Ophiolites Research Papers - Academia.edu (original) (raw)

GOLD IN LIGURIAN OPHIOLITES (Italy)
In the Ligurian Alpine-Apenninic arc, from west to south-east, there are three ophiolite complexes, known as "Gruppo di Voltri", "Zona Sestri-Voltaggio" and "Ofioliti della Liguria orientale".
The Voltri Group is an Alpine-type ophiolite association, which have been affected by the Alpine orogenesis and the consequent metamorphism, that completely canceled the stratigraphic relationships and gave rise to important green schist facies modifications which often make it difficult to recognize the original nature of the rocks. The most important forming lithotypes are: lherzolites, serpentine schists, metagabbros, prasinites, amphibolites, quartzites, shales: acid crystalline schists are present along the western margin.
The Sestri-Voltaggio and the East Ligurian ophiolites have similar petrographic and structural characteristics: in each complex it is possible to observe a normal ophiolite suite, composed of serpentinized lherzolites, gabbros, basalts, cherts, limestones and shales. Sestri-Voltaggio is a real tectonic-shared “zone”, directed North-South, which represents the geological contact between Alps and Apennines. A vast flyschoid sedimentary sequence, from the Cretaceous-Paleocene age, separates the Sestri-Voltaggio Zone from the complex of eastern Liguria: this is composed of more or less extended ophiolithe bodies, elongated in the NNW-SSE direction, emerging from the flysch.
Gold presence in the Ligurian ophiolites has been known for a long time. The most ancient records date back to the 15th century, but the most important works were carried out during the last century, mostly in the Voltri Group, where placers, eluvial soils and quartz veins were exploited. A certain amount of gold has been recognized in the most important lithotypes. Gold traces are present in pyrite and marcasite which are contained in limestone, schist, flint, gabbros, serpentinite, basalts, as well as in the contacts among them. In the copper mine of Monte Loreto, East Ligury, large native specimens of gold have occasionally been found in altered veins.
Recent studies dealing with the Voltri Group ophiolite complex, have shown that gold is unevenly distributed in the ultramafic rocks, where the average gold content ranges from 0.1 to 2.0 gr/ton. However, the most interesting mineralizations are the hydrothermal quartz veins in alterated serpentinites, that form small deposits know in ancient Italian literature as “ydrothermalites”, to day better know as “listwaenites” and similar, derived from Russian word. These are stock-work type deposits, mainly composed by quartz and Ca-Mg carbonates with gold, chalcopyrite, sphalerite, tetrahedrite, pyrrhotite, marcasite, pyrite, galena, and nickel sulphides, cementing crushed breccias near the contact between ultramafics and metabasites, or cutting serpentinites near the above contact. The single veins are sometimes more than 40 metres thick and several hundred metres in length. Gold is very unevenly distributed; the average content range from 2.0 to 20.0 gr/ton in the most altered veins, but locally the content can be 200 gr/ton and more.