Geology of the Potter Mine Property, Millstream Mines Limited, Munro Township, Ontario

Introduction

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.

General Geology

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 swarm.

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 and chloritized.

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 and sphalerite.

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 and chloritization.

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.

Structural Geology

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.

Sulphide Mineralization

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 excellent.

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.

References

Arndt, N.T. (1976)