International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 9, September 2015)
Heavy Mineral Assemblages at Cox’s Bazar Paleobeach Core Sediments, Cox’s Bazar, Bangladesh Ashraf Ali Seddique1, Ashraful Hoque2 1
Department of Petroleum and Mining Engineering, Jessore University of Science and Technology, Jessore 7408, Bangladesh, 2 Marine Fisheries and Technology Station, BFRI, Cox’s Bazar 4700, Bangladesh
Abstract: Cox’s Bazar sea beach is one of the world’s largest sandy-beaches located in the South-Eastern part of Bangladesh. Continuous undisturbed core sediments (to a depth of 18.9m), collected from Golchottor Mosque playground of Cox’s Bazar paleobeach area were characterized for their lithology and mineralogy. Result shows that the abundance of maximum concentration of heavy mineral assemblages are found at the upper sand unit sediments at the depth of 0.3-4.9m where the sediments are mainly very fine to medium sand (mean grain diameter of 45.8 to 270.2µm) with the maximum concentration of 59.34wt% at the depth of 1.2m. Heavy minerals in the core sediments are mainly garnet, magnetite, ilmenite, zircon, monazite, leucoxene, kyanite, rutile with minor amounts of hornblende, pyroxene, augite, epidote and amphibole. Studies shows that the heavy mineral assemblages are decreasing with increasing in depth of core sediments and garnet (29.57wt %) is the dominant heavy mineral among the heavy mineral assemblages. Thereafter, studies also reveal that the upper part of core sediments (i.e., 3m in depth) and indicates chemical weathering and/or diagentic alteration might be occurred dominantly in the upper part and released heavy and trace elements into the groundwater aquifers. This study may shed important light on the deposition of placer deposits thereby sedimentation that will aid the economic and sustainable management system for extraction of heavy minerals from the paleobeach areas.
In Bangladesh, the Geological Survey of Bangladesh (GSB) discovered the mineral sand deposited in the coastal region of Cox’s Bazar district in 1961, GSB reported the occurrence of heavy mineral sand deposits and published the report in 1963. Thereafter, the Atomic Energy Commission has been working on mineral sands for the last 45 years and also at initial stage aided by the Australian assistance in both technical and financial regime. Atomic Energy Commission have published a lot of valuable information and data which can attract the investors for commercialization of such deposits 6 and suggested that eight economic minerals are found in Cox’s Bazar beach area, such as ilmenite, magnetite, zircon, rutile, garnet, leucoxene, kyanite and monazite. Heavy minerals are particularly useful in studies of sedimentation related to tectonic uplift of organic belts, as the evolution and unroofing episodes are reflected in their foreland sediments 7. A number of studied 891011 on beach sands with different objectives. However, the most of these studies were based on exploitation and development of economic heavy minerals, distribution and textural characteristics in surfacial deposits of placer on top of most foredunes (recent and present beach) and backdunes (paleobeach) areas 12. As vertical extent of heavy minerals deposits in the paleobeach (i.e., backdunes) area has not been ascertained yet. Thereafter, in order to better understand the vertical distribution of heavy minerals assemblages at Cox’s Bazar, a continuous undisturbed core was performed at paleobeach area (Fig.1). The aim of the studies is mainly to identify and characterize individual heavy minerals concentration vertically of the core samples (up to18.9m) with relationship of their lithology.
Keywords: Cox’s Bazar; Heavy minerals; Paleobeach; Core sediments; Chemical weathering
I.
INTRODUCTION
Beach placer deposits around the world are known for their economic concentration of heavy minerals such as monazite, zircon, ilmenite, rutile, garnet, leucoxene, magnetite, and sillimanite. Those enriched in radioactive minerals are of special interest for their use in nuclear industry1234. It is well known that there are 17 deposits in the beach areas of the south eastern and northern coastal belt of the Bay of Bengal in Bangladesh 5. The estimated reserve of heavy minerals of these 17 deposits is 1.76 million tons 4. Cox’s Bazar beach is adorned by some of such placer deposits, as well as the area is well known for tourist attractions due to having widest sandy beach and beautiful scenery.
