Weathering Leech Cap

 

An Interactive Poster

Leach_Cap1

geothite

jarosite

"Live" hematite

chalcocite

pyrite, chalcopyrite

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W1-07

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W1-02

W1-02_macro_scale

Hyper-leeched cap weathered rock overlying Escondida . Rock intensely altered by acid ground water, removing all the copper leaving behind a kaolinitic clay and relict patches of live hematite.

W1-04_macro_scale

Leech cap rock: fine reddish spots represent relicts of hematite (“live”) formed from the replacement of early secondary chalcocite formed by replacement subsequent to further lowering of the water table

W1-05_macro_scale

pyrite veinlets altered\weathered to limonite goethite.

W1-07_macro_scale

Jarosite limonite (orangish films and veinlets. Jarosite is a common member to the limonite family, Its composition is KFe3+3(OH)6(SO4)2. Its presence in the weathering cycle infers the presence of potassium (sericite\illite\feldspar) minerals in the system.

Kemess Supergene Native Cu_macro_scale

Hematitic alteration\weathering in oxide zone suggested by the presence of native copper.

Leech
W1-01_macro_scale

Hyper-leeched cap weathered rock overlying Escondida . Rock intensely altered by acid ground water, removing all the copper leaving behind a kaolinitic clay and relict patches of live hematite.

The discovery rocks that lead to the finding of the largest porphyry copper body in the world, (La Escondida) were leech cap weathering rocks that carried less than 100 ppm copper, above billions of tonnes of copper rock

W1-02_macro_scale

Chalcocite supergene blanket: the black coating is secondary chalcocite\covellite deposited on pyrite and chalcopyrite below the water table.

110-300 m below La Escondida leech cap, > 2 billion tonnes copper ore at > 1%

W1-06_18

"Live" Hematite

Leech cap rock: fine reddish limonites represent relicts of hematite (“live”) formed from the replacement of early secondary chalcocite. The fine boxwork to fluffy texture of the hematite is characteristic of the “live hematite” of the leech cap weathering assemblages

Spence Titley, personal communication 1995 at the unmined last leach cap porphyry (Silver Bell) in Arizona: "this is a champagne rock, know it and you know you had supergene copper pass through this space"

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"champagne rock"

Supergene:

Deposits formed from weather of pre-existing mineralizaiton

– Supergene enrichment occur relatively near the surface by weathering

– Supergene processes include meteoric water circulation with concomitant oxidation and chemical weather.

– The meteoric waters oxidize the primary (hypogene) sulphide ore minerals and redistribute the metallic ore elements.
– Supergene enrichment and deposits occur at the base of the oxidized portion of an ore deposit, at and below the water table (reduction).

– Metals that have been leached from the oxidized ore are carried downward by percolation groundwater, and react with the hypogene sulphides at the supergene-hypogene boundary.

– The reaction produces secondary sulphides with metal contents higher than those of the primary ore. In particular chalcocite Cu2S, covelite CuS, digenite Cu9S5

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