'Institute of Geological Sciences of the National Academy of Sciences of Ukraine'
Abstract
Exploration for volcanogenic sulphide mineralisation
around Benglog is one of three investigations
designed to assess the metallogenic potential
of the Ordovician Aran Volcanic Group.
Detailed geological mapping in the Benglog
area enabled an interpretation of the volcanic
environment, critical to such an assessment, to be
made. The eruptive rocks are acid and basic in
composition; the acid rocks are mostly ash-flow
tuffs derived from outside the area, whereas the
basic rocks have a local derivation. They are all
interbedded with dark grey or black silty mudstone
and were probably erupted in a submarine
environment. Contemporaneous dolerite sills were
intruded into wet sediment.
This environment was suitable for volcanogenic
exhalative sulphide deposits to form and indications
of a metallogenic horizon were found at the top
of the Y Fron Formation in the form of abundant
pyrite, minor pyrrhotite and minor base metal
enrichment.
Soil samples, analysed for copper, lead and
zinc, were collected and geophysical surveys were
carried out along eleven east-west trending traverse
lines 300 m apart across the volcanic succession.
Indications were found of minor vein mineralisation
at dolerite intrusion margins and locally along
faults. Very high chargeability and low resistivity
anomalies over mudstones did not spatially
coincide with geochemical anomalies in soil, but
the secondary redistribution of metals in soils and
variable thickness of overburden precluded
confident interpretation of the source of many
soil anomalies. Geochemical drainage data, in
conjunction with rock analyses, show strong
barium enrichment in mudstones which could be
volcanogenic in origin but related to two separate
eruptive episodes.
The findings of the survey were inconclusive.
An environment suitable for the formation of
volcanogenic exhalative sulphide deposits was
established, but the geochemical and geophysical
surveys located only minor vein mineralisation and
tenuous indications of other styles of mineralisation.
Recommendations are made for further work