Geophysical evidence for a concealed eastern extension of the Tanygrisian microgranite and its possible relationship to mineralisation

A Bouguer anomaly low in the Blaenau Ffestiniog area is interpreted as being due to a concealed eastward extension of the Tanygrisiau microgranite. The geophysical evidence suggests that the granite, seen at outcrop at Tanygrisiau, 1 km south-west of Blaenau Ffestiniog, continues north-westwards at a shallow angle and that the general form of the intrusion is tabular, decreasing in width downwards The geological evidence supports this interpretation and also the existence of the extension to the east (suggested by the geophysical evidence). The magnetite-bearing granite is also thought to be responsible for a pronounced aeromagnetic anomaly which has a form supporting the gravity evidence for the eastward extension of the granite body but requires the extension of a magnetic body down to a depth of 15 km. The mineralisation in the area consists of sulphide-bearing quartz veins occupying mainly north-easterly trending faults. There are insufficient mineral occurrences to produce any clear correlation but the distribution of the veins seems to be mainly coincident with the southern flank of the concealed granite.