--> ABSTRACT: Composition of Massive Sulfide Deposits with Special Reference to Chemical Evolution of Hydrothermal Systems in the Ocean, by Sergei G. Krasnov, Tamara V. Stepanova, Sergei M. Sudarikov; #90097 (1990).

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ABSTRACT: Composition of Massive Sulfide Deposits with Special Reference to Chemical Evolution of Hydrothermal Systems in the Ocean

Sergei G. Krasnov, Tamara V. Stepanova, Sergei M. Sudarikov

Statistical analysis of all available data on the composition of modern oceanic massive sulfide ores reveals the dependence of base metal ratios versus geological position and size of the deposits. Small and

medium-sized deposits in axial zones of oceanic rifts have Fe-Zn compositions. Large (over 7 × 106 t) mature deposits on off-axial seamounts and in marginal parts of rift valleys are composed of Fe(dominantly) and Cu-sulfides. According to isotope data, Pb occurring in deposits from intercontinental and oceanic rifts resting upon sedimentary cover is of sedimentary origin. The change from Zn-dominance in ores to Cu-dominance may be explained by evolution of the primary composition of fluids in the course of the development of hydrothermal systems, inferred from thermodynamic modeling. Metal ratios in solutions and ores of the 21°N EPR hydrothermal field also show that, at the early ("black smoker") stage of development of deposits, it is Fe, Cu, and Co that pass t e barrier of sulfide sedimentation to the greatest extent and are dispersed into the water column. These metals reach their maximum concentrations in mature deposits, formed under limited dissemination of fluids. Also, enrichment of large deposits in Fe sulfides may take place as a result of zone refining during the diffuse penetrations of late-stage solutions through previously deposited ores. Such processes are traced in ancient ores. They lead to formation of zones of completely recrystallized Fe sulfides in the central parts of large modern deposits. Based on neutron-activation analysis, these zones are strongly depleted in nearly all minor elements that are transported to the outer parts of deposits and/or to the water.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990