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

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________________________________________________________________ Argilic — Intermediate Argillic This alteration zone is found between propylitic and and advanced argillic. It can be progressive outward from illite illite/smectite montmorillonite. It may also show gradations from illite + illite/smectite into a chlorite envelope around the illite vein (Comstock Lode, Virginia City, Nv). Figure 32 - Argillic alteration contains kaolinite, illite, illite/smectite, montmorillonite, quartz, and pyrite. Propylitic Alteration Propylitic can be regional, but it is usually found in the outer-most alteration zone. It will show different wavelengths for the chlorites as opposed to those chlorites in the intermediate argillic suite. 28 Figure 33 - Propylitic Alteration shows Mg-Chlorite, Fe-chlorite, epidote, calcite, pyrite.
________________________________________________________________ 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 29
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An overview of vis-nir-swir field spectroscopy - Spectral International

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