Geochemical and microbiological research was conducted on surficial (0-3 m) deposits of the Haakon Mosby Mud Volcano (HMMV), situated at 72°N in a slide valley that is cut into the alluvial fan of Bjørnøya Island on a passive continental margin at a depth of 1250 m in the Norwegian Sea. Deposits of the central zone of the HMMV crater are warmed to 15°C and contain free hydrocarbons (99.9% CH₄ with δ¹³C = -59.2‰ and δD = -243‰); surface deposits on the periphery of the crater contain up to 24 vol% CH₄ hydrates (δ¹³C = -60.6‰). No bacterial methanogenesis (in situ) occurs either in HMMV deposits or in the glacial drift that surrounds the volcano. Presumably, CH₄ is generated by heat-induced synthesis in preglacial sedimentary rocks at depths exceeding 3 km. Bacterial oxidation of methane in rocks of the outer HMMV margin at a rate of up to 18 μl CH₄ m^-2 day^-1 leads to the formation of carbonate nodules (δ¹³C = -25.8‰; δ¹⁸O = -35.4‰) and to the development of the benthic community (δ¹³C_org(biomass) = -35.0‰). Deposits with hydrocarbons contain migratory H₂S and its derivatives (δ³⁴S_FeS2 = -4.36‰). The chemical and isotopic compositions of HMMV pore waters are typical of deposits with CH₄ hydrates. They are depleted in Cl^- (up to 250 mM, M = mole per liter), SO^2- ₄, Ca²⁺, and Mg²⁺, but have high Alk and are enriched in NH⁺ ₄, Na⁺, Br^-, and J^-; δ¹⁸ O_H2O = 0.00‰; δD_H2O = -15 to -19‰, and δ¹³C_HCO3 = 1.9-2.5‰. Pore waters were found to contain seawater, juvenile fluid, and water produced by dissociation of gas hydrates. A four-stage model is proposed for the formation of the HMMV fluid: (1) a primary abyssal brine of the "oil water" type undergoes double mixing with seawater while ascending in the course of mud diapirism; (2) as a result, a subsurface layer of gas hydrates is generated and then destroyed, producing (3) a gas hydrate field on the seafloor. The last stage (4) involves the formation of pore fluids in the center of the crater owing to heat flow, which causes gas hydrates to dissociate and introduces a fluid with a higher concentration of Cl^- (353 mM). In order to refine this model and calculate the budget of the fluid, additional investigations are required.