Cupriferous bogs of the Coed y Brenin area, north Wales and their significance in mineral exploration

Abstract

A number of bog areas with richly cupriferous peats have been outlined during a geochemical soil survey investigating the distribution of Cu, Mo, Pb and Zn in the vicinity of the Coed y Brenin porphyry copper deposit. During the first half of the last century considerable amounts of peat were extracted from the bog known as “Turf Copper” and ash derived from burning this peat was smelted as a source of Cu. “Turf Copper” bog is situated immediately to the west of the main outcrop of porphyry-style mineralization in this area. In addition, three small Cu bogs were delineated in the area of Bryn Coch to the south of the main mineralized zone, where disseminated Cu mineralization outcrops. Waters draining into the bog areas are enriched in Cu up to 0.97 mg/l. Analysis of profile samples from the bogs has indicated extreme enrichment in Cu and variable enrichments in Mo, Pb, Zn, Mn, Co, Ag and Au. Mineralized intrusive rocks in this area are significantly enriched in Cu and Au. The enrichment of Cu, up to 5.72%, and Au, up to 0.13 ppm in ashed peat samples, reflects the mobilization of these elements from adjacent mineralization by ground and surface waters and their fixation by interaction with organic matter accumulated in the bogs. The recognition of cupriferous bogs and the sampling and analysis of peats, including analysis for Au, may provide useful indicators of Cu and Au mineralization, especially in glaciated areas where mineralization may be masked by transported overburden and geochemical anomalies displaced by hydromorphic agencies.