Disseminated and epigenetic Pb–Zn mineralisation in Ordovician mudstone, Galloway

Abstract

Synopsis Significant base metal anomalies in drainage and overburden in the Penkiln Burn area, near Newton Stewart, are spatially associated with the outcrop of a sequence of siliceous mudstone, siltstone and chert. Pb is particularly enriched in drainage samples, reaching approximately 1% close to a mineralised gossan zone, mainly in the secondary lead phosphate plumbogummite. Three varieties of mineralisation have been recognised: Early fine disseminations, chiefly sphalerite and pyrite, in the mudstone and siltstone characterised by Zn levels between 500 and 1000 ppm. Fine stratiform pyrite laminae adjacent to the mineralised horizons and interbedded cherts containing disseminated sphalerite support a probable syngenetic, exhalative origin. A swarm of dioritic minor intrusions carry disseminated sulphides and have elevated metal contents, most notably Zn (up to 2000 ppm). This mineralisation is considered to have a primary igneous source. Mineralisation related to veining and alteration involving both the remobilisation of the disseminated sulphides and hydrothermal effects. Thin quartz veinlets containing sphalerite and galena locally elevate base metal levels so that Pb reaches 7000 ppm in places. The veins usually contain some secondary alteration minerals which become dominant at the altered margins of dykes and in fault-bounded gossans where a low temperature mineral assemblage includes plumbogummite and beudantite. In the gossans Pb ranges up to 4.5% but Zn has been preferentially leached.