he major products from porphyry copper deposits are copper and molybdenum or copper and gold.
The term porphyry copper now includes engineering as well as geological considerations; It refers to large, relatively low grade, epigenetic, intrusion-related deposits that can be mined using mass mining techniques.
Geologically, the deposits occur close to or in granitic intrusive rocks that are porphyritic in texture.
There are usually several episodes of intrusive activity, so expect swarms of dykes and intrusive breccias. The country rocks can be any kind of rock, and often there are wide zones of closely fractured and altered rock surrounding the intrusions.
As is described following, this country rock alteration is distinctive and changes as you approach mineralization. Where sulphide mineralization occurs, surface weathering often produces rusty-stained bleached zones from which the metals have been leached; if conditions are right, these may redeposit near the water table to form an enriched zone of secondary mineralization.
DISTRIBUTION AND AGE
Porphyry copper provinces seem to coincide, worldwide, with orogenic belts. This remarkable association is clearest in Circum-Pacific Mesozoic to Cenozoic deposits but is also apparent inNorth American, Australian and Soviet Paleozoic deposits within the orogenic belts.
Porphyry deposits occur in two main settings within the orogenic belts; in island arcs and at continental margins. Deposits of Cenozoic and, to a lesser extent, Mesozoic age predominate. Those of
Paleozoic age are uncommon and only a few Precambrian deposits with characteristics similar to porphyry coppers have been described (Kirkham, 1972; Gaal and Isohanni, 1979). Deformation and metamorphism of the older deposits commonly obscured primary features, hence they are difficult to recognize (Griffis, 1979).
PORPHYRY COPPER CLASSIFICATION
Porphyry copper deposits comprise three broad types: plutonic, volcanic, and those we will call “classic”. The general characteristics of each are illustrated in photographs linked near the bottom of this page.&
1. Plutonic porphyry copper deposits occur in batholithic settings with mineralization principally occurring in one or more phases of plutonic host rock. Intrusions Associated with PorphyryCopper Deposits Intrusions associated with, porphyry copper deposits arediverse but generally felsic and differentiated. Those in island arc settingshave primitive strontium isotopic ratios (87Sr/86Sr of 0.702 to 0.705) and, therefore, are derived either from upper mantle material orrecycled oceanic crust. In contrast, ratios from intrusions associated withdeposits in continental settings are generally
2. Volcanic types occur in the roots of volcanoes, with mineralization both in the volcanic rocks and in associated comagmatic plutons.
3. Classic types occur with high-level, post-orogenic stocks That intrude unrelated host rocks; mineralization may occur entirely within the stock entirely in the country rock, or in both. The earliest mined deposits, as well as the majority of Cenozoic porphyry copper deposits, are of the classic type. The term “classic” has been applied to them because of their historical significance,because of the role they played in development of genetic models, and becauseno other term currently in the literature adequately describes them.Deposits of this type have variously been labelled simple, cylindrical, phallic(Sutherland Brown, 1976) and hypabyssal.
WHAT TO LOOK FOR IN THE FIELD
Dykes and granitic rocks with porphyritic textures.
Breccia zones with angular or locally rounded fragments; look for sulphides between fragments or in fragments.
Epidote and chlorite alteration.
Quartz and sericite alteration.
Secondary biotite alteration – especially if partly bleached and altered.
Fractures coated by sulphides, or quartz veins with sulphides. To make ore, fractures must be