image006The gold presence in the streams of Ovada territory is known for a long time, and it is certain that the pick up is initiated in the most remote antiquity.  Particularly intense has been the exploitation of the gold-bearing terraces that developed in the final parts of Stura, Piota and Gorzente rivers, which have been completely removed and, to their place, they still stay extended accumulations of residual pebbles that the popular tradition refers to Roman times.  The deposits of pebbles are entirely in effects similar to those that can be observed in other parts of the Po river basin, particularly along the external front of the Ivrea moraine amphitheater, that represent the testimony of Roman and pre-Roman exploitation of similar auriferous terraces, but while these are at notable distances by the probable primary gold ores, those of Ovada, formed in areas less exposed to the glacial phenomena, are in direct relationship with the gold lodes presents in the hilly band that extends south of Ovada, where appear on the surface some ophiolitic rocks that is part of the Voltri Group.

The primary gold mineralizations take place in tectnonized and serpentinized ultramafic rocks, derived from lherzolites, inside vertical shared fault developed in direction north-south that, generally speaking, put in contact lherzolites with other kind of rocks.  Instead, locally they assume sub-horizontal position, in correspondence of wide milonite bands that underlines slide phenomena of the more superficial lherzolites.  They are constituted by lodes and lenticular bodies of brecciated ed altered serpentinite, cemented by silica-carbonate stock work veins; only locally they appear real compact quartz.  They are extended for a few ten meters, rarely for some hundred, with varying thickness from ten centimeter to some meter, but they are numerous, and sometimes closer so much to constitute unitary layers dykes.  Overall they constitute a particular type of rock, suitable in the old literature with the name of idrotermalite and today internationally known as listwaenite, derived from ultramafic rocks carbonatization and silicization by means of hydrothermal fluids.  Inside the quartz veins, but also in the altered serpentinites, there are scattered microscopic metallic minerals: only locally there are small concentrations of microgranular pyrite or large flakes of other sulfides.  The metallic paragenesis is constituted by gold, pyrite, marcasite, chalcopyrite, pyirrhotite, sphalerite, galena and tetrahedrite, to which can join components of the serpentinites; particularly abundant are reddish iron alteration products and, in some veins, microdiffusions of a green cromium mineral (fuchsite).  The gold is predominantly free, both in the quartz matrix that in the limonitized material, as flaks that don't generally overcome the millimeter; however in some druses and geodes we can find small octahedric crystals or centimetric dendrites. The content is very various and irregular, also within the same vein: locally it is possible to reach and overcome 200 grams for ton of rock, but the average is of a few grams, also in the selected ore.  The metal is constituted by a league with about 85% of gold, 15% of silver and traces of copper and other elements.

Dykes and quartz veins are typically hydrothermal, and the gold origin must be looked in the host mafic rocks, that always contains discreet gold anomalies: in fact it was recognized in traces in lherzolites and, especially, in the dunite levels (up to 1-2 g/T); greater contents (up to 100 g/T and over) was found in friction milonite interested by talc-carbonate alteration and sometimes accompanied by graphite and by asbestos.  Locally has been seen thin gold flakes striped on the mirrors of fault.

The more developed mineralizations appear on the surface of the hills between Piota and Gorzente streams, up the artificial basin called Lavagnina Lakes, in the Mornese, Casaleggio Boiro and Lerma community territories, where four mining concessions was granted in recent times.  Here they were exploited some dykes with higt tenure, even if discontinuous, and local enrichments of superficial alteration have been recognized, sometimes constituted by some meters of lateritic saprolite.  At little distance, in the Stura valley, Ovada and Belforte Monferrato territories, they develop similar but very less mineralizations that still given however life other two mine.  Other mineralizations of minor importance, found in the Visone (of Morbello), Gargassino (of Rossiglione) and Vezzullo (of Masone) valleys, has been object of attention in past times, but they has not flowed in recent mining concessions.

