Occurrence and Origin of Chlorine in Middle Jurassic High-Cl Coals from the Sha’erhu Area, Turpan–Hami Basin, Northwest China

Exceptionally high chlorine contents (up to 1.57%) occur in the Middle Jurassic coal seams of the Sha’erhu area, Turpan–Hami Basin, Northwest China, making this coalfield one of the most Cl-enriched coal occurrences reported in China. However, the occurrence modes and enrichment pathways of chlorine in such coals remain insufficiently characterized. In this study, we integrated coal quality analyses, mineralogical characterization (XRD and SEM–EDS), geochemical measurements (XRF and ICP–MS), and an integrated Sequential Chemical Extraction Procedure–High-Temperature Combustion Hydrolysis approach to systematically elucidate the occurrence forms and enrichment processes of chlorine in the Sha’erhu coals. The results indicate that chlorine predominantly occurs in water-soluble form (78.3%–84.7% of total Cl), followed by a minor adsorbed fraction, whereas carbonate-bound and organic/silicate-bound Cl are negligible. The mineral assemblages and geochemical indicators jointly suggest that the coal seams were deposited in a semi-closed, strongly evaporative lacustrine–peat mire system, which subsequently experienced structurally controlled brine intrusion. Chlorine enrichment is attributed to the combined effects of primary evaporative concentration, externally sourced brines migrating through tectonic conduits, and diagenetic fluid activities. This study provides an important case for understanding the genesis of High-chlorine coals in continental basins.