Kerman Cenozoic Magmatic Arc Research Articles

APPLICATION OF SPECTRAL ANALYSIS IN MAPPING HYDROTHERMAL ALTERATION OF THE NORTHWESTERN PART OF THE KERMAN CENOZOIC MAGMATIC ARC, IRAN

The northwestern part of the Kerman Cenozoic magmatic arc (KCMA) contains many areas with porphyry copper mineralization. In this research, we used the advanced space-borne thermal emission and reflection radiometer (ASTER) and Enhanced Thematic Mapper plus (ETM+) images of this region to map the distribution of hydrothermally altered rocks, based on their mineral assemblages. The spectral measurements based on the spectra of field samples and on ASTER and ETM+ data, show dominantly Al-OH (sericite and clays) and FeO absorption features in the VNIR/SWIR spectral regions. Directed principal components analysis (DPCA), spectral angle mapper (SAM) and linear spectral unmixing (LSU) were performed on ASTER/ETM+ images to map zones of hydrothermal alteration and iron oxide/hydroxide minerals. The individual principal components (PC) images generated by DPCA reveal the distribution of individual alteration minerals such as sericite, illite, kaolinite, chlorite, and epidote. The best results, in terms of mapping the distribution of alteration, were obtained using the SAM and LSU methods; but the LSU method produced a more accurate map. The altered zones were sampled and then subjected to X-ray diffraction (XRD), laboratory spectral and chemical analyses. Microscopic studies of the thin sections were also conducted. Field observations reveal that more than 90% of the known copper mineralized localities occur within the interpreted alteration areas.

Lead isotope signatures of the igneous rocks and porphyry copper deposits from the Kerman Cenozoic magmatic arc (SE Iran), and their magmatic-metallogenetic implications

Copper deposits in the Kerman Cenozoic magmatic arc (KCMA) of Iran are genetically related to Miocene porphyries. The porphyries and their associated Cu deposits were intruded and emplaced into Eocene volcanic-sedimentary successions. Lead isotope compositions of the Eocene volcanic rocks exhibit homogeneous, non-radiogenic values ( 206Pb/ 204Pb = 18.40 to 18.49, 207Pb/ 204Pb = 15.41 to 15.48, and 208Pb/ 204Pb = 38.15 to 38.36). In contrast, the Miocene ore-hosting porphyries and associated sulfide minerals also show a relatively radiogenic and homogenous Pb-isotopic values with orogen character ( 206Pb/ 204Pb = 18.52 to18.60, 207Pb/ 204Pb = 15.58 to 15.64, and 208Pb/ 204Pb = 38.57 to 38.80). The shift in the Pb-isotopic composition of igneous rocks with time towards higher Pb-isotopic values precludes derivation of the ore-hosting porphyries from a reservoir of similar composition to those in the Eocene volcanic rocks. The non-radiogenic Pb-isotope signature of the Eocene volcanic rocks indicates an origin from subduction-related mantle-derived magmas, whereas the higher Pb-isotopic values of the Miocene ore-hosting porphyries suggest an increasing involvement of crustal materials (with lower U/Pb ratios than the upper crust) in the source region of these rocks. Combining the Pb-isotope data with petrogenetic considerations suggests derivation of the middle Miocene ore-hosting porphyries from a mixed reservoir with slightly radiogenic Pb values, such as a hybridized lower continental crustal source. This source probably formed because of the involvement of Cretaceous–Oligocene subduction-related mantle-derived mafic magmas at the base of arc crust. The contribution of the Miocene collision-related mantle-derived mafic magma in melting of the hybridized lower continental crust caused generation of the middle Miocene porphyries, hosting large copper deposits with the least 206Pb/ 204Pb ratios among other porphyries. The higher 206Pb/ 204Pb ratios of the late Miocene–Pliocene porphyries hosting small copper deposits, as well as barren porphyries, may reflect a decreasing involvement of collision-related mantle-derived mafic magma in the source of the late Miocene–Pliocene porphyries. The overall similarity in Pb-isotope values between sulfide minerals and ore-hosting porphyries suggests a magmatic source for most of the Pb and possibly other metals. The more radiogenic isotopic ratios of some late sulfide minerals suggest an insignificant metal contribution from an external source such as local country rocks during hydrothermal circulation.