An overview of vis-nir-swir field spectroscopy - Spectral International

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Porphyry Copper Deposits
Porphyry copper deposits form at 2-5 km below the earth's surface at
temperatures of >500-600 degreed C from hypersaline (40-60 wt % NaCl equiv.)
near neutral magmatic fluids (Hedenquist and Lowenstein, 1994). Aqueous
magmatic vapors containing CO2, SO2, H2S, and HCl separate from fluids and
ascend to the surface. Ore bodies range in size from several hundred meters to
more than a kilometer in diameter. Grades tend to be low, and the ore minerals
are usually disseminated through a quartz stockwork within the upper part of the
host porphyry intrusive.
Alteration around the ore body is usually concentric and may be described by the
Lowell and Guilbert (1970) model for porphyry copper deposits. Zoned
assemblages that were formed as reactive, metal-bearing magmatic fluid, which
moved away from the intrusion, cooled and reacted with the country rock.
In the core of the deposit, over the mineralizing intrusion, there is a potassic zone
(biotite + magnetite ± orthoclase ± albite ± anhydrite ± chalcopyrite ± bornite)
which represents late magmatic K-metasomatism of the host intrusives. This
passes upwards into extensive quartz stockwork and a transitional zone
characterized by intermediate argillic alteration (chlorite + hematite/sulfides +
quartz ± illite).
This is usually overprinted by the phyllic zone which dominates the stockwork.
The phyllic zone is acid-leached and characterized by quartz + sericite +
tourmaline + pyrite ± chalcopyrite. The sericite may be composed of either illite +
montmorillonite or fine grained muscovite. Above the phyllic zone, the rock is
capped by an advanced argillic zone where leaching is oxidized acidic water
(generated by the absorption of magmatic vapor in to meteoric waters) has been
particularly intense and may reach to the paleosurface. It is characterized by the
assemblage quartz + alunite ± pyrophyllite ± kaolinite/dickite. Surrounding the
potassic, intermediate/phyllic and advanced argillic zones is the deuterically
(propylitic) altered country rock. The propylitic zone is characterized by green
coloration and the assemblage: chlorite + epidote ± albite ± actinolite ± calcite ±
hematite.
Hypogene copper sulphide mineralization is characterized by chalcopyrite
[although bornite occurs when the sulfur activity is low as indicated by the
absence of pyrite]. Magnetite is common in the potassic zone and pyrite in
phyllic zone in the advanced argillic zone, sulphides have been decomposed and
leached, or replaced by limonite.
Supergene leaching of hypogene alteration and mineralization occurs after the
hydrothermal event. It is distinguished here from weak secondary alteration (due
to later near neutral meteoric water percolation) which does not produce strong
leaching, but may result in a modest overprint of montmorillonite, kaolinite, in the
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