Distribution and significance of Pb and Tl in the sulfides and host rocks from the hydrothermal mineralization of the Tolfa Mountains (Latium, Central Italy)

Abstract

Tl and Pb contents of 29 samples of sulfide minerals (mainly pyrite) from the hydrothermal Pliocene vein mineralization of the Tolfa Mountains volcanic district have been measured by a polarographic technique. Abundances of Tl and Pb have also been measured in selected host rocks and travertines to establish the relationship between ores and possible source rocks. Hydrothermal pyrites of the various stages of the mineralization show similar ranges of Tl contents (220–7320 ppb, X ̄ = 2101 ppb ), while the Pb content is higher in late-stage pyrites (23–3650 ppm, X ̄ = 1791 ppm ) relative to early ones (8–246 ppm, X ̄ = 95 ppm ). This latter feature suggests a different Pb content of the ore-bearing solutions of the two stages of mineralization. The Pb/Tl ratio distinguishes the pyrites of the main stage of mineralization from the different mines and is roughly constant in most pyrites from the same locality. It is suggested that Pb and Tl contents in pyrites merease with evolution of the mineralizing process. Volcanic pyrites are enriched in Tl (9000–28000 ppb, X ̄ = 16,666 ppb ) relative to hydrothermal pyrites due to the greater availability of Tl in magmatic processes. The Pb content of volcanic pyrites is as low as that of early hydrothermal pyrites. Disseminated pyrites in the unaltered flysch rocks show higher Pb and Tl levels than those expected in sedimentary pyrites and are comparable with those of hydrothermal pyrites, thus suggesting an origin or interaction with hydrothermal solutions. Tl and Pb contents in the hydrothermally-altered flysch ( X ̄ = 962 ppb and 16 ppm, respectively) are much higher than in the unaltered flysch ( X ̄ = 5 ppb and 4 ppm, respectively), indicating that Pb and Tl were carried by the hydrothermal fluids. Thus the origin of mineralization appears to be strictly related to the hydrothermal process, the ore potential being limited to the alteration zone. The source of Pb and Tl was represented by sedimentary rocks and magmatic fluids involved in the formation of the buried skarns. Tl is retained in the gossans formed on the mineralized zones, whereas the element is absent in ironstones not related to ores. This suggests its potential use as a pathfinder during prospecting for sulfide ores. On the contrary, Pb is retained in both gossans and ironstones at roughly comparable levels, and is, therefore, of limited use for geochemical prospecting. Finally, Pb and Tl are detectable in the travertines from the eastern and southern parts of the Tolfa Mountains area, where the thermal waters leached Pb and Tl, circulating deeply through volcanic rocks and/or sulfide ores. Thus, both the Pb and Tl contents in travertines may be used in prospecting.