Log InRegister

AboutSupport UsPhotosDiscussionsSearchLearnMore

Arbus Pluton (Arburese Pluton), Arburese, South Sardinia Province, Sardinia, Italyi

Regional Level Types
Arbus Pluton (Arburese Pluton)	Pluton
Arburese	Mining District
South Sardinia Province	Province
Sardinia	Autonomous Region
Italy	Country

Arbus Pluton (Arburese Pluton), Arburese, Arburese, South Sardinia Province, Sardinia, Italy

The Arbus Pluton emplaced during late Variscan post collisional phases at 304±1 Ma (U-Pb dating on zircons; Cuccuru et al., 2016), along a thrust surface separating a pile of greenschist allochtonous tectonic units from the very low-grade metamorphic para-autochtonous Foreland. Shallow crustal levels of emplacement (< 2 kb) are constrained by quite minute medium-grain rock-textures of intrusives, by a narrow contact aureole dominated by andalusite-cordierite spotted schists and as well as by the Al-in-hornblende barometer in granodiorites. The pluton is made up of an outer zone that includes several pyroxene and hornblende-pyroxene-bearing granodioritic pulses representing different degrees of magmatic evolution (GD1, GD2 and GD3 granodiorites) and of a core zone of cordierite-bearing leucogranites (LG) (Secchi et al., 1991). Field relations suggest that cordierite-bearing leucogranites post-date the granodiorites. The entire pluton forms a roughly E-W trending elliptical body, emplaced within a narrow E-W trending shear zone, that reactivate the allochtonous/para-autochtonous contact. The strong and widespread subsolidus alteration (e.g. uralitisation of pyroxene) of the granodiorites, frequent pegmatitic pods and layers, miarolitic facies with interstitial quartz/Fe-cordierite graphic intergrowths and greisens developed in GD3 and LG, are indications that magma emplacement was associated with a large and long-lasting circulation of fluids. A distinctive feature of the Arbus granitoid intrusions, recognized at regional scale, is the presence of tourmaline.

The occurrence of tourmaline in the Arbus pluton, firstly recognized in 1933 by Cavinato in the granophyres and in hornfels facies in the contact aureola, has been studied in detail only recently (Bosi et al., 2017 and 2018). The studied tourmaline samples have been collected in greisenised facies and grains in pegmatitic layers within medium-grained granodiorite (GD3), in pegmatite pods in medium-grained LG leucogranites, in fine-grained tourmaline nests in granophyric dikes, and from contact aureole. The studied tourmalines are usually dark, but a sample with two tourmaline phases (dark and bright), associated with quartz and K-feldspar, was recognised in hydrothermal-pegmatitic veinlets embedded in the schists about 100 m far from the contact with the GD3 granodiorite. All dark tourmalines have shorlitic composition whereas the bright one has a foititic composition. In particular, tourmalines from GD3 and LG host rocks are schorl; tourmalines from veinlets in the thermometamorphosed aureole are schorl and foitite; tourmaline within the aureole is Mg-rich oxy-schorl.

Close to the northern and western contacts of Arbus Pluton, a huge system of veins host one of the largest and richest Variscan Pb-Zn (Ag) ore deposits
in Europe (Montevecchio–Ingurtosu–Gennamari lode system). The veins are polymetallic associations of Pb-Zn (Cu, Ag) sulphides with accessory Ni-Co arsenides, generally included in a quartz–siderite gangue. Complex ore/gangue relationships and minor changes in composition indicate a multi-stage hydrothermal process possibly linked to changes of source and physical parameter of the fluids. South from the contact with the Arbus Pluton, hydrothermal veins hosted in the foreland unit are smaller and more discontinuous. Ni-Co arsenides are dominant over Pb-Zn sulphides (e.g. Sa Menga, S'Acqua Is Prunas, and Fenugu Sibiri). In this area, high-temperature (hypothermal) occurrences are documented as small swarms of Sn-As (W)-bearing quartz veins. These NE–SW trending bodies are mostly sub-vertical. Moreover, a roughly E–W trending vein system cut across the Arbus Pluton and, locally, the acidic and mafic dikes. It consists of sub-vertical, N-dipping, quartz veins mineralised with abundant Fe oxides (hematite, goethite) and minor Pb-Zn sulphides. Their wall rocks are frequently silicified and/or kaolinised. At Ingurtosu and Gennamari areas, this vein system offsets the peripheral veins. Finally, a very late system of NW-striking quartz veins, locally mineralized (Pb-F) occurs in the southern sector; it distinctly crosscuts all the previously mentioned systems.

Other Regions, Features and Areas that Intersect

References

Go to top of page