Exploration for porphyry-style copper mineralisation near Llandeloy

siltstones is typical of all sedimentary rock in this area,
then an intrusion the size of the magnetic sources with a
density conlast with its host of -0.04 g cm-3 would give
a maximum anomaly of about -2 mGal. There is no such
anomaly. No density is available for the Pebidian, but
around St Davidb it does not produce a gravity anomaly,
implying that its density is close to that of the
surrounding rock. The absence of a gravity anomaly at
Llandeloy therefore supports the conclusion that the
broad magnetic high is caused by m anticlinal core of
Precambrian rock, but with the qualifications that the
densities of the Llandeloy intrusive rocks are themselves
quite variable, and that the measured density of
?Cambrian sedimentary rock is rather high to be
representative of other sedimentary rocks in the area.
The acid and intermediate rocks penetrated by the
boreholes appear to form a single concordant or semi-
concordant complex of thin sheets (Treffynnon complex)
exposed by a fault-bounded horst or anticlinal structure.
The intrusions were emphced into sedimentary rocks of
probable middle Cambrian age and acid volcanic rocks,
which may be of Precambrian or lower Ordovician age.
Whichever is correct the intrusions, which are not known
to penetrate Tetragraptus Shales or Brunel Beds, are
most likely to be co-magmetic with the early Arenig
Treffgarne andesites and may comprise a sub-volcanic
intrusion complex similar to that which hosts the Coed y
Brenin copper deposit in North Wales (Rice and Sharp,
1976). The Tref fynnon complex consists of two
chemically distinct groups of rocks: tonalites (5845%
SiO2) and diorites (