Abstract

Pb-Zn ore deposits within the northern Caledonides formed in compressional environments associated with major thrust sheets that were emplaced over foreland sediments during the final closure of the proto-Atlantic. Fluid migrations of saline brines are discussed, together with Pb-Pb isotope data that are consistent with collisioninduced (Laurentia-Baltica-Gondwana) tectonic plumbing systems through which ore fluids were driven to distant structural sites. Pb- isotope data from widely separated ore deposits across the orogen fall on a Pb-Pb linear array equivalent to a 2.05 Ga isochron; μvalues (238U/204Pb) increase from the central parts of the orogen toward its forelands. The array is interpreted as a Pb mixing line for averaged basement (3.8-0.6 Ga) and juvenile Paleozoic mantle Pb within the Caledonian orogen. Continued convergence and thrusting during the closing of the proto-Atlantic activated fluids within the sedimentary column and forced the escape and focusing of ore-bearing brines along horizontal and vertical decollements. Toward the forelands, the fluids interacted increasingly with older basement or sediments derived thereof. The process may have applications in deciphering collisional events and tectonic-fluid transport directions within older orogens.

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