Exploration for porphyry-style copper mineralisation near Llandeloy

There is some variation in the alteration of primary
amphibole. One rock from near the bottom of borehole 7
contains pseudomorphs after amphibole with lath-like
tremolite crystals among the other replacement minerals
(E 53300). In another (E 53301) the pseudomorphs consist
entirely of aggregates of small crystals of pale green
tremolite-actinolite.
The amount of sericite alteration of plagioclase, in
addition to clinozosite, is locally high in boreholes 2, 4
and 6. The associated amphibole may be only mildly
chloritised (E 53235, 53255) or replaced by chlorite and
epidote (E 53294). Pseudomorphs of these two minerals
replace magmatic biotite and there are veins of epidote,
chlorite, pyrite and minor quartz. This alteration is
typical of the late stage propylitic assemblage, but in
these rocks the plagioclase has not been albitized. It is
considered possible therefore that a mild version of the
late stage propylitic alteration might have been imposed
on the earlier event here.
Late-stage propylitic alteration The full assemblage of
minerals in this alteration includes albite, sericite,
chlorite, epidote (and clinozoisi te), magnetite, pyrite and
chalcopyrite. It is the dominant alteration in the area
and appears to have been imposed on all others. The
alteration is pervasive, but there is a widespread
development of veins with it.
Typically plagioclase is altered to albite and sericite,
the latter varying from a light peppering to a dense mass
totally replacing plagioclase. Muscovite is commonly
present as well. In some rocks a rim of clear albite may
be preserved around a mica pseudomorph and it is not
unusual for original zoning to be represented by zones of
alteration minerals. Minor chlorite and clinozoisi te may
be present with the sericite and in some rocks chlorite
forms a partial rim around sericitised feldspar.
Original amphibole phenocrysts are most commonly
replaced by pale green chlorite, usually with magnetite
and less commonly with sericite. Primary biotite is
replaced by chlorite with lenticles of sphene or ilmenite
or magnetite.
Veinlets of chlorite and chlorite with sericite,
commonly with a little pyrite and magnetite, are
widespread. Larger veins of quartz, albite, epidote,
minor chlorite and pyrite are common as are veins of
qumtz alone (up to 58 cm thick) or quartz with pyrite
and chalcopyrite. Disseminated and veinlet pyrite, alone
or with sericite and epidote, in places with chalcopyrite,
is ubiquitous. Pyrite is com monly concentrated in
pseudomorphs after amphibole. Magnetite is present in
veins with pyrite and disseminated.
This alteration is both prograde and retrograde in its
effects. Quartz diorite at the bottom of borehole 4
contains an alteration assemblage characteristic of
primary propylitic alteration in its upper part and late-
stage propylitic below. In the latter rock there is no
textural or other evidence of an earlier higher
temperature alteration assemblage. This is also true of
the volcanic rocks in borehole 6 in which there is
abundant sericite, in places with only minor chlorite,
pervading the rock and totally replacing feldspar
crystals. The rocks are riddled with veins and veinlets of
quartz, some of which contain opaque dust. Veinlets of
chlorite or quartz-chlorite are uncommon. Disseminated
cubes of pyrite are rarely present. Pyrite also occurs
sparsely in veins and in a network fracture system in
borehole 1. The alteration assemblage in the volcanic
rocks has been determined largely by their original acid
composition.
Evidence of earlier biotite alteration is present in
borehole 7 where the quartz-diorite at around 22 m
depth contains plentiful green or brown chlorite (E
53297) in diffuse veins, patches and forming a close
network, all of which appears to be after secondary
biotite. In other rocks, for example in borehole 5, pale
brown and green chlorite in pseudomorphs after
amphibole also appears to be after secondary biotite.
In borehole 1 thin veinlets of quartz and K-feldspar
occur in rocks with propylitic alteration suggesting
earlier potassic alteration. In borehole 4 there is
chloritised pale green amphibole with relict cores of
bro wnish-green hornblende and abundant c hlor i tised
secondary biotite. In one of the rocks containing
unaltered secondary biotite (E 53233) a vein of epidote,
chlorite and quartz has a chlorite envelope where it
crosses a biotite pseudomorph after an amphibole
phenocryst, providing clear evidence of the retrograde
effect of this alteration.
Rocks least effected by the retrograde propylitic
alteration are the sedimentary and intrusive rocks in
borehole 8. It is reflected in the intrusions, but the
effect, particularly the alteration of phgioclase, is mild
and the feldspar does not appear to have been albitised.
In the sedimentary rocks the late-stage alteration is
most evident in zones of brecciation and fracturing
within which there is abundant veining, wall rock
alteration and *hide mineralisation. Most common is a
fine network of chlorite and chloritequar tz veinlets
with pyrite. Veins of quartz-aibite-pyrite are less
common. The quartz in them may be strained and such
veins are intersected by others carrying unstrained
quartz. The wallrock adjacent to these early veins is
sericitised locally. Sulphides may be evenly disseminated
or form a network of irregular trains of euhedral to
subhedral crystals throughout the rock, in places
associated with some chlorite erbichment. Pyrite also
occurs in quartz-chlorite veidets and in veins
intersecting them.
The quartz-chlorite-sericite-pyrite assemblage also
dominates the breccias. These minerals fill the voids in
the intrusive breccias and comprise the veins in the
"cracklen breccia. Pyrite usually concentrates at the rim
of breccia fragments. Sericitisation of fragments is
locally intense.
Epidote-bearing veins, which may be intersected by
chlorite-sulphide veins, are characteristic of the late-
stage propylitic alteration and it is not likely that
epidotisation is a separate event. There are, however,
many places in both intrusive and sedimentary rocks
where epidote veining is intense and adjacent wallrock is
patchily or totally replaced by yellow epidote. Indeed,
throughout the rocks affected by the late-stage event
yellow epidote is confined to parts of the rock near
epidote veins. Specular hematite occurs locally with
epidote in epidotised breccias.
Carbonate alteration Veins of carbonate are
consistently the last event recorded in these rocks. They
are widespread and associated with patchy replacement
of the host rock; carbonate, therefore, occurs as an
additional mineral in most types of pseudomorph. It is
present additionally in veins of all types and commonly
shows replacement texture with other minerals in the
veins.
Mineralisa tion
Because of the widespread effect of the late-stage
propylitic alteration it is difficult to determine the
extent of earlier phases of sulphide mineralistion. The
main sulphide minerals identified are pyrite and
chalcopyrite and both occur in veins with epidote, quartz
and other minerals associated with the late event. It is
likely, therefore, that this is a mineralising event, but
geochemical evidence (see later) shows that rocks little
effected by the retrograde alteration are copper rich and
an earlier m ineralisa tion, probably during the potassic
alteration, seems likely.
Pyrite is the most abundant sulphide reaching levels of
up to 4% in borehole 2. It is more or less altered to
heaatite within the weathered zone. The pyrite is evenly
or patchily disseminated, in places forming cubes up to
1 cm wide, and it occurs in veins, veinlets and
concentrated in network fracture systems. Veins of solid