The crystallisation sequence of the Campiglia Marittima skarn

The gangue of the sulphide-skarn deposit of Campiglia Marittima consists of two mineral assemblages: Hedenbergite – Ilvaite – Quartz – Pyrite ± Andradite ± Calcite ± Fe-oxides ± Sphalerite ± Chalcopyrite ± Galena ± Fluorite, and Johannsenite – Rhodonite – Quartz – Sphalerite – Galena ± Carbonates ± Fluorite. Clinopyroxenes are the most abundant minerals; they range in composition from hedenbergite to johannsenite and are chemically zoned at every scale, from the outcrop to the thin-section scale. Zoning is simple and involves mainly iron and manganese; at the crystal scale, further minor chemical exchanges may occur. The mineral assemblages identify progressive crystallisation under decreasing temperature. Skarn formed within an evolving petrogenetical environment, at pressure lower than 500 bar, under decreasing temperature, with redox conditions changing from oxidising to reducing. Earliest crystallisation originated hematite, suddenly reduced to magnetite. Then, close to 470 °C, as still less oxidising conditions were approached, ilvaite and iron sulphides crystallised. Earliest clinopyroxenes with iron-rich hedenbergitic compositions formed close to 400°C in the southernmost outcrops (Valle del Temperino). Manganese-rich johannsenitic clinopyroxenes formed later, close to 260 °C, in the outer part of the skarn complex (Valle Lanzi). Here, the meridian faults accompanying the Apennine orogeny favoured infiltration of manganese-enriched residual fluids. Most of the copper-lead-zinc ores formed during the late crystallisation stages