ontario. The
Potter Mine is a massive sulphide deposit located in the western Abitibi
greenstone belt east of Matheson, Ontario. There has been past production
of copper and zinc from the Potter Mine and of copper, zinc, gold, and
asbestos from several other deposits in the area. Ore-grade mineralization
is known to exist in and beneath the Potter Mine workings, and there is
good potential for discovery of additional reserves on the property, which
is located in one of the most mineral-rich regions of Canada.
Location and Access
The property consists of 28 claims in Lots 5 and 9 of Concessions IV
and V, Munro Township. A further 9 claims along the southern boundary of
the property have been optioned. Bedrock is exposed in the northern and
southern parts of the property, the remainder being covered largely by
alder swamps and beaver ponds. The Potter Mine is located at 48º 36'
N and 80º 12' 43" W in north central Munro Township.
Access to the area is provided by Highway 101 from Matheson, or Highway
572 from Ramore, and a gravel road extending north from Highway 101 through
central Munro Township to the Hedman asbestos mine in southern warden Township.
The Potter Mine property is located in the eastern part of the Kidd-Munro
assemblage of the Abitibi Greenstone Belt. The Kidd-Munro assemblage is
a fault-bounded package of ultramafic and mafic volcanic rocks, subvolcanic
intrusions and thin felsic and mafic fragmental volcanic units that extends
some 130 kms westward from Timmins to east of Matheson. Isotopic age determinations
indicate these rocks are about 2715 Ma old and are considered to represent
deposits of an ancient volcanic arc or back-arc basin that were tectonically
accreted, along with older and younger assemblages of similar origins,
during the Late Archean orogenic events that resulted in the formation
of the Superior Province of the Canadian Shield. The Abitibi greenstone
belt is the world's largest, best-preserved greenstone belt, and is also
one of the most mineral-rich. The Kidd-Munro sequence hosts a number of
mineral deposits, including the world-class Kidd Creek volcanogenic massive
sulphide deposit, the Dundonald and Alexo komatiitic nickel deposits and
several major lode gold deposits. In Munro Township, copper arid zinc-
bearing massive sulphides have been mined at the Potter Mine and the Potter-Doal
Mine, gold at the Croesus Mine and asbestos at the Munro Mine. The Potter
Mine property is thus located in one of the most productive lithological
assemblages in the Abitibi greenstone belt.
On the Potter Mine property, the main rock types recognized include
Archean komatiitic and basaltic volcanic rocks, gabbro, peridotite and
pyroxenite of intrusive and/or extrusive origin, and mafic fragmental volcanic
rocks. These rocks form relatively continuous lithostratigraphic units
that trend west-northwest to east-west across the property. The sequence
is folded about west-northwest trending axes, and is displaced by northwest,
north and northeast trending faults. The Archean rocks and structures are
cut by Proterozoic mafic dykes of two ages, an older (2450 Ma), north-striking
Matachewan swarm and a younger (2167 Ma), northeast-striking Biscotasing
J. A. Satterly and his associates (Satterly, 1951) produced a very accurate
map of the area at a scale of 1:12000. The komatiitic volcanics and associated
layered complexes, have been described in detail by McRae(1969), Pyke et
al (1973), Arndt (1976), and others. Paul Coad (1976) did a University
of Toronto MSc. thesis on the Potter Mine, and more recently M. Shore and
R. Theriault, University of Ottawa, completed MSc. thesis on the Pyke Hill
komatiites and the Centre Hill Complex respectively.
Geology of the Potter Mine Property
Bedrock exposures constitute approximately 30 percent of the property,
mainly in the northeastern and southeastern parts of the area. The remainder
is covered by thick, poorly drained overburden with numerous beaver ponds,
bogs, and alder swamps. The main rock types, Archean komatiitic and basaltic
volcanic rocks, gabbro, peridotite, and pyroxenite, and volcanic fragmental
rocks, form an apparently simple lithostratigraphic sequence comprising
a lower basaltic volcanic unit, a layered gabbro-peridotite-pyroxenite
complex, the Centre Hill Complex, a thin mafic fragmental unit, a peridotite
unit, and an upper komatiitic volcanic sequence. This succession is exposed
on the northern and southern limbs of a major fold, the Centre Hill Syncline,
and is cut by Proterozoic mafic dykes.
