The Minto copper deposit, Yukon Territory; a metamorphosed orebody in the Yukon crystalline terrane

Abstract

The original host rocks and form of the Minto copper deposit (lat 62 degrees 36' N; long 137 degrees 15' W) have been disguised by a succession of postmineralization events. These were: (1) regional metamorphism in the Early to Middle Triassic which generated the foliated and isoclinally folded fabric of the deposit and mobilized the sulfides, destroying all primary features except, probably, the mineral zonation (bn-cp-mt/cp-bn/cp + or - py); (2) the Late Triassic intrusion (ca 203 m.y.) of the Klotassin granodiorite, which engulfed and dispersed the deposit; (3) endoblastic potash metasomatism, as the intrusion cooled, forming orthoclase porphyroblasts throughout the deposit and its wall rock--chemical equilibrium was attained between silicates in the ore zone rocks and in the wall rocks; (4) recrystallization of biotite throughout the ore and wall rocks, an event recorded by two concordant K-Ar ages (177 + or - 9 and 180 + or - 9 m.y.; Early Jurassic); (5) cataclastic deformation, possibly related to the rapid uplift of the Klotassin suite during Laberge Group sedimentation (Early Jurassic), which fractured feldspars, strained quartz, arid probably sheared orthoclase porphyroblasts in the ore zone; and (6) intrusion of pegmatite and alaskite dikes, probably related to Middle Jurassic or younger intrusive groups of the Yukon Crystalline Terrane. Subsequently, following extensive erosion, the deposit was faulted, possibly in the Eocene, intruded by andesitic dikes related to the Eocene or younger Carmacks Group volcanics, and affected by supergene chlorite-sericite-hematite, kaolinite, and laumontite alteration.The genesis of the deposit remains in doubt. Whereas the highest grade (Cu-, Ag-, and Au-rich) quartz and K-feldspar-bearing rocks could have originated as a zone of silicification and potash silicate alteration associated with a pre-Klotassin granodiorite hydrothermal deposit, a sedimentary (i.e., red-bed copper) origin cannot be ruled out.