Southern Arburese ore deposits, Arburese, South Sardinia Province, Sardinia, Italyi

Regional Level Types
Southern Arburese ore deposits	Ore Field
Arburese	Mining District
South Sardinia Province	Province
Sardinia	Autonomous Region
Italy	- not defined -

Southern Arburese ore deposits, Arburese, Arburese, South Sardinia Province, Sardinia, Italy

The Arburese southern sector is characterised by a wide variety of mineral deposits and occurrences quite unusual for the Sardinian basement, as polymetallic Ni-Co-Pb-Zn-Cu-Ag-Bi and Sn-W-As veins. This small district has always been considered of minor or negligible economic relevance (Zuffardi, 1982) in comparison with the Montevecchio–Ingurtosu–Gennamari lode system (one of the largest and richest Variscan Pb-Zn ore deposits in Europe), although several deposits have been the object of limited mining for Ni-Co, Pb-Zn-Ag, As, Sn, and W minerals during the early 20th century.

The main ore deposits and occurrences include:

- veins with dominant Ni-Co arsenides: Sa Menga NW-SE veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Nieddoris NW-SE veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Acqua Is Prunas E-W veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Pira Inferida E-W veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Santa Barbara ENE-WSW veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Riu Mesu ENE-WSW veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Planu Is Castangias E-W veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]; Gutturu Fenugu/Prameri NE-SW veins [Ni-Co (Pb-Zn-Cu-As-Sb-Bi-Ag)]

- veins with dominanant Pb-Zn sulfides, subordinate Ni-Co arsenides: Perdas de Fogu (or Perd'e Fogu) E-W veins [Pb-Zn (Cu-Ni-Co-Ag-Bi-As-F)]; Perda S'Oliu E-W and NW-SE veins [Pb-Zn (Cu-Ni-Co-Ag-Bi-As-F)]; S'Acqua Bona NW-SE veins [Pb-Zn (Cu-Ag)]; Fenugu Sibiri E-W veins [Pb-Zn (Cu-Ni-Co-Ag-Bi-As-F)]; Monte Idda E-W veins [Pb-Zn (Cu)]

- Sn-As-W veins: Perdu Cara-Santa Vittoria NE-SW veins [Sn, As (W, Cu)]; Genna Movexi-Cuccuru Mairu NE-SW veins [As (Pb, Fe)]; Nuraghe Togoro ENE-WSW veins [W (Sn)].


The Ni-Co-Pb-Zn-Cu-Ag-Bi hydrothermal vein systems
The Ni-Co-Pb-Zn-Cu-Ag-Bi hydrothermal vein systems, hosted by Ordovician-Devonian hornfelses and very low-grade metamorphic rocks, are oriented NE-SW, ENE-WSW and NW-SE. Ni-Co minerals are found in two systems:

- a 10-km-long system of prevailing Ni-Co arsenide ad subordinate Pb-Zn-Cu-Ag sulfide quartz-siderite-calcite veins, oriented ENE-WSW, (e.g. Fenugu Sibiri and Acqua is Prunas mines); in the westernmost part, where the system form the so-called Perdas de Fogu (or Perd'e Fogu) lode, Pb-Zn sulfides are dominant (Perda S'Oliu mine); ores include nickeline, rammelsbergite, skutterudite, safflorite, gersdorffite, breithauptite, and löllingite with galena-sphalerite-chalcopyrite-tetrahedrite/freibergite, native Bi, bismuthinite, native Ag, ruby silvers, and stephanite;

- Ni-Co (Pb-Zn-Cu-Ag) sulfide/arsenide quartz-siderite-calcite veins, oriented N150°and intersecting the previous ones; these veins are more frequent in the W part of the district (Sa Menga mine); in the NW, very close to the contact with the Arburese Pluton, similar Pb-Zn (Cu, Ag, Ni, Co) sulfide veins were exploited in the S'Acqua Bona and Nieddoris mines; ores include cobaltite, nickeline, skutterudite, safflorite, gersdorffite, ullmannite with galena-sphalerite-chalcopyrite-tetrahedrite, bournonite, millerite, löllingite, arsenopyrite, bismuthinite, and native Bi.

On the field, typical orebodies are high-angle, S-SW-dipping and 1-2 m thick mineralized veins, probably hosted by normal faults (Zuffardi, 1982). They show prevailing fracture infilling (banded and brecciated) textures. Banded textures show repeated quartz and siderite layers, cemented by late calcite; ore minerals are often present in the central part of the veins, also cementing fractures in previous minerals. Brecciated textures including wallrock fragments are frequent as well, e.g. in narrow veins or in zones of interaction of different veins; they often show cockade textures. Common wallrock alterations include zones of bleaching and/or intense silicification.

In both vein systems, mineral associations are broadly similar, although variable in the relative amount of minerals species. Ore textures and mineral phases relationships allow to envisage common paragenetic sequences, essentially verified in all the localities: ore deposition started with 1) quartz-Ni monoarsenide ore (nickeline, Ni As), followed by 2) Ni-, Ni-Co and Co- di-tri- arsenides (rammelsbergite , skutterudite, safflorite, cobaltite), with native Bi and Ag, and by abundant siderite, and by 3) Pb-Zn-Cu-Ag sulfides and sulfosalts; calcite is usually very late. This depict a mesothermal depositional environment, initially characterised by Ni and As- rich fluids, subsequently evolved towards higher contents of As and Co, and, finally, enriched in S, allowing Pb, Zn, Cu deposition as sulfides and sulfosalts. This compositional evolution may have a further confirmation from field evidences, as the early veins of the ENE-WSW system are the most enriched in Ni arsenides, while in the later veins of the NW-SE (crosscutting the previous ones), Co arsenides and Pb-Zn-Cu sulfides are much more abundant (Naitza et al., 2013). The polymetallic Ni-Co-Ag-As-Bi hydrothermal veins of the small Southern Arburese district share several fundamental elements and show striking similarities with the so-called "five elements"-type deposits (e.g. the Erzgebirge/Krusné Hory Mts, Germany/Czech Republic; Cobalt, Ontario, Canada; Bou Azzer district, Morocco), starting from their unusual geochemical and mineral associations (abundant Ni-Co arsenides, Bi and Ag mineral always present and distinctive, Pb-Zn-Cu sulfides). Typical is also the general paragenetic sequence emerging from ore mineralogy, with early Ni arsenide deposition and progressive shifting towards Co- and As- rich phases up to late S enrichment and deposition of sulfide-rich mineralisation.

The Sn-As-W hydrothermal veins
The high-temperature hydrothermal As-Sn-(W) quartz veins are vertically zoned, from quartz-chlorite-cassiterite to large quartz-arsenopyrite veins. The vein texture is made of thick idiomorphic and frequently twinned cassiterite crystals, alternated with several generations of banded/geodic quartz. Clinochlore aggregates are included into the quartz. The arsenopyrite ore shows alternating bands of microgranular to crystalline millimetric arsenopyrite with quartz. Ferberite, as small crystals and grains, is a component of the tungsten-bearing quartz veins.

References

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