|On old metal-mines,
minerals and the tropical climate of 50 million years ago.....
& Dyfi Valley landscapes Slide-Library - Click HERE
my old stamping-ground, where I first started collecting
minerals as an Aberystwyth geology undergraduate in the early 1980s and
then went on to do an MPhil on the mineralogy of the old metal-mines
that are scattered through the hills and valleys of Mid-Wales. Back
then, many of the mines were still recognisable as such, although in
the intervening years there have been many land reclamation-schemes
that have changed things. However, up in the mountains there are still
plenty of mines tucked-away in remote valleys or hidden away in
forestry plantations. They are peaceful spots where the long labours of
yesteryear may be contemplated on a quiet evening......
Mining commenced in Mid-Wales during the Bronze-Age, when copper was
primarily sought in shallow surface workings. Later, the Romans mined
and smelted lead in places, whilst the 1600s saw great activity and
wealth-generation from the high silver-contents of some of the
lead-ores. The 1800s marked the heyday of the industry, which dwindled
away during the 1900s with the development of much bigger mines
overseas, and today all that is left are these silent remains.
As the image above shows, they can make striking, almost abstract
subjects for photography in the right lighting-conditions.
The mines all worked lodes. A lode is a fracture in the rocks, ranging
in width from a few inches to tens of feet and filled
with bits of smashed-up rock cemented together with minerals: the
filling is often termed "mineralised breccia". A breccia (pronounced
"brechia") is simply a mass of rock fragments cemented back together.
The image below shows an example - it's a cut and polished slab about a
foot across. Grey pieces of mudstone are cemented by zinc-blende (zinc
sulphide - brown) and quartz (white). There would have been quite an
earthquake when this lot was smashed up....
Here's an example, again about a foot across, of shattered grey banded
mudstone, cemented by the lead sulphide, galena (black) with iron-rich
dolomite (yellow) and quartz (white). This is low-grade ore: the stuff
that made the area famous consisted of solid galena, carrying 30-40
ounces of silver to the ton, in veins 1-3 feet thick or even more on
occasion. Not all mines revealed such riches and a lot of money was
lost, as well as made, at times!
Unsurprisingly, it is unusual to find boulders of high-grade ore on the
old tips of the mines, but samples representative of the ore-deposits
can still be collected for study. After I graduated in 1985, I spent
about a year wandering the hills and collecting samples and eventually
it was suggested that I did a postgraduate research-degree on the area.
The chance to use the facilities of the Geology Department at
Aberystwyth was jumped-at. Here were diamond-saws of all sizes,
grinding wheels, a polishing-machine (in bits - I rebuilt it), an X-Ray
diffractometer and a host of other useful
bits of kit. Paradise for an up-and-coming researcher with a hands-on
approach, in other words!
thing I had started noticing as I hammered my way around the old
mine-tips was that there were a number of minerals that I was
unfamiliar with and it rapidly dawned on me that previous studies of
the area had missed many interesting things. The sample below shows
golden-yellow chalcopyrite - a common ore of copper - but with it is a
fine-grained pinkish metallic mineral. I found enough of this to be
able to prepare samples for X-ray diffraction - these days a tiny speck
will do but back then about a gram was needed! The mineral proved to be
an uncommon sulphide of nickel and cobalt called siegenite.
rather fine-grained mixed minerals like this, the next step was ore
microscopy. Geologists look at the microscopic characteristics of rocks
using thin sections (below, left). But most ore minerals are opaque to
transmitted light, so that method is no good. What we do instead is to
embed a chip of ore in liquid resin. When it has cooked-off, the front
of the block is ground flat then painstakingly polished on laps in a
squeaky-clean environment with decreasing grades of diamond paste. The
final polishing is done with a diamond paste with no diamond bigger
than half a micron in size! A finished section - the research involved
making over 300 of these - is illustrated below right. Once polished
like this, the ore can be examined using a reflected light microscope.
a polished section, as seen down the microscope. From Eaglebrook mine -
the one in the photo at the top of the page - this is a particularly
interesting sample. The common ore-minerals galena (G), chalcopyrite
(C) and pyrite (P) are accompanied by bright yellow electrum (E) - a
gold-silver alloy and very rare in this part of Wales. It's about 0.1mm
across so this is high-magnification! The
buff-coloured mineral (T) is the most noteworthy, though. To identify
it, I had to take samples to the laboratories of the British Geological
Survey near Nottingham, where technical staff helped me to analyse it
with an electron microprobe and a sophisticated X-Ray diffractometer.
The mineral turned out to be the extremely rare nickel-antimony
sulphide tucekite - the first UK occurrence and only the third
occurrence anywhere in the world at the time.
