Summary:
The O’Brien Project straddles the Piché Group volcanic rocks and Cadillac-Larder Lake Fault Zone (CLLFZ) that separate Pontiac Group metasedimentary rocks to the south from Cadillac Group metasedimentary rocks to the north.
Across the Project, the CLLFZ shows a general east-west strike and dips steeply south at approximately 85°. On the property, the CLLFZ consists mainly of chlorite-talc-carbonate ultramafic schist, and ranges in thickness from 30 to 100 m in the mine area and narrows significantly to about 12 m wide to the east of Zone 36 East. The CLLFZ is in places closely associated to the Piché Group-Cadillac Group contact, but in most places, the fault is hosted by sedimentary rocks of the Cadillac Group.
Gold production at the historic O’Brien mine came from a few quartz veins, mostly hosted by the O’Brien Mine conglomerate and the northern porphyric andesitic sill. Approximately 95% of the O’Brien ore came from four veins (No. 1, No. 4, No. 9, or “F”) in the eastern part of the mine. The veins contained highgrade shoots that occasionally yielded considerable amounts of visible gold.
Mineralization is currently defined within two areas of the Project site: O’Brien East, host to both Zone 36 East (east of the historic O’Brien mine) and Kewagama (at-depth extension of the historic Kewagama mine), and O’Brien West, host to New Alger, an at-depth and lateral extension of the historic ThompsonCadillac mine.
Within the Zone 36 East area, host to the majority of the current Mineral Resources at the Project, and New Alger, the main mineralized structures (“veins”) are generally narrow, ranging in true thickness from several centimetres up to 7 m, and are similar in character to what was mined at O’Brien. Gold-bearing veins occur in different lithologies of the Piché Group, the Pontiac Group, and the Cadillac Group.
In the Kewagama area, the gold mineralization occurs in rocks of the Piché Group, but present as a series of smaller veins instead of bigger single vein as within the O’Brien deposit.
O’Brien Mine
Gold production at the O’Brien mine came from a few quartz veins mostly hosted by the O’Brien Mine conglomerate and the northern QFP dyke. Approximately 95% of the O’Brien ore came from three veins (No. 1, No. 4, No. 9 or “F”) in the eastern part of the mine. The veins contained high-grade shoots that occasionally yielded considerable amounts of visible gold. The main veins generally strike from 083° to 098°, and dip steeply to the south (-84° to -90°). The stopes averaged 0.75 m to 0.90 m wide. Gold mineralization extends vertically down to at least the 3450 ft level.
No. 1 Vein
The No. 1 Vein was the most productive in terms of tonnage and occurs mainly in the conglomerate. This vein comprises No. 1 Vein NE-SW (080° to 090° Az.) and No. 1 Vein NW-SE (090° to 095° Az.).
No. 1 Vein NE-SW extends from surface to at least the 3000' level and is over 500 ft in strike length. The richest and most productive portion of this vein was from an ore shoot 15 m to 60 m long that plunges approximately 85° to the east from about the 750' level down to at least the 3000' level, at its intersection with Vein No. 1 NW-SE, at the conglomerate hanging wall contact. A second moderate-grade shoot, about 15 m to 45 m long, plunges about 60° to the east from about the 1000' level to the 2500' level.
Vein No. 1 NW-SE extends from about the 750' level to at least the 3450' level, and ranges in horizontal length from about 15 m to 180 m. Higher grade shoots plunging about 85° to the east seem to be controlled by vein intersections and vein folds. Both of these veins average 30 cm thick.
No. 4 Vein
The No. 4 Vein is spatially associated with the North porphyric andesitic sills. It extends from surface down to at least the 3450' level and has a 1,000 ft strike length. It averaged 30 cm thick (Blais, 1954). Approximately 50% of the gold produced came from this vein. This was due to an exceptionally highgrade ore shoot, measuring only 9 m to 15 m horizontally, but extending for 190 m from the 500' level down to the 1125' level.
No. 9 Vein
The No. 9 Vein is located in the northern greywacke and volcanic units. This brown vein is rich in biotite and arsenopyrite. It is also wider than the others. The stopes were rarely less than 1.2 m wide and could reach 6 m in certain folded zones where visible gold was common. It was mined out from the 1250' level down to the 1375' level along a horizontal length of about 50 m.
Zone 36 East area
Within the Zone 36 East area, the main mineralized structures (veins) are generally narrow, ranging in true thickness from several centimetres up to 7 m, but have good continuity both horizontally and vertically. Gold-bearing veins occur in different lithologies of the Piché Group and the Pontiac Group. The veins cross the stratigraphy at low angles and are occasionally folded, particularly in volcanic and argillic host rocks. Generally, the veins strike east-west, dip steeply to the south and contain higher-grade shoots that plunge steeply to the east.
Often, the veins occur as a group of quartz veinlets scattered in a very sheared and altered zone that has no obvious main vein. Only very competent lithologies, like the conglomerate and the porphyric andesitic sills, host large veins. In some drill core, the quartz veinlets exhibit small tight folds.
Gold grades vary considerably. The gold occurs mainly as fine to coarse free grains that are heterogeneously distributed, mainly in the quartz veins, and to a lesser extent, in the wall rock. Higher gold grades occur in short, steeply plunging shoots with a similar style to those mined at the O’Brien mine.
Kewagama area
In the Kewagama area, the gold mineralization occurs in rocks of the Piché Group to the south of the CLLFZ, which strikes east-west in this area and dips 80° to 85° to the south. North of the CLLFZ lies a considerable width of tuffs and agglomerates. In the vicinity of the mine workings, the highly sheared rocks of the Piché Group have an aggregate width of 100 m to 130 m. The succession from north to the south is as follows: greenstone (15 m to 25 m); North porphyric andesitic sill (3 m to 10 m); conglomerate (12 m to 25 m); greenstone and tuffs (3 m to 7 m); South porphyric andesitic sill (3 m to 9 m); and greenstone (about 60 m).
The only gold mineralization of particular interest disclosed by extensive underground workings is found in the winze, in a 25-ft raise above the winze and in the sublevels driven from the winze. These workings revealed an ore shoot with a vertical extent of 70 m and an east-west length of 4.5 to 25 m, in which irregular and discontinuous stringers of blue quartz carry free gold. The majority of these veins are parallel and are contained within the North porphyric andesitic sill near its north margin, but some continue into the greenstone north of the porphyry. Individual veins are rarely more than 10 cm wide and 3 m long; occasionally, two or three are parallel to one another or overlap for part of their length. Some sections of these narrow veins are decidedly high grade, but in any stoping operation there would be considerable dilution.
The Kewagama ore shoot described above occurs in the same rocks as the high-grade shoot in the historical No. 4 Vein mined at the O’Brien mine, and resembles it for its short lateral extent compared to vertical, and for the fact that it contains the same type of blue quartz and associated minerals. It differs from the O’Brien shoot in that it does not follow one definite fracture, instead consisting of a series of irregular overlapping stringers, and for the fact that it is of much lower grade as a whole.
Deposit Types
Greenstone-hosted quartz-carbonate vein deposits are a subtype of lode gold deposits. They are also known as mesothermal, orogenic.