A structural-tectonic model, which was developed to assess the occurrence of undiscovered porphyry copper deposits and associated polymetallic vein systems for the Mátra Mountains, Hungary, has been expanded here and applied to other parts of central Europe. The model explains how granitoid stocks are emplaced and hydrothermal fluids flow within local strain features (duplexes) within strike-slip fault systems that develop in continental crust above subducting plates. Areas of extension that lack shear at the corners and along the edges of the fault duplexes are structural traps for the granitoid stocks associated with porphyry copper deposits. By contrast, polymetallic vein deposits are emplaced where shear and extension are prevalent in the interior of the duplexes. This model was applied to the Late Cretaceous-age porphyry copper and polymetallic vein deposits in the Banat-Timok-Srednogorie region of Romania-Serbia-Bulgaria and the middle Miocene-age deposits in Romania and Slovakia. In the first area, porphyry copper deposits are most commonly located at the corners, and occasionally along the edges, of strike-slip fault duplexes, and the few polymetallic vein deposits identified are located at interior sites of the duplexes. In the second area, the model accounts for the preferred sites of porphyry copper and polymetallic vein deposits in the Apuseni Mountains (Romania) and central Slovakian volcanic field (Slovakia).

On the cover: View of the Assarel porphyry copper deposit, the largest open-pit mine in Bulgaria. Production is more than 10 million metric tons per year of ore, from which 180,000 metric tons of copper concentrate is extracted (0.45 percent copper, 2.5 grams per metric ton gold). The mine is located in the Panagyurishte area, which is within the central Srednogorie region of central Bulgaria. The snow-covered Stara Planina Mountains in the background are seen 40 kilometers to the south. This area is famous for the discovery in the 1950s of a fabulous gold treasure dating to 2,500 B.C., and the region has a rich mining history since before Thracian time. The mine is located near the headwaters of the Luda Yana River, which flows into the Maritsa River, which then flows southward across Bulgaria and Greece, into the Aegean Sea. Photograph by Lawrence J. Drew.