GEOTHERMAL SYSTEMS IN THE NORTHERN APENNINES, ITALY: MODERN ANALOGUES OF CARLIN-STYLE GOLD DEPOSITS

Abstract

Abstract Carlin-type gold deposits in northern Nevada are inferred to overlie concealed late Eocene plutons, which are increasingly thought to have provided magmatic input to the meteoric water-dominated fluids from which the gold was precipitated. The Larderello, Monte Amiata, and Latera geothermal systems in the Northern Apennines of southern Tuscany and northern Latium, central Italy, may represent Pliocene to present-day analogues because of their demonstrated association with subsurface plutons and jasperoid-hosted antimony-gold mineralization. The plutons, which at depths of >5–7 km remain at least partially molten, continue to supply heat and magmatic fluids to the meteoric water-dominated geothermal systems. Formerly mined antimony deposits of Pliocene or younger age are exposed on the peripheries of the CO2 ± H2S-emitting geothermal systems, and antimony sulfides are still actively precipitating. Stibnite and submicroscopic gold in disseminated pyrite, along with Au/Ag of <0.5 and anomalous As, Hg, Tl, and Ba values, accompanied jasperoid formation in the Northern Apennines systems. Carlin-type mineralization in northern Nevada and the antimony-gold mineralization in the Northern Apennines are hosted by permeable carbonate rocks, particularly stratabound breccias, where they are intersected by steep normal or oblique-slip faults and confined beneath tectonically emplaced hydrologic seals. The Northern Apennines antimony-gold mineralization formed at shallow, epithermal depths, like that recently recognized in the southern Carlin trend of northern Nevada. Although underexplored, the Northern Apennines gold prospects are unlikely to ever attain the giant status of the Carlin-type deposits in northern Nevada, probably because of lower magmatic fertility (ilmenite-series rather than magnetite-series magmatism) and host-rock receptivity (less reactive iron). Nevertheless, shallow carbonate-rock aquifers within high-temperature, intrusion-related geothermal systems, be they extinct or still active, may be prospective for Carlin-style gold deposits.