1.1. Geological setting of the study area Geologically, the area separated by a longitudinal fault from the complex hilly region in the east, is a coastal area that extended upto the Bay of Bengal with a gentle slope of about 4-60 6. This hill range is tectonically located in the southern most part of the Chittagong-Tripura folded zone and forming asymmetrical anticlines of plunging in nature having a general NNW-SSE trend. Steeper western flank and a gentler eastern flank are resulted from East-West compression due to relative motions of the Indian and Burmese plates 13 14 .
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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 9, September 2015) The molasse sediments of the hilly area are of Neogene age, comprising alternating shales and sandstones in varying proportion derived from the eastern Himalayas to the north and the Indo-Burman ranges to the east 15. The litostratigraphic succession has been subdivided into upper Bhuban, Lower, Middle and Upper Boka Bil and Tipam sandstone formations 16 (Table 1). The Surma Group sediments were deposited under the strong influence of tidal currents in a delta system and the Tipam sediments were deposited under fluvial environment. The Neogene sandstones of the hill ranges are characterized by fine to very fine grained, well to poorly sorted, coarse to strongly fine skewed (Bari et al. 2003). The coastal area is attributed by series of sand dunes varies in width from rainy season to dry season and rises at its eastern boundary to about 4-5m above the mean sea level 17. The sanddunes parallel to the coastline are sparsely distributed all over the beach encompasses both recent and paleobeach. The paleobeach areas are not dynamic because they are not adjacent to the sea and composed of more compact sand than the present and recent beach. Paleobeach sands are fine to medium grained sands with silt and clay. The paleobeach area represented the actual sea beach about 5000 years ago 18. Undisturbed core samples were collected from the paleobeach area of Alluvium formation of Holocene age to a depth of 18.9m.
Fig. 1. Location map of the study area
The color of the samples was measured in situ using the L*a*b* color code system with a soil color reader (SPAD-503, Minolta, Tokyo). Samples were immediately vacuum-packed in Ο2-impermeable film bags (Escal film, Mitsubishi Gas Chemical Co.) with deoxidizer inside. Samples were kept in a laboratory freezer (-1800C) until analysis. Before analysis, samples were freeze-dried to prohibit oxidation of the sediments.
2.1. Sample Collection and Preparation Undisturbed sediment cores were collected from the paleobeach area of Golchottor playground, near Laboni sea beach point during March, 2013. The drilling site is ocated at the edge of foothills along the coast, several hundred meters away from the recent beach (Fig. 1). Undisturbed cores were collected by penetration of a 3.5cm diameter split-barrel sampler (Raymond sampler) by a private drilling company (Eastern Geotechnical, Dhaka). Samples up to a depth of 18.9m were collected at first 0.3m sampling interval which was followed up to 10m depth in 60cm long PVC tubes and then samples were collected after 1.5m interval up to the target depth in 30–60cm long tubes. A standard penetration test (SPT; British Standard BS 1377-9:1990 and ASTM D1586) was carried out at the time of sampling. The N value measured by SPT is an indicator of the density of the ground. Soon after sampling, each lithofacies was visually assessed in the field in terms of grain size, texture and structure.
2.2. Grain size analysis and heavy mineral separation Prior to analysis, the dried samples were ground gently by hand with an agate mortar and pestle, and used for the analysis outlined. Grain size distribution was determined by laser diffraction and scattering spectroscopy (SALD3000S, Shimadzu) after dispersion of the samples in sodium hexametaphosphate solution. The reproducibility of the results for the duplicate samples was within ±5% at maximum. For the separation of heavy minerals, about 50g of samples were sieved through the M50 (0.425mm), M60 (0.250mm), M100 (0.150mm), M200 (0.075mm) and pan (
