Late to post-Hercynian hydrothermal activity and mineralization in Southwest Sardinia (Italy)

Abstract

Several kinds of base metal deposits occur in the lower Paleozoic of southwest Sardinia (Iglesiente-Sulcis mineral district). This paper deals with those deposits which are generally referred to as Permo-Triassic, because they accompany and postdate the Hercynian orogeny and are related to magmatic activity.A large number of previously published geochemical data, integrated with additional new data (Sr, Pb, O, C, and S isotopes), are reviewed and discussed in the frame of the late to post-Hercynian geologic evolution of southwest Sardinia. According to geological and mineralogieal characteristics, three types of deposits can be distinguished: (1) skarn ores related to late Hercynian leucogranitic intrusions, (2) high-temperature veins, and (3) low-temperature veins and karst filling. Pervasive epigenetic dolomitization phenomena are geochemically related to the low-temperature deposits.Sr and Pb isotopes of the first and second types (0.7097-0.7140 87 Sr/ 86 Sr; 17.97-18.29 206 Pb/ 204 Pb; 38.11-38.45 208 Pb/ 204 Pb) are distinctly more radiogenic than those of the third type (0.7094-0.7115 87 Sr/ 86 Sr; 17.86-18.05 206 Pb/ 204 Pb; 37.95-38.19 208 Pb/ 204 Pb) which, in turn, are closer to Paleozoic ores and carbonates. Fluid inclusion data indicate that the fluids responsible for mineralization of the first and second types of deposits were hot and dilute (T h = 370 degrees -140 degrees C; 20 wt % NaCl equiv) were responsible for the third type of mineralization, as well for epigenetic dolomitization of the Cambrian host rocks. O isotopes measured in minerals from the first two types (delta 18 O SMOW = 12.8-18.9ppm) are 18 O depleted with respect to the third type (delta 18 O SMOW = 15.9-22.1ppm). These data, coupled with fluid inclusion formation temperatures, indicate that the fluids responsible for the first two types of mineralization were 18 O enriched with respect to those of the third type and related hydrothermal phenomena. The delta 34 S CDT in sulfides of the first two types vary between 3.7 and 10.73 per mil, whereas the values of the third type range from 12.0 to 17.9 per mil.Late to post-Hercynian mineralization is thus explained as the result of three distinct, though partly superimposed, hydrothermal systems. System 1 developed closer to the late Hercynian leucogranitic intrusions and led to the formation of the first and subsequently the second type of mineralization. The relatively hot and diluted fluids had a heated meteoric, or even partly magmatic, origin. Metals were leached from an external, radiogenic source, represented either by Hercynian leucogranites or by Paleozoic metasediments. Sulfur had a partly magmatic signature. System 2 was characterized by very saline, colder fluids which promoted dolomitization, silicification, and vein and karst mineralization. These fluids share the typical characteristics of formation waters, even though their origins remain highly speculative. The hydrothermal system was mainly rock dominated, with only a minor participation of the external radiogenic source of metals. Sulfur was derived by recirculation of pre-Hercynian strata-bound ores. System 3 records the invasion of fresh and cold meteoric waters which precipitated only minor ore and calcite gangue. It may represent the further evolution of system 2, possibly spanning a time well after the Permo-Triassic.The timing of all these phenomena is still questionable, due to the poor geologic record of the Permo-Triassic in southwest Sardinia. Nevertheless, the hypothesized scenario bears many similarities with hydrothermal processes documented throughout the Hercynian in Europe and spanning the same time interval. A comparison with the latter mineralization and hydrothermal activities leads to the hypothesis that the first two types of mineralization are linked to late Hercynian magmatic activity, whereas the third type may be related to either strike-slip or tensional tectonics which, throughout Europe, mark the transition from the Hercynian orogeny to the Alpine cycle.