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UBC Theses and Dissertations

The deformed Early Jurassic Kerr copper-gold porphry deposit, sulphurets gold camp, Northwestern British Columbia Bridge, David Julian


The Early Jurassic Kerr copper - gold porphyry deposit is hosted within a northerly striking, highly deformed and metamorphosed alteration zone in Late Triassic Stuhini Group sedimentary and volcanic rocks. Mineralization is related to west dipping calc-alkaline monzonite, syenodiorite (197 ± 3 Ma, U-Pb zircon) and K-Ba-feldspar megacrystic plagioclase hornblende porphyry (194 ± 1.5 Ma, U-Pb zircon) dykes. The Late Triassic sedimentary rocks exposed along the footwall form an east dipping, upright sequence of well bedded mudstone with conglomerate lenses overlain by conglomerate, sandstone and argillite. Feldspar fragments in the conglomerate indicate a local volcanic provenance. Copper and gold mineralization is concentrated in a region of early, texturally destructive potassic alteration, which is flanked by lower grade propylitic alteration. The potassic alteration was cut by monzonite dykes that were subsequently altered to sericite - quartz - pyrite. Alteration of the monzonite dykes coincided with widespread retrograde alteration of the potassic assemblage during the development of a banded quartz vein stockwork followed by pyrite - chalcopyrite veins and finally anhydrite - quartz - siderite - pyrite - chalcopyrite veins. The retrograde alteration of the deposit resulted in a core of chlorite - sericite - quartz - anhydrite - pyrite - chalcopyrite - magnetite, which has a halo of yellow sericite - quartz - pyrite - rutile alteration. Structural fabrics in the deposit record southwesterly directed shortening coupled with southeasterly directed extension. A whole rock K-Ar age of 124 ± 4 Ma from deformed sericite - quartz - pyrite monzonite suggests that the deposit was deformed and metamorphosed in the Cretaceous. This age and style of deformation correlates with widespread deformation in the Skeena Fold belt during the Late Jurassic to Early Tertiary. The deformation caused widespread recrystallization of the deposit with only local redistribution into extension gashes. Boudinaged dykes and left lateral displacement of the fabric across westerly dipping faults suggest that this deformation was followed by additional southeasterly directed contraction. Undeformed kersantite dykes (51.5 ± 2 Ma, whole rock K-Ar) intrude these westerly dipping faults. Supergene alteration was focused where anhydrite occurred in the chlorite core of the deposit. Hydration of the anhydrite to gypsum caused the rock to fracture parallel to foliation, and resulted in a flaky rubble zone once the gypsum dissolved. Three supergene alteration zones developed in this permeable rubble: (i) leached hematite/jarosite, (ii) minor native copper and coatings of chalcocite/covellite, and (iii) stable chalcopyrite and pyrite without gypsum or anhydrite. The supergene alteration developed in the chlorite core mainly in response to the dissolution of the gypsum.

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