cu porphyry

Porphyry Copper Deposits

What is a Porphyry?

What is a Porphyry Copper Deposit?

Porphyry (por’phy-ry)

An igneous rock of any composition that contains conspicuous phenocrysts in a fine-grained groundmass.

Porphyry

Porphyry Copper Deposit

A large low- to medium-grade deposit, of primarily of pyrite, chalcopyrite and

molybdenite with characteristic concentric zoning of mineralization and

alteration around calc-alkaline porphyritic intrusion (typically quartz

monzonite to granodiorite)

Typical Grades and Size• Copper grade is usually in the range of 0.5 to 1.0%.

But can be as high as 1.5 or as low as 0.3%. The lower limit is defined by production costs.

• Zones of local supergene enrichment can contain as much as 20% copper

• Typically 1 to 2 square kilometers in size, but some deposits is in excess of 10 square kilometers. Mineralization has been found to continue to depths exceeding 1km.

• The total amount of ore may be in excess of 3 billion tons (Chuquicamata)

Deposit Tonnage and Grade

Bajo la Alumbrera (Argentina)

Chuquicamata pitapproximately 2x4x0.8km

Bingham Pit

La Escondida Chile

Importance of Porphyry CU Deposits

• Porphyries produce approximately 60% of the worlds copper

• In addition in conjunction with porphyry molybdenum deposits almost all of the world’s molybdenum is produced from porphyry deposits

• Porphyry deposits produce a significant amount of the worlds silver and gold

World’s Leading Copper Producing Mines

World Copper Production/Consumption

Associated Metals

• Molybdenum• Gold• Silver• Rhenium

Examples of Associated Metal Production

• In addition to 320,000 tons copper, in the year 2000 Bingham also produced four million ounces of silver and about 500,000 ounces of gold and 21 million pounds of molybdenum

• The Grasberg porphyry mine in Indonesia has an annual production of 1.5 billion pounds of copper and 2.5 million ounces of gold

• Bajo la Alumbrera 2001 production was 674,000 oz of gold and 423 million pounds of copper

Are Porphyry Cu Deposits the Ideal Source for Copper?

Why not Sedimentary Base Metal Deposits?

Why not Sedimentary Base Metal Deposits? Part 2

Global Distribution of Porphyry Cu Deposits

Distribution in Time

Generalized Geology of a Porphyry Cu Deposit

Proposed Geology of Porphyry Mo Deposit

Geology of the San Manuel Kalamazoo Deposit

Geology of the Chuquicamata Deposit

Geology of the Bajo la Alumbrera

Alteration Zones - Lowell and Guilbertfrom core of porphyry stock outward

•Potassic Alteration: Always present. Contains secondary k-feldspar, biotite and/or chlorite, replacing feldspar and plagioclase and mafics. May also contain serecite.•(Ore Zone)•Phyllic Zone: Often present. Characterized by vein quartz, sericite, pyrite and lesser amounts of chlorite and illite replacing k-feldspar and biotite.•Argillic Zone: Sometimes present. Characterized by montmorillonite and kaolinite replacing plagioclase and the replacement of biotite by chlorite.•Propylitic Zone: Always present and usually has the largest areal extent. Chlorite, calcite and edpidote replacing mafic minerals and to a lesser extent plagioclase.

Alteration and Ore Zoning

Supergene Enrichment

Fluid Inclusions

More Fluid Inclusions

Fluid Inclusion Findings

• Fluid inclusion data suggests a two end-member system with mixing

• Magmatic fluid inclusions have very high salinities (30 to 60 wt % NaCl equivalent) and very high homogenization temperatures

• Meteoric fluids have lower salinities (<15 wt % NaCl equivalent) and much lower homogenization temperatures

Fluid Circulation Models

Hydrothermal Alteration

Mineral Stability

Stable Isotope Data

What do Stable Isotopes Suggest

• Hydrothermal alteration (and most likely metal transport) is the result rock interaction with both magmatic and meteoric water

• Early potassic alteration is most likely the result of rock interaction with magmatic water

• Later quartz-serecite alteration was caused by meteoric water

Mineralization Environment

Radiogenic Isotopes

• Most Porphyry Cu intrusives often have low 87Sr/86Sr ratios on the order of 0.703 to 0705 - this is unlike tin and moly porphyries that have much higher Sr ratios.

• Intrusive biotite is usually low in fluorine • These as well factors as well as the geologic setting point

to mantle derived sources for the intrusives - unlike Tin and Moly porphyries which seem to have a much larger crustal component (s-type granites)

Tectonic Controls

Tectonic Control