POTENTIAL FOR EPITHERMAL GOLD MINERALIZATION IN EAST AND CENTRAL SUTHERLAND, SCOTLAND - INDICATIONS FROM RIVER BRORA HEADWATERS

The eastern part of Sutherland in northern Scotland is well known for its alluvial gold localities, which provided the impetus for a gold ru sh in 1868. Deep pre-glacial weathering and Recent alluvial reworking were responsible for the final concentration of the gold in the rich p lacer deposits, but the original source of the gold has remained enigm atic despite sporadic exploration. Bedrock gold mineralization that ha s now been located in association with an extensive float train some 2 5 km to the west of the known alluvial gold provides a clue to the sou rce. Mineralization is dominantly in the form of open, matrix-supporte d breccias comprising pyritic quartz/sericite clasts in a vuggy cherty , chalcedonic and crystalline quartz matrix. Fluid inclusions in pyrit ic clasts within these breccias indicate trapping temperatures for gol d-mineralizing hydrothermal fluid of 170-240 degrees C. These textures and the indicated temperature of the hydrothermal fluid suggest that the mineralization represents the upper part of the feeder zone to an exhalative hydrothermal system. The proximity of a Lower Devonian outl ier unconformably overlying the Precambrian injection complex that hos ts the mineralization indicates that the Lower Devonian land surface i n the area is not far removed from the present erosion level. A direct analogy with the Rhynie Devonian hot-spring locality in Aberdeenshire is thus implied and the mineralization is taken to represent the feed er zone to an analogous epithermal system. Since such Lower Devonian o utliers are scattered over most of east and central Sutherland, the Lo wer Devonian land surface must be close to the present erosion level o ver a large part of the district. The possibility therefore exists tha t other high-level feeder zones to such hydrothermal systems could be preserved elsewhere in this terrain. Deep preglacial weathering of suc h a system could provide a source for the alluvial gold localities nea r Helmsdale. The deep weathering could well have removed the bedrock s ource of this particular alluvial gold completely, but the potential f or further bedrock gold occurrences in this terrain cannot be overlook ed.