The results of a reconnaissance geochemical drainage Introduction
survey across an area of 850 km2 in the Cheviot Hills are
reported. At each of the 708 sample sites, the - 0.15 mm
fraction of the stream sediment, a panned heavy mineral
concentrate derived from the - 2 mm fraction of the
stream sediment and a water sample were collected. Be,
B, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Zr, MO, Sn, Ba,
Ph and Lo1 were determined in stream sediment samples;
Ti, Mn, Fe, Ni, Cu, Zn, As, Sr, Zr, MO, Sn, Sb, Ba, Ce,
Pb, Bi and U in panned concentrate samples and Cu, Zn
and Pb in water samples.
The survey area is composed largely of andesitic
volcanic rocks of Lower Devonian age. Underlying _
Silurian greywackes and shales and overlying Devonian
and Carboniferous sedimentary rocks and basaltic lavas
are exposed around the periphery of the volcanic complex.
The Cheviot granite crops out on the eastern margin
of the area.
Anomalous concentrations of metals recorded in the
drainage samples are, in general, related to (i) known
mineral occurrences, (ii) hydrous oxide precipitation and
scavenging processes, (iii) the relatively _ widespread
occurrence of baryte, (iv) contamination, (v) tourmalinisation
and other hydrothermal activity and (vi) high
background levels in unmineralised rocks. At several
localities, however, the source of an anomaly is uncertain
and some of these may reflect hitherto unknbwn mineralisation.
Anomaly groupings and regional variation patterns
in the data are influenced by major structures such
as the Gyle-Harthope fracture zone.
Follow-up investigations into a wide range of metal
anomalies in the the Kingsseat area involved more detailed
drainage sampling, geological mapping at 1: 10 000
scale and petrographic, mineralogical and lithogeochemical
studies. Rocks, previously mapped as extrusive
mica felsites, are reinterpreted as a high level intrusion
complex, named the Cock Law Complex, which contains
live distinct types of porphyry. Many of the intrusive and
extrusive rocks are highly altered and seven alteration
assemblages are identified, arising from sericitisation,
kaolinisation, silicification, tourmalinisation, haematisation
and carbonate alterations. All except haematisation
are related to Lower Devonian igneous activity. Metal
enrichments were recorded in many of the analysed rocks.
The greatest enrichments, for the widest range of
elements, occur in samples taken from a gossanous structure,
where the mineralisation has features in common
with the epithermal precious-metal style of mineralisation
associated with sub-aerial volcanism