Aeromagnetics, geology and ore environments, II. Magnetite and ore environments

Abstract

“Ore environment”, in the sense that is usually used by geophysicists, signifies a region that is perceived to be favourable for certain kinds of ores and is large enough to be recognizable from airborne surveys. The aim of this article is to inquire whether or under what circumstances such regions can be identified from airborne magnetometer surveys as a result of characteristic features of their magnetic mineralogy. Greenstone belts and former greenstone belts can usually be recognized in large-scale aeromagnetic surveys due to their overall deficiency in magnetite compared with normal granitoid rocks. The low magnetite capacity of greenstone belt rocks is recognizable even under high grade metamorphic facies conditions. Massive, concordant, pyritic CuZn sulfide ores are found within greenstone belts (or former greenstone belts), usually within felsic volcanic or sedimentary zones, and are frequently associated with small beds of metasediments that are rich in iron or in iron and manganese. These beds are 1–10 m thick, continuous over distances of 100 m to a few kilometers, and may extend to distances of 10 km or more away from the ore zone along its strike. The iron may be in the form of pyrite, hematite or magnetite; but magnetite is the most common iron mineral in medium or high grade metamorphic terrains. These units often cause weak anomalies which can be detected by high quality aeromagnetic surveys. Porphyry CuMo sulfide ores are associated with felsic-intermediate plutons and are characterized by concentrically zoned patterns of alteration and mineralization. Aeromagnetic exploration guides to these deposits include the identification of potential porphyry districts by large-scale zoning of igneous rocks from mafic to felsic types as one proceeds towards the centre of the district, and the recognition of concentric patterns of alteration and mineralization having diameters 1–10 km. Some porphyry deposits have a weak “magnetite halo”, and some have magnetite-bearing skarns around the rim of the pluton. Volcanic-associated gold ore deposits occur within greenstone belts or former green-stone belts. They are spatially associated with ultramafic flows, alteration, major fault zones, and feldspar-porphyry intrusions. The ores are believed to besyngenetic-diagenetic Aeromagnetic exploration guides to deposits of this type, which include most of the Canadian deposits, include identification of greenstone belts and delineation of mafic, felsic and sedimentary volcanic phases within the belt, recognition of alteration zones and fault zones, identification of small, felsic porphyry stocks, and delineation of altered mafic/ultramafic layers within the volcanic regions. These and other examples suggest that the “ore environment” concept as previously defined, namely, a region several tens of kilometers in extent in which an experienced interpreter can recognize favourable patterns in the opaque oxide mineralogy, is valid. Used in this way, magnetite can be a useful pathfinder to certain types of ores, and the “aeromagnetic ore environment” has the potential of becoming a practical exploration concept.