Summary of As, Pb, and Zn behavior and typical assay ranges in porphyry copper systems

This is a clear summary of As, Pb, and Zn behavior and typical assay ranges in porphyry copper systems, based on major USGS porphyry deposit models, Economic Geology literature, and peer-reviewed geochemical studies.

As, Pb, and Zn in Porphyry Copper Systems

These elements are typically pathfinders and peripheral halo indicators, not main economic metals.

1. Arsenic (As)

Arsenic is common in pyrite-rich phyllic zones and is often associated with arsenopyrite and enargite (in high-sulfidation zones).

Typical As Ranges

• 50–300 ppm in most porphyry systems

• 300–1,000 ppm in pyrite-rich phyllic zones

• >1,000 ppm near high-sulfidation epithermal overprints

Interpretation

• High As commonly indicates:

  • Phyllic (sericite–quartz–pyrite) alteration

  • High-sulfidation style Cu mineralization (enargite)

  • Boiling zones or lithocaps above porphyry intrusions

2. Lead (Pb)

Pb is usually low in the porphyry core and increases outward toward the epithermal transition or propylitic–base metal halo.

Typical Pb Ranges

• <50 ppm in potassic core

• 50–150 ppm in phyllic zone

• 150–500+ ppm in outer propylitic/base metal halo

• >0.1–1.0% Pb only in distal polymetallic veins (not porphyry core)

Interpretation

• Increasing Pb indicates:

  • Moving away from the porphyry center

  • Approaching carbonate replacement bodies or polymetallic veins

3. Zinc (Zn)

Zn behaves similarly to Pb and increases outward. Zn occurs mainly as sphalerite and is enriched in the epithermal–porphyry transition.

Typical Zn Ranges

• <80 ppm in potassic core

• 80–200 ppm in phyllic zone

• 200–800 ppm in propylitic/base metal halo

• >0.5–5% Zn only in distal epithermal / skarn / CRD systems

Interpretation

• Zn is one of the strongest indicators that you are in:

  • Outer halo of porphyry

  • Transition to epithermal veins

  • Skarn or carbonate replacement (CRD) environment

Summary Table (As, Pb, Zn in Porphyry Systems)

Element	Porphyry Core (Potassic)	Phyllic Zone	Propylitic / Base Metal Halo	Distal Epithermal / CRD
As	50–200 ppm	200–1000+ ppm	Variable	1000–5000+ ppm (enargite, As-rich veins)
Pb	<50 ppm	50–150 ppm	150–500+ ppm	0.1–1%+
Zn	<80 ppm	80–200 ppm	200–800+ ppm	0.5–5%+

Exploration Meaning

Low Pb–Zn + Moderate Cu–Mo = Porphyry Core

Moderate Pb–Zn + Increasing As = Phyllic Halo

High Pb–Zn (ppm to %) = Outer Halo / Distal Epithermal

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