The more recent and deeper explorations, performed in the 1980’s from doctor G. Pipino in collaboration with Canadian mining companies, have underlined the presence of discreet mineralizations in the area between Piota and Gorzente rivers, particularly in the zones of C. Ferriere sup., Nebbie and Argion, where some superficial bodies have been individualized, 500 to 600.000 tons, with gold content varying from 2 to 4 grams per ton.

Despite the pluri-secular levelling and removal of the pebbles, along the lower Gorzente and Piota courses the presence of the accumulations are still observable with a certain continuity, for a linear development of around 14 kilometers, while in the final part of Stura course there are limited edges, because the same alluvial terraces are discontinuous and a little wide.  In the low Piota valley it is also possible to observe the presence of residual pebbles accumulation in a lower terrace and in an high one.  The pebbles directly lean on the rocky substratum that, proceeding toward north, from the hills downstream, just the Orba valley, he is constituted first from greenschists and chalceschists of the Voltri Group, then from the basal sediments of the Piedmontese Tertiary Basin.  In the upper part, close to the hills, they are deprived of vegetation and can reach the 10 meters high, while downstream they are being less elevated and covered by a sharp pain brushwood: in all the cases it is still possible to observe the disposition in parallel alignments, separated by depressions toward the neighbor course of actual strams.  The pebbles are very coarse and rounded; the dimensions go since 10 to 50 centimeters and more, with total absence of more minute elements, and their composition reflects the Voltri Group one, from which they originate, with prevalence of ultramafic rocks, metagabbros, metabasalts, anfibolites and eclogites: the pebbles of quartz, today discreetly diffused only in depth, were certainly very more abundant in past, before their collection for the glass manifactures and for the use as flux in the furnaces.

The accumulation of pebbles are the residues of the great style gold terraces washing, happened in uncertain epoch but certainly ancient.  In the lack of any literary sign, it is reasonable to suppose that activity has been developed in pre-Roman times or, better, to the times of the first ligurian wars (197-172 b.C.).  Of coarse they are not to confuse with heaps of gavel deposed in recent times by local quarrymen, as instead they have made officials of the Piedmont Archaelogic Superintendence.

The working system, that the Romans found on the place and they increased with their military organization, consisted of washing portions of alluvial ground in channels to the necessity dug: water was carried deriving it from mountain streams or from artificial basins previously predisposed, and the material to be washed was poured in the channel directly demolishing it from the banks: the coarsest pebbles, that prevented the slide, were eliminated by hand or with the help of pitchforks, and piled up to the sides, while sand and gravel were dragged away up to the underlying stream.  The heavy minerals contents in the sediment, comprise the coarsest gold, were trapped by the pebbles, whether to trap the fine gold, at least partly, opportune obstacles were predisposed on the fund of the channels. When it became difficult and few practical to directly pour in the channel the material from the banks, running water was interrupted, the residual pebbles completely eliminated and the trapped heavy minerals recovered and sets in safety, for to be finished up under overseeing.  Subsequently, or also contemporarily, according to the availability of water and workers, other parallel channel were dug to the first one and, after having carried the water, was proceeded with the demolition of the banks, and so on.  When the whole alluvial terrace was washed , rested powerful parallel accumulations of pebbles, separated by the channels served for the washing.

lavaggio

Demolition of the gold-bearing terrace and accumulation of the pebbles, idealized during an interview to doctor Pipino (Scienza e Vita Nuova, giugno 1990)

 

As broadly observed and described in recent times, with this washing system a man could demolish and put in the channel from 5 to 10 meters cubes of sediment a day; the volume of necessary water varied from 100 to 1000 per cent of the material to wash, that is from 10 to 100 cubic meters day for worker, and the speed of slide had to be at least 2-3 meters per second.  The recovered gold was about the 60-80% of that content in the washed material, and mostly was constituted by the biggest pieces (nuggets) trapped in the pebbles: the greatest part of the fine gold escaped the washing and went to deposit along the underlying streams, where allowed for centuries the modest activity of panning.