porphyry deposits

Porphyry Deposits

Based on Notes From The Prospecting School on the Web
By W.J. McMillan for the B.C. & Yukon Chamber of Mines


Introduction
Distribution and age
Classification
What to look for in the field
Structure
Alteration
Mineralisation
Theory
Conclusion

INTRODUCTION

 The 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.                                                                                    

Plutonic porphyry copper deposits occur in batholithic settings with mineralization principally occurring in one or more phases of plutonic host rock. Volcanic types occur in the roots of volcanoes, with mineralization both in the volcanic rocks and in associated comagmatic plutons.

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.

Their characteristics, particularly for deposits in the southwest United States, have been extensively described (Titley and Hicks, 1966; Lowell and Guilbert, 1970). 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 because no 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.

  Intrusions Associated with Porphyry Copper Deposits Intrusions associated with, porphyry copper deposits are diverse but generally felsic and differentiated. Those in island arc settings have primitive strontium isotopic ratios (87Sr/86Sr of 0.702 to 0.705) and, therefore, are derived either from upper mantle material or recycled oceanic crust. In contrast, ratios from intrusions associated with deposits in continental settings are generally


WHAT TO LOOK FOR IN THE FIELD

 1.Dykes and granitic rocks with porphyritic textures.

2.Breccia zones with angular or locally rounded fragments; look for sulphides between

fragments or in fragments.

3.Epidote and chlorite alteration.

4.Quartz and sericite alteration.

5.Secondary biotite alteration - especially if partly bleached and altered.

6.Fractures coated by sulphides, or quartz veins with sulphides. To make ore, fractures must be
closely spaced; generally grades are better where there are several orientations (directions).                                                                                                     

If you are doing geochemical soil or stream silt sampling, copper is the best pathfinder element but beware of glacial cover, which may mask the geochemical response.

 Deposits in the Cordillera are mainly of Triassic to Jurassic or Tertiary age. Some of the older ones occur entirely within the granitic host rocks, most are in both the qranitic body and the enclosing country rock. The deposits can be huge; worldwide, some are more than two billion tonnes; at Valley Copper, reserves are nearly one billion tonnes.
 

STRUCTURAL FEATURES

 Mineralization in porphyry deposits is mostly on fractures or in alteration zones adjacent to fractures,so ground preparation or development of a 'plumbing system' is vitally important and grades are best where the rocks are closely fractured. Porphyry-type mineral deposits result when large amounts of hot water that carry small amounts of metals pass through permeable rocks and deposit the metals.


Strong alteration zones develop in and around granitic rocks with related porphyry deposits. Often there is early development of a wide area of secondary biotite that gives the rock a distinctive brownish colour. Ideally, mineralized zones will have a central area with secondary biotite or potassium feldspar and outward 'shells' of cream or green quartz and sericite (phyllic), then greenish chlorite, epidote, sodic plagioclase and carbonate {prophylitic) alteration. In some cases white, chalky clay (argillic) alteration occurs.

MINERALIZATION

 Original sulphide minerals in these deposits are pyrite, chalcopyrite, bornite and molybdenite. Gold is often in native found as tiny blobs along borders of sulphide crystals. Most of the su1phides occur in veins or plastered on fractures; most are intergrown with quartz or sericite. In many cases, the deposits have a central very low grade zone enclosed by 'shells' dominated by bornite, then chalcopyrite, and finally pyrite, which may be up to 15% of the rock. Molybdenite distribution is variable, Radial fracture zones outside the pyrite halo may contain lead-zinc veins with gold and silver values.

THEORY

 The spectrum of characteristics of a porphyry copper deposit reflects the various influences of four main and many transient stages in the evolution of the porphyry hydrothermal system . Not all stages develop fully, nor are all the stages of equal importance.         

Various factors, such as magma type, volatile content, the number, size, timing and depth of emplacement of mineralizing porphyry plutons, variations in country rock composition and fracturing, all combine to ensure a wide variety of detail. As well, the rate of fluid mixing, density contrasts in the fluids, and pressure and temperature gradients influence the end result. Different depths of erosion alone can produce a wide range in appearances even in the same deposit.

 No single model can adquately portray the alteration and mineralization processes that have produced the wide variety of porphyry copper deposits. However, volatile-enriched magmas emplaced in highly permeable rock are ore-forming processes that can be described in a series of models that represent successive stages in an evolving process.

End-member models of hydrothermal regimes attempt to show contrasting conditions for systems dominated by magmatic (waters derived from molten rock) and meteoric waters (usually groundwater), respectively.

Both end-members are depicted after enough time has elapsed following emplacement for water convection cells to become established in the country rock in response to the magmatic heat source.

The convecting fluids transfer metals and other elements, and heat from the magma into the country rock and redistribute elements in the convective system.              

 The two models represent end-members of a continuum. The fundamental difference between them is the source and flowpath of the hydrothermal fluids.

CONCLUSION

 The search for porphyry copper deposits, especially buried ones, must be founded on detailed knowledge of their tectonic setting, geology, alteration patterns, and geochemistry. Sophisticated genetic models incorporating these features will be used to design and control future exploration


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