The Porphyry Type Ore Deposits in Western Canada

Abstract

Distribution of the porphyry copper-molybdenum deposits in British Columbia, Canada, is confined to the area where eugeosynclinal volcanic rocks were developed during late Triassic and early Jurassic age. These volcanic rocks contain significant amount of copper minerals and syngenetic origin for this type of mineralization is now widely accepted. It is also noted that the contact type ore deposits, as well as "Kieslager" or volcanogenic ore deposits, have been found on a certain stratigraphic horizon of upper Triassic formations. The porphyry deposits, mostly located to the east of the quartz diorite line, are accompanied by the acidic to intermediate intrusions of the Nevadan or Laramide Orogeny. Regarding chemical composition, the typical porphyry deposits are associated with intrusions of calc-alkali suite but more alkalic suite is the case for the Copper Mountain deposits. Cu-Mo ratio is closely related to the mineral composition of intrutions. The deposits of higher copper content occur in close relation to the intrusions of which mineral composition ranges from granitic to quartz dioritic, while the intrusions associated with the porphyry "molybdenum" deposits are closer to the acidic extrimity in mineral composition. Hydrothermal alteration, typical for the porphyry mineralization, has been observed but alteration zoning has not been recognized in most of the deposits in British Columbia.
In conclusion, ore fluid for the porphyry copper deposits may have been formed in the process of crystallization differentation of magma which originated in relatively shallow depth of the earth's crust and assimilated those highly cupriferous formations of upper Triassic and lower Jurassic. Copper, with unusually larger ionic radius, precipitated as sulphides independent to rock forming minerals, while molybdenum, with similar ionic radius as of iron or magnesium, replaced ferro-magnesian elements of mafic minerals in relatively basic magma and precipitated as sulphides only in association with highly acidic magma.
The argument may be extended to the possiblity that metallic elements, one introduced in the earth's crust, repeat to form various types of ore deposits in accordance with geological events.