Lower Mafic Volcanic Unit
Mafic volcanic rocks are exposed in the southern part of the property
in the axial part of an anticlinal structure adjacent to the Centre Hill
Fault. They consist of massive and pillowed basalt which near the fault
are sheared and altered to chloritic and amphibolitic schists. Outside
the property, Satterly (1952) mapped thick sequences of basalts and andesitic
volcanics with minor dacite-rhyolite layers and mafic and ultramafic sills
and dykes. Massive, pillowed, amygdaloidal and spherulitic lavas are present
as are units of flow-top breccia, pillow breccia and hyaloclastite. There
are also thick mafic units with coarse gabbroic or diabasic textures that
may be intrusive sills or thick ponded flows.
Centre Hill Complex
The Centre Hill Complex is a layered mafic-ultramafic complex that forms
Centre Hill in the south and is repeated on the north limb of the Centre
Hill Syncline in the northern pan of the property. The complex is approximately
450 metres thick consisting of a lower cyclical sequence (200 m) of interlayered
peridotite, and pyroxenite units 10 to 50 m thick and an upper, 250 m thick,
gabbro member. The ultramafic rocks display well-developed magmatic layering
and typically have poikilitic textures with large pyroxene phenocrysts.
The primary textures and mineralogy of these rocks are generally well-preserved
except for minor serpentinization.
Mafic Fragmental Unit
Satterly (1952) mapped a number of thin volcanic fragmental units which
he described as rhyolite agglomerate and tuff consisting of angular grey
rhyolite flagments up to 0.5m in length. He noted that these rocks arc
commonly heavily mineralized with disseminated and massive sulphides. In
the Potter mine property arid surrounding area, the fragmental rocks are
mafic, not felsic, consisting of basaltic rock fragments and have a composition
intermediate between olivine basalt and picritic basalt. The mafic fragments
are commonly silicified and have a bleached, greyish cherty appearance.
The mafic fragmental unit, which is host to the Potter Mine ore, is
approximately 40 to 70 metres thick and is exposed over a strike length
of about 1.5 km in the southern part of the property. West of the Potter
Mine, the unit can be traced in outcrop to the middle of Lot 6, Con V,
where it is dragged southward along north-trending faults and possibly
by folding to the southern boundary of Concession V. Sulphide mineralization
is exposed in several old pits in the area.
East of the Potter Mine, the mafic fragmental unit can be traced in
outcrop to the eastern part of Lot 6, Con V, where it is displaced southward
along north-trending faults and is exposed again near the Lot 5- Lot 6,
Con V- Con IV corner. Sulphide mineralization is exposed in several pits
and there are geophysical anomalies in this area.
In the northern part of the property, the south-dipping, south-facing
sequence is similar to that in the south, consisting of massive gabbro
overlain by a mafic fragmental unit which is in turn overlain by peridotite
and komatiitic flows. The mafic fragmental unit is exposed over a strike
length of about 2.5 km and is approximately 15 to 40 metres thick. As in
the south, it commonly contains sulphides and graphite and is locally silicified
The mafic fragmental unit consists mainly of tuff-breccia and tuff,
some possibly of hyaloclastic origin, and a variety of graphitic, chloritic,
siliceous and sulphide-rich rock that probably represent altered tuffs,
epiclastic rocks and chemical (exhalative) sediments.
The tuff-breccia arid tuff consist mainly of close-packed basaltic fragments
ranging from millimetres to 0.5 metres in maximum dimension; most are 2
to 5 centimetres. Crude stratification is imparted by variations in fragment
size, packing and alignrnent of the long axes of the fragments. Layer-parallel
foliation is present in some localities. The mafic fragmental rocks generally
contain disseminated sulphides, commonly 1% to 5% pyrrhotite and pyrite,
and not uncommonly, chalcopyrite. Massive sulphides, which occur sporadically
in metre-scale pods, consist of layered pyrrhotite, pyrite, chalcopyrite
The mafic fragments and matrix are commonly silicified and have a bleached,
greyish, cherty appearance. Zones of chlorite alteration are present, notably
in the Potter Mine area. Massive sulphide mineralization, notably with
chalcopyrite and sphalerite, may correlate with zones of silicification
There are also altered, mineralized gabbroic rocks within and marginal
to the fragmental unit. The upper part of the gabbro unit commonly contains
1-2% sulphides and the mafic minerals are chloritized. There are sill-like
and dyke-like bodies of gabbroic rocks within the mafic fragmental unit.
The mafic fragmental rocks and the gabbro are compositionally similar,
closely spatially related and probably genetically related. I could find
no good evidence that the gabbro-mafic fragmental contact is a fault. The
mafic fragmental unit is probably a breccia or hyaloclastite formed at
the top of a large ponded mafic extrusive complex.
