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Montserrat

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Great summary from the excellent Brett Davis of his site visit and structural geology findings at the San Jose Mine in the Novales-Udias Project in Spain 🇪🇸 

Great to work with a genuine subjectmatterexpert and thoughtleader well supported by the Variscan Mines Limited technical team and Frank Bierlein.

📢 “This is a truly massive zinc system that is strongly structurally controlled. “

I was privileged to be asked to visit Variscan Mines Ltd’s Novales-Udias Zinc Project in the Basque-Cantabrian Basin, in Spain last year. This is a truly massive zinc system that is strongly structurally controlled. 

The interpretations presented here pertain to the Novales portion of the mineralised system – Udias was not reviewed. These interpretations are based on compilation of a geological history that honours overprinting relationships and that has been married with structural orientation data. As such, there is a lot more information behind these interpretations, but there’s only so much you can put in a LinkedIn post before people’s eyes glaze over. So, you can read the summary of the structural controls below, or you can simply enjoy some images of a truly huge mineralised system and see the variability inherent in the various syn-mineralisation structures.

Mineralisation is hosted by zones of dolomitic alteration within Reocin Formation, a thick sequence of limestone. The permeability framework responsible for the localisation of mineralisation comprises bedding, two tectonic cleavages (S1 and S2), and a suite of coevally developed faults. Mineralising fluids accessed the dolostone during the D2, with alignment of historical exploitation volumes reflecting the S2, D2 transfer-style faults, and reactivated S1. Post-D2 modification of the mineralised volumes has been negligible. The mineralisation is typically coincident with voluminous, white, crustiform dolomite veining, the emplacement of which spanned D2, commencing prior to sphalerite deposition and ceasing after it. The dominant dolomite vein orientation is steeply dipping to the east, reflecting the influence of coeval, subparallel S2.

S1 is the interpreted axial plane to the regional macroscale folds. Orientations form a tight NE-SW trending population. The S1 is subtle and can't always be recognized (as opposed to the S2, which forms persistent penetrative fractures). In one of the photos of the main access drive, the S1 is responsible for a stepped geometry of the wall on the side of the main water channel. 

The carbonate host-rocks have sponsored extensive karst development, typically expressing as open, brown, water-bearing faults. Fault orientations vary because they have exploited earlier and coeval structures, including the S1 and S2.

Insight into the deposit-scale structural controls to mineralisation is given by the geometries of historical zones of extraction represented by massive, elongate open stopes. Stope elongations and stope terminations are controlled by D2 structures, including the S2 and the karstic faults, indicating they are long-lived events that have both controlled the ingress and localization of mineralisation as well as potentially truncating or terminating mineralised zones. S1 is a minor contributor at the deposit-scale, but has appreciable influence at the orebody-scale.

Site Visit by Brett Davis