The Koman dawsonite and realgar-orpiment deposit, northern Albania: Inferences on processes of formation

The deposit of dawsonite and realgar-orpiment in the Koman area, northern Albania, is aligned along the NE-SW-trending tectonic line joining the Krasta-Cukal and Mirdita structural-tectonic zones. The deposit contains the following main paragenetic assemblages: i) marcasite - greigite (Fe-sulfide stage), ii) stibnite - realgar - orpiment (As-Sb-sulfide stage), iii) dolomite calcite - dawsonite - aragonite - barite - gypsum (carbonate-sulfate stage), iv) native As - gibbsite (supergene stage). There was lithostratigraphic control of mineralization; carbonate-rich wallrocks reacted with the mineralizing fluids emanating from a buried magmatic body and migrating along Albanian transversal faults, rather than argillaceous lithotypes. Values of delta(18)O and delta(13)C indicate that dawsonite and hydrothermal dolomite are derived at the expense of carbonate rocks, which occur extensively in the stratigraphic sequence of the host rocks. The water:rock ratio during the carbonate-sulfate stage of deposition was probably small. Moreover, oxygen and carbon isotopic exchange during metasomatic transformation of the rocks, recrystallization and late involvement of groundwater, probably all occurred. The sulfur involved in realgar and orpiment deposition may be of magmatic origin, and there was enrichment of heavier S during late hydrothermal processes. Both marcasite and greigite have low Co/Ni values reflecting low-temperature conditions during their formation; the amount of As in the fluids was relatively high during both Fe- and As-Sb-sulfide stages of deposition. The wallrocks were the source of At and Na involved in dawsonite deposition. Al. Na, K and Si were the elements most effectively leached from rocks during hydrothermal alteration.