Upper Peridotite and Komatiitic Volcanic Rocks
The massive peridotite above the mafic fragmental unit probably represents
the base of a thick succession of komatiitic volcanics that overlies the
Centre Hill Complex. The peridotite is generally serpentinized. The komatiitic
volcanic rocks have been described in detail by Pyke et al (1973). They
consist of a series of ultramafic flows 0.5 m to 15 m thick with chilled,
fracture type and well-organized internal structure comprising lower olivine-cumulate
and upper olivine-spinifex zones.
The structure of the Abitibi greenstone belt, though complex at regional
scales, is probably relatively uncomplicated at local scales. Regionally,
the belt consists of a large number of juxtaposed panels with differing
isotopic ages, and structural and metamorphic histories. Within these fault
bounded panels, however, the rocks are relatively unstrained and commonly
have simple open folds and well preserved primary structures and textures.
The Potter Mine property lies in the central part of one of these panels,
bounded on the south by faults of the Porcupine-Destor system and on the
north by faults of the Lake Abitibi system. On the Potter Mine property,
the dominant structure is the Centre Hill Syncline, here interpreted as
a simple, upright fold with limbs dipping at 70º and a
subhorizontal axis striking west-northwest. The southern limb of the fold,
and a satellitic anticline, are truncated by the Centre Hill Fault. The
Centre Hill Fault strikes west-northwest and has apparent left-lateral
displacement of unknown magnitude. It is probably a member of the Porcupine-Destor-Lake
Abitibi fault system.
There are also north, northwest arid northeast trending crossfaults,
In the south, north-striking faults at either end of Centre Hill displace
the mafic fragmental unit southward approximately l00m. In the north, the
mafic fragmental unit is offset approximately 200 m by a northeast striking
fault with left-hand apparent displacement.
Most of the sulphide mineralization in the area occurs within the mafic
fragmental unit, which generally contains 1 to 5% sulphide minerals, mainly
pyrrhotite with some pyrite, and, locally chalcopyrite and sphalerite.
There is however great variation in sulphide mineral content. For example,
east of the Potter Mine, the mafic fragmental unit is poor in sulphides,
generally containing 1 % or less pyrrhotite. West of the Potter mine, the
unit is more heavily mineralized with disseminated and massive sulphides
in the north; the mafic fragmental unit generally contains 1 to 5% sulphides
and there are metre-scale pods with 25% or more sulphides. The sulphide
minerals, mainly pyrrhotite and pyrite with minor chalcopyrite, occur as
disseminations in altered (chloritic, graphitic, silicified) rocks and
as massive lenses which commonly have mm-scale banding.
Graphite is associated with sulphide mineralization and may be present
in amounts of 5% to 10% or more. In the graphitic rocks, pyrite and pyrrhotite
commonly form framboids, zoned circular to elliptical bodies up to 3 cm
in diameter. Zones of chlorite alteration and silicification are also closely
associated with sulphides, notably in the Potter Mine area.
Plans of the underground workings at Potter Mine (Figure 1) show that
massive to disseminated sulphides form a number of bodies, mainly within
the upper part of the mafic fragmental unit. These bodies are lensoid,
1 to 3 m thick, 40 to l00m long, and extend for at least several hundred
metres down dip (700) within the mafic fragmental unit. The ore shoots
resemble the fingers of a glove in that they thicken and coalesce downward
forming larger bodies of mineralization. It appears that there is considerable
mineralized material in the lower levels of the mine and that even more
may exist at depth. One of the best ore shoots was on the 6th (850') level
some 1000 ft. west of the shaft. It was approximately 10 ft x 100 ft. and
had grades of 5% copper and 2% zinc.
Correlation Between Geology and Geophysics
A preliminary analysis of correlations between the geology as mapped
at surface and geophysical data, including VLF electromagnetic, magnetometer
and Fraser Filter data, shows that the main anomalies trend generally east-west
parallel to structural-stratigraphic trends.
There are VLF, Fraser Filter and magnetic anomalies over the mineralized
mafic breccia unit in the Potter Mine area and in the northwestern part
of the property. There are also several VLF anomalies over the swampy area
north of the Potter Mine. The mineralized mafic breccia immediately west
and south of the potter Mine is also marked by VLF, Fraser Filter, and
magnetic anomalies. The zone of mineralized mafic breccia in the southeast
(450W/750S) is marked by a strong positive magnetic anomaly. However, a
strong Fraser Filter anomaly is located to the north over a tailings-covered
area between Pyke Hill and the east end of Centre Hill.
On the south side of Centre Hill, some VLF anomalies may correlate with
the Centre Hill Fault. Other VLF, Fraser Filter, and magnetic anomalies
in this area of known mineralization and may be related to magnetic conductors
within or beneath the southern margin of the Centre Hill Complex.
