GRSG Conference 2022: Orbit to Outcrop

Title: Alteration mapping of a porphyry copper deposit using airborne (HyMap) and spaceborne (EnMap) Imaging Spectroscopic data

Author: Saeid Asadzadeh


Porphyry copper deposits represent large, relatively low-grade, epigenetic, intrusion-related deposits that are associated with vast hydrothermal alteration footprints. Hyperspectral imaging spectroscopy is a proven technology for the systematic mapping of the mineralogical footprints of hydrothermal systems. The capability of spaceborne hyperspectral imaging data, including the recently launched EnMap satellite system, however, is yet to be demonstrated. To achieve this goal, the Shadan porphyry Cu-Au deposit was thoroughly studied using airborne HyMap and spaceborne EnMap hyperspectral data collected in 2006 and 2022 at 5 and 30 m ground sampling distances, respectively.

Shadan is a typical porphyry deposit with near-perfect zonation that is hosted by monzonitic intrusions located in the volcanic belts of eastern Iran. The atmospherically and geometrically corrected HyMap and EnMap data were processed using a combination of spectral unmixing algorithms and feature extraction techniques available in the EnGeoMap toolbox. The EnGeoMap toolbox is a spectral processing tool for mineral identification and abundance quantification using hyperspectral remote sensing data.

The feature extraction methodology uses a variety of spectral parameters for the accurate identification/quantification of minerals in geological environments. The chemical composition of a number of minerals including white micas’ Tschermak substitution was mapped by tracking the wavelength minimum of the diagnostic absorption feature at 2200 nm using polynomial fitting. The employed spectral processing approach revealed a rich variety of mineralogic products, including white mica abundance, composition, and crystallinity, kaolinite abundance and crystallinity, amphiboles (actinolite) abundance and composition, ferrous/ferric iron abundance and composition, together with biotite, jarosite, gypsum, chlorite-epidote, and tourmaline abundance maps.

The potassic alteration was characterized by high proportion of biotite-chlorite in the center of the system. The jarosite-rich areas mapped around the potassic core were attributed to the pyrite halo. Actinolite, epidote, and chlorite abundance maps helped define three subzones within the propylitic alteration zonation. The resulting mineral maps revealed that the mineralized parts of the system are spatially connected with well-crystalline, Al-rich white mica (with wavelengths ranging from 2187 to 2200 nm) as well as high proportions of biotite, goethite, jarosite, and ferrous minerals.

The findings of this study were validated by field observations, reflectance spectroscopic measurements in the laboratory, and bulk rock geochemistry. This study demonstrated that spaceborne hyperspectral remote sensing data coupled with advanced processing algorithms could be effectively used to delineate the alteration patterns of exposed porphyry copper systems in great detail and assist in targeting the most promising zones for follow-up exploration activities.