Abstract

The Pb-Zn-Fe ores of La Union, Spain, occur in an area of about 50 square km in three facial types of parageneses: (1) sulfides-carbonates-silica, (2) sulfides-carbonates-silica-greenalite-magnetite, and (3) oxides-hydroxides-sulfates-carbonates-silica. The distribution of the ore deposits shows a central zone with parageneses (3) and (1) in direct association with late-Tertiary subvolcanic rocks and breccia pipes, an intermediate zone characterized mainly by paragenesis (2), and an outward zone characterized mainly by paragenesis (1). The occurrence of paragenesis (3) in the three zones at higher levels than parageneses (2) and (1) suggests a vertical zoning. A geochemical zoning with, from the central zone outward, Sn, Cu, Be, and Ba richer zones is also apparent. Mineralogical data indicate formation temperatures largely below 250 degrees C and, within range of probable values of f (sub S 2 ) and f (sub CO 2 ) , an increasing importance of f (sub O 2 ) toward the central and higher zones. The absence of crosscutting relations between deposits of different paragenetic type suggests a monascendant or facial zoning related to a center of Late Tertiary subvolcanic rocks and superimposed on the pre-Pliocene geology of the region. The ore deposits are syngenetic fissure veins in Late Tertiary volcanic rocks and epigenetic veins and peneconcordant replacement orebodies in older formations. Geological relations and the structure of the ore deposits indicate strong structural and lithological controls and shallow depths of formation within the zone of ground-water movement.

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