Date of Award
Master of Science
The gold mineralisation at Darius Mine, Cadillac Quebec, is associated with quartz veins within the rocks of the Piché group. The Piché group consists of both volcanic and sedimentary rocks, older than 2.68 Ga, which crop out along the southern side of the Cadillac break at the interface between the flyschoid sedimentary rocks of the Pontiac group to the south and the volcano-sedimentarv succession of the Abitibi belt to the north. The Cadillac break probably represents an ancestral fault zone along which movement occurred during the deposition of the adjacent Archean rocks. The evolution of the Piché group has been influenced by both Pontiac group sedimentation and Abitibi belt volcanism. It consists of three cycles each of which comprises distinctive volcanic and sedimentary rocks. In the first cycle mafic tholeiitic lavas and tuffs conformably overly the Pontiac grouo grevwackes. Subvolcanic andesite sheets and lenses of qrevwacke occur within the mafic extrusive rocks which are overlavn bv a thin but persistent conglomerate unit. This is interpreted as having been deposited by a mass flow of unconsolidated sediment. The second cycle consists of a unit of calc- alkaline pillowed andesite flows in which auriferous pvrit- ic cherts are developed locally in the pillow interstices and on the upper contact of the unit. Overlying the andesite unit is a thin lens of siliceous greywacke which, ii 1 in part, consists of andesitic tuffaceous material. This is overlavn by an epiclastic biotite grevwacke at the top of the second cycle. The third cycle is composed of intercalated units of mafic tholeiitic tuffs, lavas and sub- volcanic intrusions and volcaniclastic and epiclastic greywackes. Lenses of pyritic chert and carbonaceous argillite are developed on the contacts of the various members of the cycle. In the north the rocks of the third cycle abut against the ankerite talc-chlorite schists of the Cadillac break. Previously it has been considered that the gold and quartz in the veins was deposited by metasomatic fluids. This study however indicates that two stages of mineralisation have occurred. In the first stage of mineralisation, which is considered to be syngenetic, gold mineralisation is associated with pyritic cherts which are developed as interflow sedimentary rocks and are characterised by a relative enrichment of Cu and Zn and depletion of Cr and As. Here it is suggested that gold was precipitated with chert and sulphides from hydrothermal fluids discharged on to the sea floor during the volcanism of the second and third cycles. The gold quartz veins of the second stage of mineralisation transect the rocks of the Piche group at low angles and are considered to have been emplaced subsequent to the deposition of the Piche group rocks, and are therefore epigenetic. The veins are characterised by a relative enrichment of Cr and As and a IV depletion of Cu and Zn and are commonly surrounded by a potassic alteration zone. It is suggested that the gold and cruartz were deposited in dilatent openings within the rocks of the Piche group from ascending hydrothermal fluids generated at depth by dehydration reactions during prograde regional metamorphism. It is probable that the ore shoots represent the principal channels for the ascent of fluids. Where the veins are developed adjacent to the auriferous pyritic cherts of the first stage of mineralisation it is possible that gold was remobilised and concentrated in the veins.
Comline, Stuart Robert, "A STUDY OF THE PICHE GROUP AND VEIN SYSTEMS AT DARIUS MINE, CADILLAC, QUEBEC" (1979). Digitized Theses. 3995.