Magnetite-apatite-amphibole-uranium and silver-arsenide mineralizations in lower Proterozoic igneous rocks, East Arm, Great Slave Lake, Canada

Abstract

The East Arm of Great Slave Lake contains a great thickness of Aphebian (2,460-1,750 m.y.) sedimentary and igneous rocks that are little deformed or metamorphosed. Three separate periods of intrusive magmatic activity have been recognized: (1) early Aphebian ( approximately 2,100 m.y.) alkaline complexes and probably related breccia pipes; (2) middle Aphebian volcanism ( approximately 1,870 m.y.) with hypabyssal intrusions; and (3) late Aphebian ( approximately 1,790 m.y.) calc-alkaline diorite stocks and laccoliths. These intrusive rocks all contain examples of the Ag, Ni-Co arsenide ore-type and the later ones particularly contain magnetite-apatite-amphibole with uranium and rare earth element minerals. A model for the genesis of the arsenide ore fluid is postulated from the study of one of the early Aphebian complexes and involves essentially hydrothermal leaching principally of mafic minerals. Models for various modes of deposition are presented and principally involve fluid migration up early joints and deposition in open spaces. The magnetite-apatite-amphibole deposits were emplaced earlier in joints, sometimes passively and sometimes forcefully, and were probably generated as immiscible liquids.The similarity of the ore deposits in the rocks of different ages suggests a common petrochemical process for their formation. The metallogeny of the late Aphebian Great Bear batholith is also closely similar which justifies the assumption of some geotectonic relationship. It has been proposed that the two ore associations may serve as geotectonic fingerprints for ancient subduction zones. These data from the East Arm add no definite evidence for or against the hypothesis but appear only to confuse the issue.