Abstract

Mineralogic and geochemical studies were carried out on hydrothermal alteration zones associated with two types of volcanogenic massive sulfide deposits in Turkey. The first type includes Cu-Zn(-Pb) deposits in the eastern Black Sea region associated with Upper Cretaceous dacitic rocks. The second type includes the Kre pyritic Cu orebodies in the western Black Sea region that are associated with paleo-Tethys ophiolites (Triassic-Lower Jurassic) and overlain by carbonaceous argillites and turbidites.The eastern Black Sea deposits are characterized by clay alteration of both the footwall and hanging wall. The clay zones in the hanging wall range up to 200 m in thickness and from 300 m to 2 km in lateral extent. Alteration beneath the deposits consists mainly of sericite, quartz, and pyrite, with carbonate and kaolinite present in some deposits. This central zone is surrounded by an illitc-mica + Mg-rich chlorite + gypsum zone that grades outward into a zeolite zone. Kaolinite, mixed layer illite-smectite, hematite, and local chlorite occur in the hanging-wall rocks. Carbonates comprise dolomite, siderite, and local calcite. In the footwall rocks, the crystallinity of illite shows an increase with depth. Similarly, in the hanging-wall rocks there is a sharp increase in the proportion of illite in mixed layer illite-smectite, and a change in the kaolin polytype from kaolinite to nacrite as the orebody is approached. These variations in alteration assemblage suggest steep upward-decreasing temperatures during the hydrothermal alteration of the footwall and hanging-wall sequences. The mineralogy, chemistry, and size of the alteration zones associated with the eastern Black Sea deposits are similar in many respects to those of the Japanese kuroko deposits.The alteration pipes below the Kre deposits consist of chlorite, quartz, and sulfides, with some local illite, epidote, calcite, and siderite. Chlorite is Fe rich and becomes increasingly so toward the massive ore. Relative to unaltered basaltic host rocks, the pipes are enriched in Fe 2 O 3 and MgO and depleted in Na 2 O, K 2 O, and CaO.In the eastern Black Sea deposits, a model involving the mixing of ascending hydrothermal fluids with cool seawater below the paleosea floor could explain the formation of the chlorite + anhydrite assemblage in the outer alteration zones and the quartz + sericite assemblage at high a (sub K (super +) ) /a (sub H (super +) ) ratios in the inner footwall zones. In some deposits, decreases in a (sub K (super +) ) /a (sub H (super +) ) ratio and temperature resulted in the formation of kaolin minerals.

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