The shale-hosted Castellanos sedimentary exhalative Zn-Pb-Ba deposit of Jurassic age in northwestern Cuba consists of pyrite, sphalerite, and galena in a hydrothermal quartz-carbonate-barite matrix with very little clastic or shale matrix. The northeastern end of the Zn-Pb-Ba lens overlies a transgressive Cu stringer zone which is interpreted as the vent or feeder channel for the exhalative mineralization. Abundance patterns in the sulfide assemblage define layering within the deposit. In the southwestern part of the deposit sphalerite and galena increase and pyrite decreases stratigraphically upward within individual layers. To the northeast, near the vent, sphalerite and galena decrease upward, parallel to pyrite. In the area of the vent the basal carbonate is exclusively dolomite; however, calcite abundance increases rapidly upward. Remote from the vent, dolomite is the dominant carbonate from the base to the top of the deposit. Carbonate and barite are the main gangue minerals near the discharge site; quartz is the major gangue mineral in the southwestern section. These and other variations result from differences in a Ca (super 2+) , f OH2 , temperature, and Sigma S relative to sulfide-forming cations, especially Fe (super 2+) , at the discharge site and in the depositional basin some distance from the vent.