This paper discusses the manner in which Upper Silesian zinc-lead ores of Mississippi Valley type were introduced into, and deposited within, the ore-bearing dolomite, the host rock of the ores. The ore-bearing dolomite is a neosome developed in Triassic carbonates through dolomitization of limestones and recrystallization of primary or early diagenetic dolomites. It contains three types of ore: (1) ore deposited in rock openings; (2) metasomatically emplaced ore; and ($) ore that crystallized in disaggregated, i.e., delithified, dolomite. The ores were emplaced by mineralizing solutions circulating through an aquifer(s) in lithified rocks. The action of these solutions, combined with that of mobilized ground waters, accounts for the formation of the ore-bearing dolomite. The sulfide ores definitely are epigenetic in relation to their host rocks, and there is no evidence that they were ever part of primary bottom sediments. Further, no evidence is known to us of lateral deposition within the Triassic strata. Mineralization worked outward from those rock volumes that contained the greatest abundance of solution voids. Ample geologic evidence, discussed in this paper, confirmed by measurements of temperature ranges for sulfide precipitation, clearly points to ascending hydrothermal solutions as the progenitors of the ore mineralization. These solutions are thought to be responsible for the formation of conspicuous mineralized karst structures (hydrothermal karsts) that are among the most important ore hosts in the Upper Silesian ore district. The geologic evidence (as detailed in this paper) also indicates that the hydrothermal orebearing solutions rose on a broad front along the northeastern margin of the Silesian basin and, after gaining access to the Triassic aquifer, spread laterally to the south and southwest. larska, 1972). Not until the last decade, however, has the direct relationship of a considerable part of the Upper Silesian ores to underground karst structures been recognized. This direct relationship implies that the eraplacement of ores and the formation of underground karst were parts of the same formative processes and were essentially simultaneous. Such an implication led to the interpretation of Upper Silesian ores in terms of hydrothermal karst processes (Bogacz et al.,

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