The Genetic Link Between Amber and Associated Coal from Kachin State, Myanmar: Evidence from Multiple Spectroscopy and Geochemistry

Amber and coal seams from Hukawng Valley, Kachin State, Myanmar, co-occur within fine-grained clastic rocks, exhibiting a close symbiotic relationship. Although research on the formation age, gemmological characteristics, and geological background of amber from Myanmar has been extensive, investigations from the perspective of this symbiotic relationship remain relatively scarce. This study employs conventional gemmological testing, Fourier-transform infrared (FTIR) spectroscopy, laser Raman spectroscopy, scanning electron microscopy (SEM), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to obtain detailed gemmological, spectroscopic, surface micromorphological, and compositional characteristics of amber-coal symbiotic samples, aiming to reveal the differential elemental selective absorption and geological preservation mechanisms between resinous bodies (amber precursors) and plant matter (coal precursors). The results demonstrate that the amber and coal coexisting sample is disk-shaped, with a layered structure and a "coal-enclosed amber" structure, and white vein-like impurity minerals are developed on the surface. The amber shows light yellow to reddish-brown colour with a resinous luster and a relative density of 1.07; the coal is black to brownish-black, exhibiting a pitchy luster after polishing, with a brownish-red streak, and displays a porous structure in cross-section. Microscopic observation reveals that the contact boundary between amber and coal is not linear and continuous, but rather exhibits corrosion- erosion fragmentation features and heterogeneous characteristics. FTIR spectroscopy reveals distinct organic compositional features: characteristic absorption peaks are present in both the functional group region (4 000-1 300 cm^-1) and fingerprint region (1 300-600 cm^-1). Raman spectroscopy indicates the presence of impurity minerals (calcite, barite and pyrite) in both amber and coal from Myanmar. Under SEM, the contact zone between amber and coal shows interpenetrating phenomena; organic particles in the coal become progressively smaller when approaching the amber region. Additionally, numerous inorganic pyrite microcrystalline aggregates (3-5 μm) are dispersed on both amber and coal substrates, indicating formation in an anoxic and stable depositional environment. LA-ICP-MS data demonstrate that the coal is enriched in trace elements, with uranium (U) enrichment closely associated with organic matter reduction conditions during diagenesis. Significant trace element differences exist among opaque amber, coal, and transparent amber—the variety and concentration of trace elements in opaque zones and coal far exceed those in transparent regions. The opaque amber was affected by intrusion of exotic fine mineral veins and elemental substitution, and the distinction between the elements of coal and transparent amber was mainly attributed to differences in inherited parent material and geochemical coordination mechanisms during subsequent diagenetic processes.