Petrology, mineralogy and geochemistry of Hemrin Coal Seam, Hemrin South Mountain, Northern Iraq

Abstract

A newly-discovered 12cm thick coal seam within the Late Miocene Injana Formation in the anticlinal Hemrin South Mountain of northern Iraq was studied geologically, geochemically and petrographically. This previously unnamed coal is herein designated the Hemrin Coal Seam (HCS). To date, this coal seam has been analyzed for its proximate, ultimate, petrological, geochemical, and mineralogical characteristics. The proximate analysis showed that it contains (as median values) 0.8% moisture, 61% volatile matter (daf), 38% fixed carbon (daf), 1.9% ash yield (dry), 9% total sulfur (dry), 80% total carbon (daf) and 16,000 BTU/lb (=37MJ/kg) (dry) calorific value. The ultimate analysis showed that it contains (as median values) <0.01% moisture, 2.2% ash, 9% sulfur, 80% carbon (daf), 0.9% nitrogen (daf), 8.1% hydrogen (daf), and 0.7% oxygen (daf). Organic petrography studies showed that the maceral content of HCS is dominated by huminite (96.4%mmf) with minor liptinite (3.6%mmf) and negligible intertinite. The huminites are mostly ulminite B (95.2%), very little attrinite, and negligible densinite and corpohuminite. The liptinites are mostly resinite (3.6%) and minor solid bitumen. Intertinite macerals are extremely rare, with only one sample containing minor fusinite. The mean random vitrinite reflectance is 0.37% Ro, which is indicative of a thermomaturation level at the lignite/subbituminous boundary. Mineralogically, the HCS consists of quartz, clay, pyrite, calcite, rozenite, anatase and gypsum which are the source of ash and most major elements. The HCS was geochemically analyzed on a whole coal and ash basis for 11 major elements and 51 trace and rare-earth elements (REE). The HCS is a high-sulfur coal and the source of sulfur is shared between organically-bound and ash. In comparison to the Clarke of the world coal, and average of USA and Chinese coals, the coal of HCS is strongly enriched by a factor of 10's in environmentally hazardous elements V, Cr, Ni, and Mo. The ash of the HCS is also significantly enriched by a factor of 10's to 100's in V, Ni, Mo, Cd, and Ta; and less significantly by a factor of 1–10 in many other trace elements, while it is depleted in all REEs and many trace elements relative to the Clarke of the world coal ashes. The mean of coal affinity index and enrichment factor shows that the HCS has comparable values for most major and REEs relative to the Clarke of upper crust and sedimentary rocks, while it has high values for most trace elements including significantly high values for Mo, V, Se, Ni, Cd, Sb, Br, Hg, Ag, As, and Ta. The REEs distribution pattern show decreasing trend from LREE towards HREE and has very low total of 2.29ppm relative to the world coal and international reference rocks. The Ni/Co versus V/(V+Ni) diagram showed a suboxic/anoxic environment of deposition, possibly under lacustrine or flood-plain conditions. The rank of HCS is probably bituminous coal as indicated by very high calorific value and total carbon and very low ash yield.