TUCHOLITE AT THE SALAMÓN SEDIMENT-HOSTED GOLD DEPOSIT (LEÓN, N OF SPAIN): MINERALOGICAL AND CHEMICAL CHARACTERISTICS

The Salamón gold deposit is an epithermal mineralization hosted mainly in limestone interbedded with bituminous shales formed during the latter events of the Hercynian orogeny. It is formed by a complex paragenetic sequence of Au-As-Sb-Hg-Cu-Ni-Co-Pb-Zn-U minerals in jasperoid and quartz-dolomite host rock. This paragenetic sequence can be divided into two hypogenic phases: the first phase is characterised by very fine grained gold-bearing sulphides and sulphosalts as major ore minerals and tucholite and uraninite disseminated in subordinate quantities. Samples of tucholites from brecciated pockets in subvertical N110ºE fractures of the Valle mine (West Salamón) have been studied.

Tucholite is a carbonaceous material, black and very brittle, uranium bearing, with a massive texture and retraction cracks. Two components with a heterogeneous distribution were identified during the microscopic studies: one isotropic, dark-grey, and the other anisotropic, light-grey and highly graphitizated with numerous sigmoidal microfractures filled by massive cinnabar and numerous uraninite crystals distributed along the boundaries of both components. Uranium-bearing organic matter, originating probably through the interaction of ore solutions with organic matter from the host rocks, was subjected to gamma radiation, which caused polymerisation and partial graphitic restructuring, forming both components. Uranium was released from the organometallic bonds at the boundary of both the components and crystallised in the form of uraninite

Chemical characterisation by organic means (Infrared Spectrometry analysis) of insoluble residue in acetone and chloroform shows a very strong graphitization with destruction of most functional groups.