Chavez 1

Supergene Enrichment and Ore Deposits: Geochemical Mobility and Accumulation of Metals

William X. Chávez, Jr. New México School of Mines Socorro, New México 87801 Spence, Chile: porphyry system oxide profile



SUPERGENE METALS MOBILITY IS A FUNCTION OF NUMEROUS GEOCHEMICAL - AND PHYSICAL FACTORS: Source/Sink Region Characteristics ♦ High S:Me (usually = high pyrite content, sulfide abundance) ♦ Low reactivity ⇒ alteration assemblages having stable silicates (through hydrolysis and cation leaching) ♦ High fracture permeability (allows fluid movement) ♦ Oxidizing/reducing environment over extended period of time → structural, climatic implications

Quebrada Blanca, Chile

Exhumation of Miocene age porphyry systems in the Tethys Belt shows variable enrichment and present removal of leached cappings → protolith via aggressive erosion: ♦ Ovajik region, eastern Turkïye active ferrihydrite precipitation

♦ SarChesmeh and Meduk, I.R.I. ♦ various Himalaya examples

WORLDWIDE SOURCES OF COPPER BY ORE DEPOSIT TYPE

1% 7% 14%

57% 21%

Porphyry VMS + Sedex Skarn Vein "IOCG"

Leached capping development in a high-pyrite environment…

Fortuna de Cobre, Chile

Do all high-pyrite environments promote significant metals mobility?

Atalaya Pit, Rio Tinto, Spain

Most economically-important supergene copper occurrences are the result of: ♦multiple periods of migration and accumulation of metals (e.g., see Adler, 1956) ♦⇒ ⇒ represent multiple ages for weathering and metals transport… (not necessarily characteristic of other base metals)

Chuquicamata, Chile

“enriched keel” (phyllic to advanced argillic) Fortuna Igneous Complex

Chuquicamata, Chile

Cambrian Coronado Quartzite ~1000 meters higher than in the mine area

Coronado (CuOx)

Northwest Extension (mixed CuOx-sulfide)

Sulfide accumulation as chalcocite >> covellite Morenci, Arizona

Mid-Oligocene age rhyolitic ignimbrite

Santa Rita, New México Photo: Dr. J. Richard Kyle

Paleosurface with hematite goethite leached capping

descent of cappingenrichment contact

cover sequence hematitic capping jar + gt capping

enrichment zone (grey = reduced)

Cuajone, Perú

volume of essentially complete Cu removal and replacement by red hematite residual chalcocite with variable replacement by red hematite

LEACHED CAPPING with residual Cu as relic chalcocite and adsorbed Cu on hematite, goethite; Cu concentration varies significantly because of residual and diverse copper mineralogy, so up to 2000+ ppm Cu available….

Porphyry systems characterized by well-preserved K-silicate constructive alteration usually display geochemically limited metals mobility because of hostrock reactivity…so, age dating may rely on oxide-associated minerals, notably Mn oxides.

Radomiro Tomic, Chile

In-situ oxidation of sulfides: chalcocite → atacamite + hematite

Note red hematite halos adjacent to chalcocite… from where comes the Fe+++?

1 cm

Spence, Chile

Mobilization of metals: ♦…to what distances can one expect metals to be transported?

MnOx in Recent alluvial sediments

…where this does not work….

♦precipitation of malachite on boulders from pH = 6.1, low total sulfate stream waters

Southwestern China

♦ source porphyry-skarn system with variable supergene enrichment

El Salvador leached capping

Cretaceous andesites with exotic chrysocolla

Damiana, Chile

Early, mb-only quartz veins

Later highgrade cv + py + en veins

Chuquicamata, Chile

lavendulan

chenevixite ceruleite

El Guanaco, Chile

→ ilsemannite as a product of molybdenite oxidation

Precipitation of silica ⇒ pH ≈ alkaline (why?)…

Eh

Al+++(aq)

Al2O3.H2O(s)

AlO2(aq)

Al

pH Stability of aluminum in a supergene environment

Conclusions ♦ Supergene metals mobility is a function of a series of geochemical and physical parameters… ♦ …each of which are significant in determining how, and how much, metals are transported during weathering. ♦ Structural considerations are notably important for controlling – and preserving – metals accumulation