Porphyry Copper Systems Notes

Porphyry Copper Systems Overview

  • Definition:

    • Large hydrothermally altered rock volumes (10 to >100 km³) centered on porphyry Cu stocks.
    • May include related deposits such as skarn, carbonate-replacement, sediment-hosted, high- and intermediate-sulfidation epithermal mineralization.

  • Geographic Distribution and Characteristics:

    • Found primarily at convergent plate boundaries (magmatic arcs).
    • Develop in belts, may occur in isolation.
    • Associated with composite plutons at depths of 5-15 km.
    • Life spans of individual systems: ~100,000 years to several million years.

  • Geological Context:

    • Alters from barren to potentially ore-grade zones from sodic-calcic to potassic to advanced argillic.
    • Potassic zones often host high-grade mineralization (chalcopyrite, bornite).
    • Peripheral mineralization includes base metals and Au-bearing veins.

  • Alteration Zones:

    • Early Sodic-Calcium: Ore grade may be low, characterized by minerals like albite and actinolite.
    • Potassic: Centers on the porphyry stocks; high-grade Cu and Au mineralization present.
    • Chlorite-Sericite: Develops outward from potassic zones, may cause depletion of Cu/Au.
    • Sericitic: Formed from sericite and quartz, often overprints existing alteration zones.
    • Advanced Argillic: Deposition of pyrophyllite, alunite, and quartz; often associated with shallow high-sulfidation deposits.

  • Fluid Dynamics:

    • Injection of oxidized magma saturated with S- and metal-rich fluids initiates alteration and mineralization sequences.
    • Transition from high-temperature conditions (>700°C) to lower temperatures (<350°C) yields changes in fluid character, impacting mineralization patterns.
    • Progressive cooling leads to formation of various alteration types, affecting economic viability of deposits.

  • Mineralogical Sequences:

    • Characteristic sequence of quartz-bearing veinlets and disseminated mineralization.
    • Hydrothermal breccias can host high-grade mineralization due to intrinsic permeability.
    • Phreatomagmatic breccias often poorly mineralized; mainly formed later in system evolution.

  • Exploration Implications:

    • Recognizing typical porphyry Cu characteristics assists in mineral exploration strategies.
    • Understanding spatial relationships of deposits aids in evaluating mineral potential in untested areas.
    • Exploration should consider geological and alteration zoning, mineral types, and surrounding host rocks.

  • Notable Characteristics of Deposits:

    • Grades typically range from 0.5-1.5% Cu; possible accompanying Au, Mo.
    • Low-sulfidation Cu-rich mineral assemblages in potassic alteration zones.
    • High-sulfidation deposits often yield Au up to 3 g/t, with minor or no recoverable Cu.

  • Life Span and Stability:

    • Systems can remain active for 10 million years or longer (due to continuous magmatic activity and alteration).
    • The survival of ore-forming processes is contingent on regional geology and the presence of existing infrastructure for exploration.