Geological Setting

Stratigraphy of the Belt Supergroup in the Troy area (from Harrison & Cressman, 1983) Click image for larger chart.

The geology of the general area is characterized by a thick sedimentary sequence Proterozoic in age. Within this sequence, there are four major conformable groups: Lower Belt, Ravalli, Middle Belt Carbonate and Missoula Groups. The Troy Mine and Rock Creek project are found within the Ravalli Group and specifically in the Revett Formation.

These formations represent a mature, clastic sandstone of varying thicknesses with the sulphide mineralization being stratabound and disseminated. Copper and silver mineralization occurs throughout the Revett Formation (in excess of 2,000 feet thick).

Stratabound Copper Deposits
Base and precious metals mineralization is associated with sulphide dissemination occurring within selected portions of the Precambrian Belt Supergroup, and more specifically within the Revett Formation. This group of mineralization, referred to as stratabound disseminated copper deposits, is interpreted to result from the migration of hydrothermal solutions through unconsolidated porous sediments prior to or during diagenesis.

This class of copper and silver deposits have received considerable research to understand their genesis and determine the fundamental controls on ore distribution. All deposits are very similar and exhibit consistent lateral metal and mineral zoning which were interpreted to derive from primary ore-forming processes.

Source: Balla 2000 Click image for larger chart.

Copper-Silver Mineralization
Copper is found in the sulphide minerals bornite and chalcocite and most often occurs as fine-grained disseminations with concentration of these minerals (less than two percent up to approximately six percent) along fractures, veinlets, and bedding planes. There are two adjacent copper sulphide zones: the bornite-calcite zone and the chalcocite-chlorite zone. Significant amounts of silver are found only in these two copper sulphide zones, primarily as native silver.

Typical Style of Copper and Silver Mineralization in the Revett Formation at the Troy Mine. (A, B, C) Middle Quartzite, North Ore Body; (D) Upper Quartzite, East Ore Body. Note Sub-vertical Sulphide-rich Fractures in D. Source: Balla 2000 Click image for larger version.

The thickness of these zones and their copper and silver grades are generally quite continuous across large areas, while locally there are segments considerably thinner or of lower grade.

Enveloping the bornite-calcite and chalcocite-chlorite zones are four additional concentric mineral zones that generally have no economic interest. In concentric shells away from the core these are: the chalcopyrite-ankerite, chalcopyrite-calcite zones, galena-calcite and pyrite-calcite zones.

Troy Mine Deposit
At the Troy Mine, economically significant mineralization occurs within three distinct stratigraphically adjacent quartzite sub-units (Lower, Middle and Upper Quartzites) located within the Upper Member of the Revett Formation.

In plan view, the enveloping surface of the stratiform deposit measures approximately 7,500 feet long by 1,800 feet wide and is approximately 60 feet thick. Each quartzite sub-unit delimits the top and bottom of individual mineralized zones. In the vicinity of the mine, the stratigraphy is generally flat with a shallow dip of four degrees (7% grade). There are two styles of faults in the mine area. Northwest trending fault are brittle-ductile structures with common clay gouge. This family is exemplified by the East Fault. The East Fault displays a close spatial relationship with the Cu-Ag mineralization. In addition, higher grade stockwork and sulphide veins are known to exist within this fault zone. It is not known whether these sulphides represent hypogene mineralization or local remobilization of stratabound disseminated mineralization.

The second type of faults represents late brittle and generally open faults with sandy infill typified by the Cross Fault. These faults trend ENE to ESE and have steep southerly dips. These faults are late structures offsetting the mineralized sedimentary units. The Cross Fault offsets the East Fault.

The deposit has been subdivided into three separate mining areas: being the North Ore Body (NOB), South Ore Body (SOB) and the East Ore Body (EOB), delineated primarily by two faults dissecting the mineralized quartzite sub-units. The NOB and SOB are developed in the Lower and Middle Quartzite while the Middle and Upper Quartzite sub-units are host to the EOB. No economic copper and silver mineralization was delineated in the Upper Quartzite west of the East Fault and similarly in the Lower Quartzite east of the East Fault. The East Fault represents the eastern boundary of the NOB and SOB. The Cross Fault separates the NOB from the SOB. The South fault delineates the southern margin of the SOB. All other lateral ore boundaries are assay delimitated and do not represent hard geological surfaces.

Rock Creek Deposit
The stratigraphy in the vicinity of the Rock Creek project is identical to that found at the nearby Troy Mine. Bedrock exposed in the area consists primarily of the Revett and St. Regis Formations. In this area, Belt Supergroup rocks are gently folded and cut by several northwest-trending faults. In the vicinity of the deposit, two faults – Copper Lake and Moran Faults – sub-divide the deposit into three segments. The Chicago Peak, St. Paul and North Basin blocks.

The more significant portion of the Rock Creek deposit forms an oblong body measuring at least 16,000 feet (4,900 metres) along the long axis by 7,200 feet (2,200 metres) along the short axis. The long axis of the copper and silver mineralization is generally oriented along the north-south direction. The copper and silver mineralization occurs within a very open anticlinal structure, plunging slightly to the northwest. The copper and silver mineralization occurs between elevations of 4,300 and 6,000 feet above mean sea level. On average, it dips shallowly towards the north and west. Mineralization occurs primarily within quartzite units of the Lower Member of the Revett Formation and subordinately within siltite and argillite sub-units of the lower and middle Revett Formation. The Lower Member of the Revett has been locally subdivided into individual units named the “A” through “I” Quartzites (top to bottom). The bulk of the mineralization is confined to one layer, but locally there may be up to four vertically stacked, potentially minable layers. The copper and silver mineralization ranges in thickness from six (6) feet up to a maximum of 235 feet, near the Copper Lake Fault. The average thickness is approximately twenty-seven (27) feet.