Stratabound barium and base-metal mineralisation in Middle Dalradian metasediments near Braemar, Scotland

Stratabound mineralisation comprising baryte, barium silicates, sphalerite, has been discovered in the upper part of the Ben Eagach Schist Formation This Formation forms part of the Middle Dalradian (late Precambrian metasedimentary and meta-igneous rocks. The new mineralisation occurs position as the Aberfeldy and Loch Lyon deposits, 45 km and 90 km galena and other sulphides 1 l- 13 km SSE of Braemar. to Cambrian) sequence of at the same stratigraphical SW respectively along the regional strike in the Grampian Highlands. One of the Aberfeldy deposits is presently being worked for baryte at Foss Mine by M-I Great Britain Ltd. Up to 10% Zn+Pb is present in Coire Loch Kander in quartzite which also contains hyalophane and armenite, the rare hydrated barium-calcium aluminosilicate. Bedded quartz-baryte rock is exposed in the headwaters of Allt an Loch, l-2 km south of Loch Kander where it is some 5 m thick. A barium anomaly in the overburden extends over 1.6 km along the strike of the bed. The mineralisation was found as a result of integrated geochemical-geophysical-geological investigations northeastwards along the presumed strike of the Ben Eagach Schist from the Glenshee district. Overburden sampling and geophysical (VLF-EM, magnetics, IP and SP in part) measurements were conducted along 40 km of across-strike lines running for 11 km from Glen Brighty in the south, across the mountains of Glas Mao1 and Cairn of Claise to the Allt an Loch district and Coire Loch Kander. Host-rocks are graphitic schists and quartzites regionally metamorphosed to amphibolite grade lying at or within a few tens of metres of the top of the Ben Eagach Schist against a thick, sill-like amphibolite body incorporated into the Ben Lawers Schist Formation. Sharp variation in the thickness of the Graphitic Schist Member from 0 m to 300 m may be partly attributable to folding but along-strike facies variation is probably of greater significance. Younger igneous rocks include a stock-like diorite body which has contact metamorphosed both the bedded sulphide mineralisation and a thin baryte-galena vein unaffected by the regional metamorphism. The geochemical and geophysical information provides an excellent guide to the bedrock geology which is very poorly exposed except in Coire Loch Kander. The zinc-lead sulphide enrichment in quartzite is accompanied by pyrite, actinolite and diopside as well as by armenite, hyalophane and traces of baryte, while the massive fine-grained quartz component of this rock is rich in fluid inclusions. The precursor assemblage may have been a hydrothermally altered sediment or a chemical exhalite. The sulphidic quartzite is interdigitated with 15 m of highly pyritic graphitic quartz-schist, regarded as a distal exhalative iron-sulphur concentration in carbonaceous mud, and banded talc-silicate schist. In contrast, the bedded, quartz-baryte rock occurs in a gossanous elastic quartzitic sequence containing very little graphitic schist 1 km to the south along strike.