The Calabona Intrusive Complex (Sardinia, Italy) - Evidence for a Porphyry Copper System

The overall relationships between alteration assemblages, mineralization and fluid phases in the Calabona dacite-andesite porphyritic intrusive complex typify a porphyry copper system associated with low SiO 2 rocks and represent a "variation on a theme" of classic porphyry copper deposits (Gustafson and Hunt, 1975). The hydrothermal alteration assemblages define concentric patterns centered on the inner part of the intrusion. These alteration zones vary from a potassic core through a well-developed phyllic shell, a spotty argillic zone, and an outer propylitic fringe. The hypogene subeconomic copper mineralization (0.09% Cu) occurs at the boundary between the potassic and the phyllic zones,in disseminations and in veins. A shallow supergene sulfide zone (0.04% Cu) is mainly located in the argillic alteration. The well-preserved zoning of fluid inclusion populations within the alteration-mineralization pattern, and their P-T-X properties have led to the identification of a multiple-stage hydrothermal event. The earliest recorded fluid (fluid I) was a high-temperature brine characterized by significant concentration of Na, K, Ca, Cu, Fe, and S. This fluid formed the potassic assemblage and possibly represents the transporting agent for the copper. An important boiling episode of the fluid phase (fluid II) occurred probably as a result of a drop in pressure and was responsible for the evolution from potassic to phyllic alteration and for the main stage ( hypogene) of copper deposition. The latest stage (fluid III) was marked by increasing and continuous cooling and dilution. Argillic alteration and deposition of the supergene mineralization occurred at this time.