Spatial and Metallogenic Relationships between Different Hydrothermal Vein Systems in the Southern Arburèse District (SW Sardinia)

Abstract

The SW Sardinian basement hosts various ore deposits linked to geological processes active from Cambrian to post-Variscan times. In particular, the Southern Arburèse district hosts several granite-related W-Sn-Mo deposits and a 10 km-long system of Ni-Co-As-Bi-Ag ± Au bearing five-element veins. New investigations into the eastern and central parts of the district (Pira Inferida mine sector) were performed to understand the poorly documented spatial and metallogenic relationships between these systems. The granite-related deposits consist of massive wolframite-quartz (W-Bi-Te-Au) and molybdenite-quartz veins, linked to the early Permian (289 ± 1 Ma) Mt. Linas granite, that are cross-cut by the five-element veins. The wolframite-quartz veins, observed by optical and electron (SEM-EDS) microscopy, show abundant native Bi, Bi-Te phases and native Au suggesting a W-Bi-Te-Au hydrothermal system. The five-element veins exhibit breccia and cockade textures, enveloping clasts of the Ordovician host-rocks and locally small fragments of the earlier W-Mo-quartz veins. The five-element vein paragenesis includes three main stages, from older to younger: (1) native elements (Bi ± Au); (2) Ni-Co arsenides-sulfarsenides in quartz gangue; and (3) Pb-Zn-Cu ± Ag sulfides in siderite gangue. The mineralogical, geochemical and isotopic features of the five-element vein swarm are closely comparable to five-element deposits elsewhere in Europe (Germany, Switzerland, Italian Alps). While the source of Ni and Co is still unknown, the high Bi contents, as well as Au enrichment in the five-element veins, suggest selective remobilization of these elements, and perhaps others, from the granite-related W-Bi-Te-Au veins. The five-element vein system was likely formed during a post-289 ± 1 Ma and post-Variscan metallogenic event.

Vein intersection zones and host rock composition as controlling factors in polymetallic ore shoots genesis: insights from the Southern Arburèse five-element (Ni-Co-As-Bi-Ag) vein system (SW Sardinia)

“…The five-element veins of the Arburèse district show many features in common with similar occurrences in other districts of Central-Western Europe (e.g., Southern Germany: Burisch et al, 2017Burisch et al, , 2021. Similar mineral assemblages, origin from comparable low-temperature and highly-saline fluids and a probable post-Variscan age indicated by geological constraints (Deidda et al, 2021), suggest that they are probably related to the same largescale geological frame (breakup of the supercontinent Pangea: Burisch et al, 2022). However, some non-negligible differences with the better known occurrences of Central Europe may be highlighted.…”

“…Another important evidence reinforcing this assumption is the occurrence of abundant native Bi droplets enclosed in the Ni-Co arsenides/sulfarsenides at the intersection zones. In fact, the temperature of fluids must have been greater than 270°C (Ramdohr, 1969;Ciobanu et al, 2005) to allow the remobilization of bismuth from quartz-wolframite veins, where it mostly occurs as complex mineral phases such as Bi-tellurides and sulfotellurides (Deidda et al, 2021).…”

“…Three main ore shoots are known in this area: namely the Speranza, the Pira Inferida and the Santa Barbara mineworks (Fig. 1c); the last two are of particular interest for this study, as they match intersection zones with granite-related, high-temperature systems: 1) the ENE-WSW directed and N dipping quartzwolframite (Bi-Te-Au) vein swarm (Nuraghe Togoro veins: Deidda et al, 2021); 2) quartz-arsenopyrite-pyrite veins. Moreover, old mine reports pointed out that during the ore exploitation in Pira Inferida mineworks some quartz-arsenopyrite-pyrite veins were encountered.…”

Post-Collisional Tectonomagmatic Evolution, Crustal Reworking and Ore Genesis along a Section of the Southern Variscan Belt: The Variscan Mineral System of Sardinia (Italy)

“…(1) Skarn systems, including compositionally heterogeneous Fe-Pb-Zn-Cu ± F ± Sn ± W ± Bi ± As oxidized skarns [128] associated with GS1 suite granites, like the Sulcis and Monte Linas plutons [40,[129][130][131][132][133], and Fe-Cu ± Pb-Zn, ±Ag-Au, ±W, ±LREE reduced skarns, are more diffused in central-eastern Sardinia [79,134,135]; mineralized ores of both types of skarn systems are typically hosted in retrocessed exoskarns, forming massive to lentiform or vein-type bodies [129,133,135]; high contents of rare metals are also reported in primary skarns and related hornfelses [79,133]. (2) Mo-(W-Sn-Bi ± Cu ± Au) endo-and exo-greisens; in 1980s studies [124,125], these ores have been interpreted as porphyry-Mo deposits; in S Sardinia, Mo-bearing deposits and occurrences are exclusive of the GS1 suite of granites, but they are also present in E Sardinia (e.g., Goene: Table 3) and in the Oschiri-Alà dei Sardi district of N Sardinia [125], always associated with YMP granites; (3) Greisen-related Sn-W-Mo-Bi (±As, ±F) vein systems, constantly related to GS1 suite granites [40,136].…”