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The following papers were presented at the Northeastern Ontario Mines & Minerals Symposium (Apr. 18-19, 2000)

SUBSEAFLOOR REPLACEMENT SULPHIDES AND ZONE REFINING;
EVIDENCE FROM THE POTTER MINE, ONTARIO

GAMBLE, A.P.D., Dave Gamble Geoservices, 70 First Street, Kirkland Lake, ON P2N 1N3; GIBSON, H. L., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6

Two basic types of base metal sulphide mineralization (477,572 tonnes grading 1.67% Cu; 1967-72 are recognized at the Potter Mine, subseafloor replacement and seafloor sulphide. In both types the predominant sulphide mineral is pyrrhotite with lesser sphalerite and calcopyrite. The deepest massive sulphide intersection encountered during Millstream Mines Ltd.'s 1997-98 drill program returned values of 5.34% Cu, 3.24% Zn over 7.75 metres. The mineralization is open at depth.

Subseafloor sulphide consists of disseminated and semi-massive sulphide mineralization (10-80% sulphides) within the matrix of hyaloclastite. Mineralization ranges from disseminated sulphide, often replacing an earlier carbonate cement, to semi-massive sulphide where black, chloritized, wispy hyaloclastite shards occur within a massive sulphide matrix. The delicate, wispy nature of chloritized hyaloclastite shards within sulphide is not a primary feature but a product of sulphide replacement along shard margins and perlitic cracks. Although textural and field evidence is limited to a few cross cutting relationships it is tentatively interpreted that early formed pyrrhotite and sphalerite were replaced by chalcopyrite. This paragenetic sequence is typical of many volcanic-associated massive sulphide deposits, and by analogy, may reflect original temperature gradients and sequential replacement during formation of the sulphide lenses where an early, "lower temperature" pyrrhotite/sphalerite mineralization was progressively replaced by "higher temperature" chalcopyrite.

Thus, the hyaloclastite acted as a trap for sulphide mineralization. Thc semi-massive sulphide lenses are interpreted to have grown below the seafloor by the precipitation of sulphides within the permeable hyaloclastite matrix and by replacement of the matrix and, to some extent the hyaloclastite shards. Subseafloor replacement is a mechanism common, but not restricted to, the formation of many large volcanic-associated massive sulphide deposits.

THE GEOLOGIC SETTING OF Cu-Zn-Co-Ag VMS MINERALIZATION AT THE POTTER MINE, MUNRO TOWNSHIP, ONTARIO.

GAMBLE. A. P. D., Dave Gambia Geoservices Inc., 70 First Street, Kirkland Lake, ON P2N 1N3; GIBSON, H.L., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6

The former Potter mine (477,572 tonnes grading 1.67% Cu, 1967-72) is located within the east-southeast trending Archean (2714 Ma) belt of mafic to ultramafic komatiitic to tholeiitic volcanic and intrusive rocks referred to as the Kidd-Munro Assemblage. In the mine area, the volcanic succession is divisible into three lithostratigraphic and chemostratigraphic units occurring, from the oldest to youngest, as follows: 1) a Lower komatiitic ultramafic flow sequence; 2) a Middle tholeiitic basalt hyaloclastite sequence, including comagmatic peperitic basalt sills, argillaceous sediments and komatiitic flows; and 3) an Upper komatiitic ultramafic flow sequence. Strata strike easterly, dip steeply and face north at 85 degrees, and lie on the south flank of the regional west plunging McCool Hill synclinal structure. The Center Hill ultramafic to mafic layered igneous complex, a part or the Munro Sill, has intruded into the volcanic pile southeast of the Potter mine. Sulphide mineralization at the former Potter mine, as well as sulphide mineralization encountered during Millstream Mines Ltd.'s recent 1998 deep drilling are hosted within "hyaloclastite" of the Middle unit. The sulphide mineralization infills a primary topographic depression -graben- that immediately flanks the western side of a tholeiitic eruptive centre (fissure), within the flat komatiitic lava plain. Sulphides, predominantly pyrrhotite with lesser amounts of chalcopyrite and sphalerite, occur as: 1) a seafloor massive sulphide lenses associated with carbonaceous argillite; and 2) as semi-massive to massive replacement deposits that formed in a subseafloor environment within the hyaloclastite. The deepest massive sulphide intersection to date occurs at a vertical depth of 605 meters in DDH S-98-01 and returned 5.34% Cu, 3.24% Zn over 7.75 meters and is open at depth. The stacked multi-lens nature of the sulphide mineralization suggests a relatively long-lived hydrothermal event was active throughout deposition of the hyaloclastite. Alteration consists of palagonitization, carbonitization, and silicification, and chloritization. The semi-massive replacement sulphides, and the massive sulphide lenses, are enveloped by semi-conformable black chlorite alteration that is characterized by a depletion in SiO2, Na2O, CaO, and MgO and an enrichment of Fe2O3 and base metals. The mineralization and alteration encountered to date is largely stratabound and may represent the fringe of a larger hydrothermal system located down plunge, within the inferred subsidence structure.

