IT Permotrias Z Carboniferous i Silurian Ordovician )A Ashgill X Caradoc Id Llandeilo , Scale I r . .: : :.:::::~:~:~.~~~>,:~ ~~~~: Basic intrusions ‘..+::.. ..:.:..: .,.,., --- Major faults Simplified geological boundary 0 3 4Km ,1T, .* 1 FIG.39: Contoured greyscale plot of lead in panned concentrate Contour values in ppm I I I I I 1 10 50 90 99 probabi I it y
trials for slate. Analysis of a quartz-veined siltstone from one of the workings showed no metal enrichment above average values recorded in siltstones, but a pyritiferous .shale from the same site contains high levels of Ni, As and Fe (Table 6, Nos. 1 and 2). Pyritiferous shales and abun-. dant float blocks of barren quartz were also noted in the catchment. This and other streams in the area contain anomalous or high levels of Co, V, Zn and Mn in sediment and, in the upper reaches, Zn,. The assemblage suggests that the anomalies are caused by hydrous oxide precipitation processes in combination with a high background generated by mudstones and perhaps tuffaceous horizons within the Mynydd-tarw Group. The _ -_ weak anomalies in panned concentrates are also ascribed to these causes with the addition of contamination sites. at some To the east of Llanarmon, anomalous levels of Cup, Pb,, and Sn, in streams draining Llangadwaladr ([SJ 185 3451, [SJ 176 3411, [SJ 169 335)) were shown by mineralogical study to be entirely the result of contamination. Weakly anomalous levels of Ba, at these sites (600- 750 ppm) and high but not anomalous levels of Cu, were attributed to the background geology. Although not examined mineralogically, contamination is also believed to be the most likely cause of anomalies for Sn, Cu and Zn in the panned concentrate collected south of the village at [SJ 157 3231, for upstream are not anomalous. samples taken above the roads 7 Anomalies in the Iwrch Valley. Anomalous levels of Cu,, Pb, and Zn, in samples collected near and downstream of Craig-y-glyn are related to old mine workings hereabouts. -4 brief examination of the sites indicated that quartzcarbonate veins with galena and sphalerite within limestones, calcareous siltstones, ashes and shales of the Llandeilian Craig-y-glyn Group have been exploited. Details of the mines are not known, and the workings are collapsed and at least partly grassed over. Exploitation appears to have been from a series of levels and shafts along the Iwrch Valley and the tributary by Pen y Graig farm, extending from [SJ 122 2901 in the north to [SJ 124 2841 in the south. Collapsed adit directions suggest that one vein trends SSW from near Pen y Craig farm, but otherwise the controlling structural directions and style of mineralisation remain in doubt. The presence of a gravity low in this area is an interesting feature. It may simply reflect the relative closeness to basement, but might also be construed as indicating the presence of an intrusion, such as a volcanic plug with which the volcanism and mineralisation might be associated. Examination of tip material suggests that sulphide mineralisation is weak, but analysis of a composite tip sample (Table 6, No. 3) inferred the presence of appreciable baryte (10 % Ba) in addition to minor galena (0.9% Pb) and sphalerite (0.3% Zn). With the exception of Pb, the panned concentrate anomalies are weak and the absence of a Ba, anomaly suggests that the majority of tip material is not entering the drainage system. The Ni, and some part of the Pb, anomalies are related to the high levels of Ce, and not to mineralisation. All samples taken in the Iwrch catchment contain high Ce,, Ni, and Pb,, related to the presence of monazite nodules and coatings. Several sites in the upper catchment also contain subanomalous levels of Zn,, Co, and Zn,. These are attributed to a high background generated by mudstones and ashes of the Mynydd-tarw Group enhanced by hydrous oxide precipitates. . 3191 contains anomalous Cus, Zn, and a very high level (0.12 ppm) of Cu,. The catchment is mapped as Mynydd-tarw Group and contains shales, ashy shales, ashes and sandstones (Wedd and others, 1929) cut by several northerly-trending faults. A brief examination of the site revealed abundant float quartz but repeat and ad- ditional water samples taken upstream failed to yield anomalous results. The cause of the majority of the anomalous metal values is probably the same as that thought to generate anomalies across the interfluve to the north, i.e. a combination of background geology and hydrous oxide precipitates, but the possibility of mineralisation cannot be eliminated at this stage as the cause of the Cu, anomaly in particular is unresolved. 8 Baryte south of Llandrillo. A panned concentrate from Nant Crechwyl [SJ 025 3 191 contains an isolated Ba, anomaly which mineralogical examination related to barium in shale fragments and rare, rounded grains of baryte. The site also produced weak Cu, (0.02 ppm) and Zn, (340 PP m ) anomalies. The source of the baryte is uncertain. It is possibly derived from fault-associated veins in the Swch Gorge Tuff Formation, similar to those discovered in Cwm Rhiwarth. 9 Cobalt and associated anomalies in the upPer Afon Rhaeadr. Sample sites in first and second order streams close to and above Tan-y-pistyll contain high or anomalous levels of Co, Fe, Mn, Cu, Pb, Zn and Ni in stream sediment and Cu and Zn in water. The sediment anomalies are at- tributed to a combination of hydrous oxide precipitates and a high background from the Llyn Lluncaws, and Cwm Dywyll basic intrusions. This conclusion is sup- ported by high or weakly anomalous levels of Ti, at these sites and an analysed rock sample from the margin of the Llyn Lluncaws intrusion, which contains relatively high levels of Fe (7.67%), Mn (0.14%), V (221 ppm) and Co (28 ppm) compared with most of the sedimentary rocks (Table 6, No. 4). At the two sites near Tan-y-pistyll, levels may also be enhanced by contributions from the volcanic rocks. Anomalous levels of Zn, were confirmed by re-sampling and are attributed to a combination of acid upland waters and a high available background. At [SJ 052 3021 an exceptional level of Cu, (0.12 ppm) was recorded, but resampling failed to reproduce the anomaly and the site is contaminated. High and anomalous levels of Ce, Ni, and Pb, are attributed to monazite coatings and nodules. Weakly anomalous levels of Zn, and perhaps some of the other anomalies such as Cu, and Pb, may have yet another source, for a brief visit to the catchment revealed the presence of quartz veining and pyrite apparently associated with both the volcanics and the basic intrusion. One of the quartz veins, at [SJ 057 3001, had an adit cut into it but no sulphide besides pyrite was seen. Some of the volcanics were quite strongly veined, possibly altered, and merit further investigations. 10 Anomalies associated with base-metal mining centred on Llangynog. Many of the anomalies in the Rhaeadr, Tanat and Hirnant valleys are caused by old base-metal mines. Generalised sites of these workings are shown on the anomaly maps. Sites known to be contaminated by old workings were avoided, wherever possible, whilst maintaining the sampling density, reducing the overall impact of the known mineralisation on the results. The old mine workings generate anomalies characterised by In addition to these anomalies, the site at [SJ 104 ’ high levels of Cu, Pb and Zn in concentrate and lower 43
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