Val Vedello

For a long time we feel necessity for a quality change of Orobic Alps Geology that is not tied to hold ideas and models.

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Passo della Scaletta uranium bearing outcrops

 

General

Less important uranium vein like mineralization have been found in Orobic south-vergent  thrust system Alpine reverse faults and fractures of the Collio formation (1). The most important one, located near the “Passo della Scaletta” (Scaletta Pass), has been explored in 1978 by means drillings. It is located on the left hydrographic side of the upper Caronno valley (Piateda – Sondrio - Valtellina). The most important uranium mineralizations near to “Passo della Scaletta” are in the val Vedello (Sondrio province) e in Novazza area (Bergamo province).


Geology

The Collio formation rocks outcropping in the area are green grey tuffaceous conglomerates in 1 m to 1,5 thick beds, interbedded to green grey compact fine grained micaceous sandstones, in thin, up tomaximum of 10 cm tick beds.These rocks of the Collio formation show an almost constant bearing to the North - East and dip 50 - 60° to the North-West. The tectonic is prevailing disjunctive: several fault system with different bearing a little displacement have been mapped. The mineralized fault has been traced at surface for a total of 80 m along the strike and for 14 m along the dip, and is cut into two parts by a small creek.

The Eastern portion of the mineralization is 28 m long in outcrop, and result connected whit a fault whose strike may vary between the West and North-Northwest, dipping 60° to 80° to the North. The anomalous band is 1-2 m wide.

To the West of the creek the mineralized fault outcrops for a distance of 55m; the pitchblende is distributed in the form of a stockwork along several North-Northwest trending fractures associated with siderite. This leads to a widening of the radiometrical anomalous band (up to 7 m). In siderite-bearing veins occur boudinage.

The drillings planned in the area were aimed to define the possible prolongation of the uranium mineralization both in strike and in depth. By means of 2 Diamec 250 rigs, 26 inclined drillings for a total of 1,655m have been performed in a four drilling sites (P1, P2 , P3, P4). A total of 211 samples have been taken for analytic determinations. Stratigraphic-radiometric logs of exploration drillings has been performed.

The results are hereunder resumed:

Most of the drillings sites P1, P2 resulted to barren or poorly mineralized and the fault get barren at the depth. The drilling located in the P3 site (figure 1) has found that here the mineralization extends both in strike and in depth. The ore has been traced to a horizontal distance of 60m and to a total depth of 60 to 80 m. The drilling located in P4 site gave negative result, showing that the fault gets barren also in this sector.

 

Uranium geology

The uranium deposits in the Alps are related to the end of the Hercynian orogeny. Uranium tends to be abundant in the last acid igneous rocks of an orogenetic cycle.  The permotriassic remobilized uranium ore is related to the structural control of to the Alpine cycle (by thermal metamorphism or hydrotemalism?). “Passo della Scaletta” uranium bearing outcrop it is an alpine E-W high-angle reverse fault. Uranium minerals are controlled by concentration centers as fractures, breccias and shared-zone. “Passo della Scaletta” ores occur in veins and breccias (figure 2), or as dissemination in the host rock. The principal uranium mineral is pitchblende associated with abundant secondary uranium minerals made by supergene alteration.


To the "Passo della Scaletta" uranium bearing outcrops my friend Amilcare (in figure 3) has found with Pitchblende many type of secondary uranium minerals related at the weathered zone along fractures as: Gummite, Becquerelite, Uranophane, Kasolite, and Autunite.


(1)The Collio Formation are the lower Permian continental volcanoclastic rocks and the Permian cover over the “Basal conglomerate” (where they exists, however not in val Vedello e val Caronno) or over the metamorphic basement. Recently the Orobic Collio Formation has assumed rank of Gruppo dei Laghi Gemelli (Group of Twin Lakes) superiorly constituted by the Formazione del Lago del Diavolo (Formation of Devil’s Lake) and inferiorly by the Vulcaniti del Monte Cabianca (Vulcanites of Mount Cabianca).