USING A PORPHYRY COPPER DEPOSIT MODEL, ASTER DATA, AND A GIS DATABASE TO HELP DETERMINE THE NUMBER OF UNDISCOVERED DEPOSITS IN A MINERAL ASSESSMENT

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data have been used for regional mapping of hydrothermally altered silicic, argillic and phyllic rocks typically associated with porphyry copper deposits in the Southeastern Kazakhstan region. This study illustrates the use of ASTER- mapped alteration patterns based on the porphyry copper deposit model for identification of potential porphyry copper deposits in a mineral resource assessment. A Geographic Information System (GIS) database of physical characteristics of potential porphyry copper deposit sites is used to help assess the number of undiscovered deposits.

Mineral deposit models form the basis for mineral resource assessments because they document the important characteristics of a deposit type. In the idealized porphyry copper deposit model, a core of quartz and potassium-bearing minerals, mostly K-feldspar and biotite, is surrounded by multiple hydrous zones of alteration minerals (Lowell and Guilbert, 1970). Hydrous zones include phyllic-altered rocks which typically contain sericite, a fine grained form of muscovite that has a distinct Al-OH spectral absorption feature at 2.2 micrometers, and advanced argillic-altered rocks that contain kaolinite and alunite, which exhibit Al-OH 2.165- and 2.2-micrometer spectral absorption features (Hunt and Ashley, 1979). Hydrothermally altered silica-rich rocks, also associated with porphyry copper deposits, contain a prominent restrahlen feature in the 9.1 micrometer region.

A regional ASTER alteration map was compiled from 225 ASTER scenes of the Kazakhstan region, using AST05 emissivity data and Level_1b radiance data converted to reflectance. Interactive Data Language logical operators were used with emissivity and reflectance data to map silicic-altered rocks and were also used with reflectance data to map argillic- and phyllic-altered rocks. The ASTER alteration map typically illustrates known deposits, such as the Qonyrat mine, as elliptical to circular patterns of argillic- and phyllic-altered rocks with minor amounts (< 10%) of silicic-altered rocks. Over 300 potential deposit sites were identified on the ASTER alteration map based on similar alteration types and patterns of known deposits. Physical descriptions of potential deposits were recorded in a GIS database using the ASTER alteration map, an ASTER image (R=4, G=6, B=8), a mineral occurrence database, and a 1:1,000,000 scale geologic map. Physical characteristics of potential deposits include shape of alteration patterns, types and percentages of alteration, diameter of alteration pattern along the long axis, argillic and phyllic alteration density determined by a low pass filter, age, lithology, observed structure or landforms in the ASTER image, and associated occurrences and deposits. The number of potential deposits with similar physical characteristics to known deposits are identified using the GIS database and used in assessment, along with other considerations, in estimating numbers of undiscovered deposits within geology-based permissive areas.

Hunt, G. R., and Ashley, R.P., 1979, Spectra of altered rocks in the visible and near infrared: Economic Geology , v. 74, no. 7, p. 1613-1629.

Lowell, J.D., and Guilbert, J.M., 1970, Lateral and vertical alteration-mineralization zoning in porphyry ore deposits, Economic Geology and the Bulletin of the Society of Economic Geologists, v.65, no.4, pp.373-408, 1970.