Autumn 2010 part 3 - A
Geological Interlude - Indian Summer at Coed Y Brenin and Parys
Mountain....
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Finally,
some very welcome work came along in mid-October!
The first job was a 3-day field trip to North Wales with third-year
geology students from Cardiff University. A large group indeed - 42
students, three postgrad demonstrators, the leader, Prof. Pearce,
myself and my colleague Tom from the National Museum. Our group visited
ten different ore-deposits over a three-day period, so a lot was
crammed in, but I'll concentrate on the photogenic highlights.
Day one saw us meet in Coed y Brenin to the north of Dolgellau, home to
the largest copper deposit in the UK, discovered by Riofinex in the
late 1960s. The idea was to study the various types of mineralisation
and alteration associated with the copper deposit. The students were split into groups
whilst we drove around meeting them at key exposures, one of which is
at one of the highest points in the forest. On a clear day this is an
excellent viewpoint but on this occasion the light was fantastic and I
excused myself for a few minutes' photography. The first shot shows the
road along this high hill, Moel Hafod Owen, with Cadair Idris forming a
magnificent backdrop:
From a little further along, looking down the valley of Afon Wen, on
down the Mawddach valley below the confluence of the two rivers to
Cadair Idris' summit with Cyfrwy on the RHS. The light was ethereal:
And a shot directly towards Cyfrwy, with Foel
Ispri and Cefndeuddwr in the foreground:
The reason for
dragging the students up to this hillside is the rock-exposure
illustrated below:
And in close-up:
This is a pipelike structure, within which the local sedimentary rocks
- mudstones belonging to the Upper Cambrian Ffestiniog Flags Formation
- have been shattered, cemented and partially replaced by a mix of
silica and pyrite. Other sulphides are also present but are too
finely-grained to see except with a microscope. The pipe is believed to
be the roots of an ancient fumarole (geyser) system, formed when
groundwaters explosively reacted with magma deep underground, during
the volcanic activity that took place hereabouts at the end of the
Cambrian and beginning of the Ordovician periods. That makes it getting
on for 500 million years old. The site is a SSSI for its fascinating
geology, which has been covered by a PhD thesis but not in
mineralogical detail by any work published in the peer-reviewed
literature - it deserves that coverage!
The
following day dawned cloudy but brighter conditions were forecast for
the afternoon. We piled onto the huge coach and had an interesting
journey up into Snowdonia, the excellent driver doing an impressive job
of negotiating some awkwardly narrow bridges. After spending the
morning at the Sygun copper mine near Beddgelert, we headed north to
Anglesey and the famous copper-mines of Parys Mountain.
Mining at Parys dates back to the Bronze Age but the heyday of mining
was in the late 1700s, when the site was as important as any in Europe.
Geologically, it is classified as a Volcanogenic Massive Sulphide (VMS)
deposit - meaning that the ore-minerals were deposited by
"black-smokers" on an ancient sea-bed during a period of volcanic
activity. The mineralisation consists of quartz with abundant sulphide
minerals: two types of ore were worked - "yellow ore" being a mixture
of pyrite and chalcopyrite (the source of copper) and "blue ore" being
a mixture dominated by sphalerite (zinc ore) and galena (lead ore).
Also present are arsenic, antimony, bismuth, silver and gold. A
considerable amount of unworked mineralisation has been found to the
west of the existing workings by the company Anglesey Mining, who have
drilled boreholes and driven new underground workings to explore this
new ground.
For the geologist, there is plenty of interest here, but for the
photographer it presents a rather unique experience in the right light
- bright sunshine is best as it allows the vivid colours to stand out.
Although a bit hazy on this occasion, the colours were not bad at all:
These are all images of the Great Opencast, dating back over 200 years.
The amazing colours are due to various oxides of iron, formed by the
weathering of the abundant pyrite. Some of the weathering post-dates
the mining but some pre-dates it: the deposit was originally covered by
a thick "gossan" - a hard mixture of rock-fragments, quartz and the
iron oxide goethite. In the gossan, likely formed over the past few
tens of millions of years, there also occurred the mineral Anglesite -
named after Anglesey, a sulphate of lead and one of the few other
minerals able to withstand the harshly acidic environment.
Close to this point (above), a viewing-platform has been erected, but
there are many spectacular points for the photographer. The three shots
below are from the RHS of the opencast:
No wonder that
the site has been used for location filming for things that need an
"extraterrestrial" look to them, such as Dr Who!
That night the weather went downhill but the following day saw the Coed
y Brenin teaching wrapped up and with a final visit to a zinc mine near
Aberystwyth the job was completed.
The following week saw a day trip to Taffs Well just north of Cardiff
to help the Countryside Council for Wales assess a couple of working
quarries for their mineralogical importance as potential SSSIs. With
working quarries, one cannot just go in and shut them down because
their work has uncovered something interesting - that would be
ridiculous! But, when a quarry closes for the last time, it can be
reassessed to see if anything of interest is on show and - if it is and
safe access can be provided, then features may potentially be preserved.
While a quarry is working, everything can be expected to get crushed
into aggregate. The best policy during that phase is to collect
specimens of anything of interest and preserve them for posterity -
which is exactly what the National Museum in nearby Cardiff does. The
Museum has pretty much everything from these quarries now, so when I
found a good cavity in a boulder I was able to collect for myself:
The image above is post-collection - the crystals deeper in the pocket
would be difficult to extract owing to the very hard nature of the
dolomitised limestone matrix. The specimen is illustrated below: the
main crystal is 55 mm long and undamaged. I don't do a lot of mineral
collecting these days but couldn't resist this one!
Other, larger cavities are often filled with dark brown clay: these are
also lined with crystals, as in the image below, but on cleaning they
are found to be badly etched by groundwaters and nothing more than
"garden rocks".
Some even
bigger, multi-metre sized cavities were visible in the quarry walls,
filled with huge crystals to over 20cm in length. Although impressive
in size terms, they are generally, in the words of the mineralogist
F.J. North, writing in 1916, to be "of such a nature as to be looked at
rather than collected". He was right then and is right now!
The bigger of the two quarries is Taff's Well itself, which has been
worked for decades. It has undergone some major modifications in recent
years. Formerly, the vehicle access and the crushing/classifying plant
was all located at the top of the quarry's western side. In recent
years that has changed: an access tunnel has been constructed, bringing
vehicles out to a roundabout a fraction of a mile from the main A470
(below):
Here's an overview of Taff's Well Quarry - the main working area is out
of sight to the R:
With Cardiff, the Bristol Channel and the
Mendips as a backdrop:
Previously, the rock had to be hauled up out of the quarry to the
plant. Now, it is hauled down to the primary and secondary crushers,
linked by conveyor:
And thence to the brand new
classifying/loading plant:
And off into the wider world via the tunnel:
Chatting to the quarry management, I was impressed by the energy
savings this has achieved. The previous crushing plant was old and worn
out, so it needed replacing: having the quarry's fleet of huge dumpers
driving uphill empty and taking their loads downhill has brought a fuel
saving of 75%!
That's a fantastic achievement, and the tunnel means that the lorries
taking aggregate from the site no longer have to negotiate the rather
narrow lane from Tongwynlais to Pentyrch - an area also busy with
residential traffic. This is a pragmatic approach that has solved
several problems all in one fell swoop.
On the weather-front, things have been pretty quiet: the first Autumn
frosts occurred during the North Wales field trip mid-October, and
there have been a few since, but as of a couple of days ago, a mild
moist sou-westerly regime took over and looks to continue as we move
into November. But it remains a remarkably storm-free year to date - at
least here in the UK - other parts of the world have had more than
their fair share!
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