Geology, occurrence and gemmology of Khamti amber from Sagaing region, Myanmar

Abstract

A large quantity of Burmite (or Myanmar amber) is produced at Tanai in the Hukaung valley in Kachin State and at Hti Lin (Tilin) in the Magway Region. Another occurrence of amber is found in Pat-tar bum (also called Pat-ta bum) which is located near the Nampilin stream, about 40 km southeast of Khamti (Hkamti), Khamti Township, Sagaing Region. The present mining sites in Pat-ta bum are Laychun (Lachun) Maw (most productive), Kyat Maw, Shan Maw, Gyar Maw, Kyauk Tan Maw and Nameindra Maw. Low grade metamorphic rocks, Kanpetlet schists and similar schists to the Naga Hills are exposed in the eastern part which include glaucophane schist, graphite schist, and epidote schist. Sedimentary units of Miocene age of the Upper Pegu Group are widely exposed in the western, middle and northeastern part. Amber is found in Orbitolina (mid-Cretaceous) bearing limestone which ranges from a few centimetres to up to two metres in thickness. This limestone is intercalated with sandstone and carbonaceous shaley limestone and sometimes together with carbonaceous materials. The bedding dips vary from 20˚ to 35˚ and amber production follows the bedding plane. Amber is also found in sandstone and carbonaceous shale. The primary amber mining is carried out by blasting the amber-bearing limestone, sandstone and carbonaceous shaley limestone along their bedding planes and aditing. The colour of Pat-tar bum (Khamti) amber varies from yellow, greenish-yellow, orangy-yellow, golden yellow, brownish-yellow and brown. Gemmologically, it is transparent to opaque and the refractive index ranges from 1.53 to 1.54 (spot reading), and the specific gravity ranges from 1.03 to 1.09. Ultraviolet radiation analyses show that very strong chalky yellowish-blue under long wave and weak chalky yellowish-blue or greenish weak chalky yellowish-blue or greenish under short wave. Some of the deep brownish material displays weak chalky blue or yellow under long wave ultraviolet light and inert under short wave ultraviolet light. Inclusions that identified in amber samples in the present study are flattened gas bubbles, flow marks, some brownish organic debris and various organic inclusions (spiders, flies, feather-like and plant-like inclusions and other organic materials). Eleven analysed specimens of Pat-tar bum amber were quite similar to one another and the IR features are dominated by a group of absorption bands at around 2800– 3000 cm–1 , relatively narrow bands in the range of 950–1750 cm–1 overlaying a broad hump at 800–1400 cm–1 , and a weak broad band at around 3420 cm–1 . Pat-tar bum amber does not show the characteristic IR and Raman bands of young copal (which lie in the 1050–1250 cm–1 region and at 1764 cm–1 ), which provides the confirmation for the older age of the amber i.e., mid- -Cretaceous as confirmed by Orbitolina sp. in the host limestone.