In the western part of the property, there are VLF and Fraser Filter
anomalies immediately north of the large gabbro outcrop (2700W/350S) that
may correlate with the westward extension of the mafic breccia unit. There
is, however, no coincident magnetic anomaly; the main magnetic anomaly
in the area is a bullseye located at 2850W/400S immediately west of the
gabbro outcrop. In the northwest, there are several linear VLF, Fraser
Filter and positive magnetic anomalies and some of these may correlate
with the westward extension of the mafic breccia unit on the north limb
of the Centre Hill Syncline.
Conclusions And Recommendations
The Potter Mine property is strategically situated in one of the most
productive lithotectonic units in what is undoubtedly the most mineral-rich
greenstone belt in the world. Within and immediately around the property,
there has been past production of copper, gold, zinc, and asbestos. The
potential for discovery of additional reserves of these commodities is
On the Potter Mine property, the mineralized mafic fragmental unit is
repeated by folding and displaced by faults. It is present over a strike
length of at least 2.5 km and possibly over 4 km in two parallel zones
across the property. In addition to the Potter Mine itself where the mineralization
is apparently open to the west and down dip, there are several areas warranting
further exploration, including drilling. To the west and south of the Potter
mine shaft, there is a relatively large area of infolded/faulted mafic
breccia. There is appreciable sulphide mineralization, and graphite, chlorite
and silica alteration exposed at surface, strong geophysical anomalies
and mineralized mafic breccia bodies are known to exist at depth from underground
workings and drilling.
To the east of the Potter Mine, at the eastern end of Centre Hill, the
mafic fragmental unit is displaced southward by a north striking fault.
On the east side of the fault, the mafic breccia contains appreciable sulphide
mineralization, as well as graphite and chlorite alteration. The zone is
marked by magnetic and VLF anomalies. Further exploration could include
geophysical surveys (extension of previous surveys) followed by drilling.
In the northeast, further drilling of the mafic fragmental unit is warranted,
especially in the areas around line 1275W/700N and line l800W/700N. Two
angle holes drilled from the south would probably be the best way to test
these areas. In Lots 8 and 9, Con V gabbro in an isolated outcrop may represent
the upper part of the Centre Hill Complex. If so, geophysical anomalies
immediately to the north may indicate the presence of the mineralized mafic
breccia unit. Similarly, in the northwest, geophysical anomalies in heavily
covered areas may correlate with the mafic fragmental unit on the north
limb of the Centre Hill Syncline.
Of the foregoing exploration targets, I would rate the area immediately
around the Potter Mine as having the highest potential. There is ore-grade
mineralization present in the mine, and the mineralized zone is open both
to the west and beneath the existing workings. Although there has been
some drilling, the mineralized zone to the south of the Potter Mine workings
has not been fully tested. The possibility that some of the rocks in the
mafic fragmental unit are of exhalative origin, are highly silicified,
have abundant disseminated sulphides, and are in a gold-rich region, would
warrant checking some of the drill core and surface samples for gold.
Distribution of sulphides in the mafic fragmental unit is highly erratic,
and the definition of potential ore zones, which will probably be relatively
small targets, will not be simple. Detailed studies of the alteration associated
with mineralization, and to define local structural controls should be
carried out in conjunction with further ground geophysics and drilling.
Possibly some geochemical work to define geochemical indicators of related
alteration would be useful.
Finally, a detailed review and synthesis of the existing geological,
geophysical and geochemical data, which is extensive. should be carried
out to arrive at a better understanding of the stratigraphy and structure
of the Munro area and the genesis of the Potter-type ores. The Potter ores
are not typical volcanogenic massive sulphides, being associated with mafic
rather than felsic volcanic rocks. As Paul Coad noted, they have some characteristics
in common with Cyprus-type copper-bearing pyritic deposits associated with
ophiolites. Exploration based on this type of deposit model could be rewarding.
Arndt, N.T. (1976)
Ultramafic rocks of Munro Township; economic and tectonic implications.
Geol. Assoc. Can. Spec. Paper 14, p. 617-657.
Arndt, N.T. and Nesbitt, W. (1982)
Geochemistry of Munro Township basalts; in Komatiites, George Athen
and Unwin, London, p.309-329.
Coad, P.R. (1976)
The Potter Mine. Unpublished MSc. Thesis, University of Toronto, 239p.
MacRae, N.D. (1969)
Ultramafic intrusions of the Abitibi area. Can. Jour. Earth Sci. 6,
Pyke, D. et al (1973)
Archean ultramafic flows, Munro Township. Geol. Soc. Am. Bull. 84,
Satterly, J. (1951)
Geology of Munro Township. Ont. Dept. Mines Ann. Report, 1951, 60(8),