Tracking magmatism and metallogeny in continental crust traps (southern Tuscany)

The reconstruction of magmatic, metasomatic and ore body geometries plays a major role in understanding the emplacement mechanisms for magmas and hydrothermal fluids circulation in the upper crust. The Gavorrano Intrusive-Hydrothermal Complex (GIHC; Tuscany, Italy) is a notable case study where intrusive, hydrothermal rocks and sulfide ore bodies, are spatially associated. The evolution of the GIHC starts in the early Pliocene with Castel di Pietra granitoid intrusion (4637.5 – 4606.1 ka, biotite ages), leading to the thermal aureole formation in the Paleozoic rocks of ~200 m of thickness. A few kilometres, an analogue intrusion is inferred to emplace below the outcropping Gavorrano intrusion leading at 4602.3±20.8 ka (phlogopite age) to the thermal aureole mainly made of phlogopite-olivine marble. At 4520±24 (biotite age), the granitoid GIHC intrusion overlapping the previous thermal aureole leads to an irregular and apparent thermal aureole, but not genetically linked to it. The intrusion of the GIHC (~1.7 km3) consists of sequential emplacement of a cordierite-biotite granite and a tourmaline microgranite. The granitoid bodies exploited the contact between the Permian phyllites and the overlying Mesozoic evaporite-carbonate rocks. The granite is highly porphyritic with megacrysts of K-feldspar and phenocrysts of quartz, plagioclase, biotite, and cordierite. The microgranite is rich in euhedral microliths (10-500 μm) of black tourmaline set in a quartz-feldspars groundmass. At the contact with the intrusion, marbles were overprinted by a discontinuous (0.1-1 m thick) layer of vesuvianite-garnets exoskarn. Host sedimentary and metamorphic rocks, exoskarn and - partially - granitoids rocks were subsequently affected by multiple hydraulic brecciations with late-stage chlorite overprinting. The hydrothermal breccia mantles the granitoid intrusion with a shell shape in paraconcordant attitude. The closing stage of the evolution of the complex is characterized by mineralizing fluid circulation, producing at 3885.2±14.2 ka (adularia) widespread silicification and decametre-hectometre pyrite bodies with smaller amounts of base metal sulfides, Fe-oxides, adularia and fluorite. The integration of mining reports, drill logs, as well as surface and underground mapping allows to reconstruct the attitude and shape of magmatic and hydrothermal bodies. The NW-SE elongated intrusion is characterized by a pronounced asymmetry. The western part has an overall subvertical, west-dipping attitude. Such an asymmetry is shown by each of the two intrusive units and highlighted by second-order features: the Crd-Bt granite unit reaches its maximum thickness (0.8 km) in the central-western subvertical zone. Whilst the subhorizontal eastern branches is a few hundred meters thick, and the subhorizontal Tur-microgranite bodies are made of steep to sub-horizontal west-dipping sheets with minor steeply-dipping offshoots. The GIHC asymmetry is also exhibited by the hydrothermal system: the pyrite ore bodies mantle the top and the western flank of the intrusion. In the vertical section, the pyrite bodies have a sigmoidal shape with a steep west-dipping thick portion connecting upper and lower tails gently dipping to the west. The collected data indicate the west side of the GIHC as the zone where both magmas and hydrothermal fluids did concentrate, focusing on metasomatic/hydrothermal processes. The overall geometries of the intrusive units and pyrite bodies suggest a sense of movement top-down-to-the-West. This close spatial and shape relationship between intrusive rocks and hydrothermal bodies suggests a common extensional tectono-magmatic regime capable to produce asymmetric crustal traps (dilational structures) for magmas and fluids active for ~0.7 Ma in the shallow crust (~ 5 km) during the post-orogenic extensional regime affecting the entire area of the Tuscany. The minero-chemistry study of the Fe-bearing minerals crystallized during the GIHC evolution permits to speculate about the source of iron. A concealed intrusion - probably richer in Fe than outcropping intrusion – is speculated to be under GIHC and able to release Fe-fluids for the huge pyrite crystallization. Moreover, also for the hydrothermal breccia, a deep magmatic source is in more agreement than from the granitoids (boiling) origin. Therefore, the directly investigated part of the GIHC is the place of trapping fluids (magmatic and hydrothermal) rather than the crustal level where the fluids form. Only small portion of fluids were produced for the on-site magmatic-hydrothermal process (e.g. leached granite). The GIHC appears as the upper part of a multi-intrusive - hydrothermal system that was active for more than 0.7 Ma, where the magmatic and hydrothermal fluids were trapped in the post-collisional extensional tectonic regime.