GEOLOGICAL, GEOCHEMICAL AND REMOTE SENSING STUDIES FOR IDENTIFICATION OF SOURCE ROCKS FOR GOLD IN SELECTED AREAS OF SKARDU AND ASTOR, NORTHERN, PAKISTAN.

Abstract

Gilgit-Baltistan region is covering the northern most part of Pakistan where the rocks of the Kohistan-Ladakh island arc and Karakoram plate are exposed. Detailed geological work has been done on the main lithological units of this region in regard to their mineralogy, geochemistry and genesis. However, there is a great need to explore the economic potential of the region through advanced level geochemical, spectroscopic and remote sensing techniques. The main aim of this study was to find source rocks for gold mineralization using these modern techniques in three areas of Gilgit-Baltistan region including Bubin and surrounding areas in Astor valley, Machulu, and Ranthak in Skardu area. Geologically these three areas belong to western part of Ladakh island arc. Spectral reflectance study identified all the characteristic minerals of the unaltered and sulfide bearing altered rocks through their specific absorption and reflectance features. Almost all the absorption and reflectance features of minerals are within the range of 0.35-2.35 μm of electromagnetic spectra that covers visible, near infrared (NIR) and short wave infrared (SWIR) region. Results demonstrates that most of the absorption features at 1.4 μm, 1.9 μm and 2.3 μm were caused by hydroxyl ions (OHˉ) and water (H2O) while the absorption bands between 0.4-1.0 μm were caused due to ferrous ions (Fe+2) and ferric ions (Fe+3). At 0.52 μm, 0.9 μm and between 0.4-0.6 μm are the absorption features shown by Fe+3 while at 1.1 μm the strong absorption minima is due to Fe+2 ions. Minerals olivine, anorthite, antigorite, sillimanite, jarosite, illite, serpentine and calcite were identified through their respective absorption bands of the grab unaltered rocks of Astor and Machulu area. Among the copper and lead mineralized rocks minerals malachite, chrysocolla, azurite, jarosite, limonite, hematite, goethite and montmorillonite were identified through their unique absorption features at various wavelengths. In sulfide-bearing altered rock samples minerals epidote, muscovite, albite, kaolinite, amphibole, muscovite and jarosite were identified. Besides identification of different types of minerals of unaltered rocks and sulfide bearing altered and mineralized rocks the reflectance spectroscopy study helped in selection of different absorption and reflectance bands for processing the remote sensing data. Landsat 8 and ASTER remote sensing data were used for mapping the lithological units and targeting the sulfide bearing alteration zones. ASTER Level 1B data were acquired and the log residual technique was used for calibrating the data. Various band combinations, band ratios, maximum noise fraction (MNF) and principal component analysis (PCA) techniques were applied on Landsat and ASTER data, but the most useful information were obtained by using colour composite combinations and the band ratio techniques. These two methods helped in mapping different lithological units in all the three study areas. The band ratio combinations 7/5– 6/3–4/3 and 7/5–4/3–6/3 for Landsat 8 and 4/7-4/3-2/1 for ASTER were very effective in mapping lithology. Also simple colour composite combinations like 7-5-4, 7-6-5 and 6-5-4 of Landsat 8 and 12-5-3 of ASTER remote sensing data were helpful in mapping different lithological units. Detail petrographic study was carried out for determination of mineralogical composition and different types of textures. About 110 thin sections of different rock types were studied. Petrographic study shows that the granitoids display variations in textures which are fresh in Astor area while slightly altered in Machulu area. Both hornblende and clinopyroxene bearing diorite were identified in the study areas. Serpentinized ultramafic rocks have the same mineralogical composition in Machulu and Ranthak areas. Studied volcanic rocks are slightly metamorphosed to greenschist facies. Whole rock major, trace and rare-earth elements geochemistry of 37 selected rock samples was carried out to understand the tectonic and petrogenesis of these rocks. Geochemistry data show high values of LREE, low values of HREE, depletion in Nb, negative trend of TiO2, enrichment in Sr and slope in trend from LILE to HFSE indicate calc-alkaline nature for the studied rocks that formed in subduction related island arc setting. In addition, 14 pure quartz samples were analyzed for oxygen isotope to find the source for ore fluids involved in the generation of ore minerals associated with quartz veins. This data supports that the fluids involved in the generation of hydrothermal ores is magmatic in nature. Mineralized samples and sulfide bearing altered rock samples were analyzed using ore microscopy, SEM-EDX, QEMSCAN and atomic absorption spectrophotometer techniques. Bulk sample analysis for gold, silver and base metals concentration demonstrate that almost all bulk samples have variable amounts of gold, silver and base metals. Samples from Astor area show promising results and gold concentration of up to 96 ppm was found in mineralized quartz veins. The mineralized veins are few cm to ten meters wide. These quartz veins are associated with dioritic rocks. Dominant minerals in the mineralized zone include galena, chalcopyrite, limonite, pyrite and malachite. The enrichment and depletion of different types of minerals of Bubin (Astor) area suggests that the magmatic hydrothermal fluids played a key role in copper and lead mineralization with gold concentration. Concentrations of gold, silver and base metals were much lower in samples analyzed from Machulu and Ranthak areas. Enrichment and depletion study of ore minerals in these samples show association with sulfide bearing altered zones in shear zones. Further work is recommended to evaluate economic feasibility of these sites and to locate more mineralization zones.