Fluid evolution of the Monte Mattoni mafic complex, Adamello batholith, Northern Italy: Insights from fluid inclusion analysis and thermodynamic modeling

Abstract

The magmatic-hydrothermal fluid evolution in the Monte Mattoni mafic complex in the Adamello batholith was reconstructed based on a combination of fluid inclusion studies (microthermometry and laser ablation inductively coupled plasma mass spectrometry microanalysis of individual fluid inclusions) and thermodynamic modeling of subsolidus fluid-mineral equilibria. The mafic complex consists of two main units, the Monte Mattoni and the Cadino gabbros, both of which show textural evidence for fluid saturation. In the Mattoni gabbro, which is crowded with equant amphibole phenocrysts forming local cumulate layers, fluid saturation was probably reached as a result of rapid magma ascent and pressure decrease. Some of the exsolved fluid phase was trapped in ocelli (miarolitic cavities of subcentimeter size) occurring in the matrix of the porphyritic gabbro. The overlying and locally transgressive Cadino gabbro consists of needle-shaped amphibole grains that crystallized together with plagioclase at the emplacement level of laterally extensive sills. The Cadino magma reached fluid saturation after progressive crystallization as shown by miarolitic cavities that formed in the center of local pegmatitic pods. Five distinct fluid inclusion types (A to E) are present as texturally and compositionally consistent assemblages in a clear relative chronology. The earlier fluid inclusion types, A to C, are three-phase aqueous-carbonic and show a decrease in salinity (from 7.9 to 5.5 wt % equivalent NaCl), coupled with a decrease in CO2 concentration, with time. The salinity in later two-phase aqueous fluid inclusion types D and E increases again (to 9.5 and 27.1 wt % equivalent NaCl), whereas the CO2 concentration drops to very low values. The Ca/Na ratio in the aqueous fluids increases and the late-stage type E fluids are concentrated calcic-sodic brines. The initial decrease in salinity and CO2 content can be explained by fluid-melt partitioning during successive stages of fluid exsolution from the magma at a minimum pressure of 150 MPa. The reversal to increasing salinity in late-stage aqueous fluids is the consequence of water-consuming subsolidus reactions of the fluid with the surrounding rocks, forming epidote, chlorite and calcite at low fluid/rock ratios. The subsolidus evolution is quantified by fluid- mineral equilibria modeling, which predicts the observed mineral transformations, the systematic variation in chemical composition of the fluids, and the decrease in their CO2 concentration as a result of calcite precipitation. Isochores constructed for the five inclusion types indicate an approximate pressure-temperature path starting with isobaric cooling for the early aqueous-carbonic fluid inclusion types at 250 MPa, followed by a substantial pressure decrease during the entrapment of increasingly saline aqueous inclusions. The late-stage calcic brine inclusions were entrapped at temperatures as low as 250°C, where brittle fracturing marks the transition to near-hydrostatic fluid pressure. The ore metal contents (Cu, Pb, Zn, W, Pb and Mn) and the S content of aqueous- carbonic fluid inclusions in the Monte Mattoni complex are comparatively low, in comparison with typically more differentiated intrusions associated with porphyry copper deposits worldwide. Low concentrations in ore-forming components, even in this mafic magma, indicate a weak endowment with ore-forming components already determined in the mantle source of the Adamello magmas. This lack of fertility for ore formation may be explained by restricted subduction metasomatism in an orogen dominated by collision and slab break-off.