SUBAQUEOUS FIRE-FOUNTAINING, HYALOCLASTITE AND MASSIVE SULPHIDE MINERALIZATION

TARDIF, N.P., GIBSON, H.L., WHITEHEAD, R.E.S., MacDONALD, C.A., Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6; GAMBLE, A.P.D., Dave Gamble Geoservices Inc., 70 First Street, Kirkland Lake, ON P2N 1N3

The former Potter VMS deposit, Munro Township (477,572 tonnes @ 1.67% Cu; 1967-72) occurs within an Archean (2714 Ma), tholeiitic volcanic centre contained within an east-southeast striking ultramafic komatiitic succession that comprises the Kidd-Munro Assemblage. At the Potter mine the Kidd-Munro Assemblage is divisible into 3 informal lithostratigraphic chemostratigraphic units: 1) a Lower ultramafic komatiite unit; 2) a Middle tholeiitic basalt unit; and 3) an Upper ultramafic komatiite unit. The Middle tholeiitic unit contains bedded volcaniclastic breccias, previously referred to as hyaloclastite, as well as intact and autobrecciated, comagmatic, peperitic sills (dikes) of massive quench-textured basalt, carbonaceous argillite, massive sulphide and komatiitic flows. The "hyaloclastite" consists of framework supported, normally to reversibly graded, depositional units (centimetres to metres in thickness) of densely packed, amygdaloidal, globular to angular, plate-like lapilli (<lmm to 5mm) of palagonitized and chloritized sideromelane. Fragments of olivine porphyritic basalt, amygdaloidal aphyric basalt and plagioclase microlitic basalt are less common. Vent proximal deposits contain coarse fluidal bombs, cored bombs and armored lapilli. Accessory fragments of chert, carbonaceous mudstone, argillaceous mudstone, and massive su1phide account for <1% of the breccia. The matrix, which rarely exceeds 20% by volume of the hyaloclastite, consists of: 1) carbonate; 2) broken crystals of quartz, plagioclase and pyroxene; 3) fine, massive chlorite; 4) carbonaceous sediment; and 5) massive su1phide. The hyaloclastite was derived by quench fragmentation within a subaqueous fire-fountain where rapidly erupted, low viscosity Fe-tholeiitic magma was literally torn apart and quench-fragmented. The resulting "hyaloclastite" was transported as high particle concentration mass or grain flows and deposited within a linear graben in the underlying flat, komatiitic lava plain. Thin, discontinuous deposits of argillite and chert signify breaks in hyaloclastite deposition dominated by fine suspension sedimentation and hydrothermal discharge (chert, sulphides). Clasts of these sediments within the hyaloclastite probably represent rip-ups from underlying sediments that were completely removed during emplacement of subsequent mass/grain flows. The occurrence of massive sulphide clasts indicates that hydrothermal discharge and sulphide deposition occurred during breaks in hyaloclastite deposition, although the majority of the sulphide may have precipitated below the seafloor within the hyaloclastite.

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