By the time the research was completed, the thesis submitted and papers
prepared for the peer-reviewed literature (references 1 and 2), a lot
of minerals new to the area had been discovered and the sequence of
events leading to the formation of the primary ore mineralisation had
been unravelled to a great extent. Later, the British Geological Survey
and I worked on isotopic dating using the lead isotopes in the galena.
We found that there had been episodes of mineralisation beginning in
the Middle Devonian Period (~390 million years ago), in the early
Carboniferous Period (360-330 million years ago) and the Permian and
Triassic Periods (250-225 million years ago).
the same time, the old mines had drawn a steady stream of visitors
interested in the area's secondary minerals. Secondary minerals form
when the primary metal sulphides in a mineral deposit, such as a lode,
react with rain and ground-water. Such waters are slightly acidic
already - they contain dissolved carbon dioxide - and when they react
with sulphides the sulphur is mobilised as even more reactive sulphuric
acid, which can also attack the rocks surrounding a lode, releasing
things like phosphorus, silica and so on to the system. The metals go
into solution and are then redeposited as colourful secondary minerals
- for example, carbonates, sulphates, oxides and phosphates in various
The example below is well-crystallised pyromorphite, a chloro-phosphate
of lead, for which the area is famous.
well-crystallised specimens of secondary minerals were never that
common, though, and with increased availability of affordable binocular
microscopes, many collectors turned their attention to unusual and rare
formed microcrystals of 1mm or less in size. As a consequence, a host
of rare minerals was again recorded: the image below shows crystals of
the very rare copper-zinc sulphate ramsbeckite, from a chance find that
I made in 1992 (reference 3). Through the 1990s, discovery after
discovery occurred, so that when, in 2004-5, we updated the Mineralogy
of Wales in website format (reference 4; the original book was
published in 1994),
there was plenty of stuff to add!
like the ramsbeckite below reveal thin crystalline coatings involving
fractions of a gram of mineral. I started to get interested in finding
out why, at certain mines, there was much more secondary mineralisation
than at others. We knew that minerals like ramsbeckite formed over the
few centuries since sulphide-bearing rocks were brought to surface and
dumped in mine-tips. Why, at some mines, were there just thin coatings
of secondary minerals, whilst at others, they occurred in big, solid
masses? This question started off
another line of research.
sample below, the size of a really large fist, is just such an example
of a big, solid mass of secondary minerals.
Here we have brown iron oxides with lots of pale blue chrysocolla
(copper silicate) and green malachite (copper carbonate). Material like
in abundance in a wide lode at the bottom of one valley - yet just four
away, at the
entrance to a shaft in Cwm Rheidol, unweathered sulphides, including
the incredibly unstable mineral marcasite, were present at surface.
What was going on?
Another thing I
started to record on a regular basis was the coincidence of
of secondary minerals with bleached mudstone. In the
image below, iron oxide contains large amounts of the white lead
carbonate, cerussite. At the bottom right is a 2-inch fragment of
mudstone, but it is not the drab grey colour of the rocks around here -
it has a much paler, bleached appearance.
Below is a
typical outcrop of the local grey mudstones....
Here is a part-bleached
chunk from a mine-tip: the bleaching has worked its way into the rock
And here is a fully-bleached example - from
dark grey to shades of pinkish-buff.
some places, the bleached rock was accompanied by manganese oxides that
were mined where they were present in quantity. The photo below is the
hill called Drosgol, looking across the Nant-y-moch reservoir. There
are tips just above the small beach across the lake, beyond which a
prominent gully heads off uphill. The gully marks the course of one of
the main lodes of the area and just over the skyline, 450-500 metres
above sea-level, there are collapsed tunnels and pits where the
manganese was dug....
this is the ore (below) together with pinkish, bleached mudstone. The
ore mineral is mostly chalcophanite - an oxide not only of manganese
but also of zinc - which diluted the manganese grade, as a consequence
of which the operations were not successful.
Looking further afield, the same thing kept on cropping up. At various
scattered locations across Snowdonia, for example, there are similarly
pale, bleached rocks
accompanied by manganese oxides.
The black mineral in this image is
hollandite - a barium manganese oxide, veining volcanic tuff
(that should normally have been a greeny-grey colour), from the
Arenigs, between Bala and Trawsfynydd, where again, manganese was mined
in the 1800s.
So: in some places, totally bleached rocks
with masses of secondary oxides, but
in others, fresh, unweathered rocks and even fresh, unweathered
sulphides outcropping at surface.
This image shows just such an occurrence: it
is at Ceulan mine in the NE of the mining district. Just a metre below
the original ground surface, this is a pillar left in a small opencast
working. The grey mudstone carries a lode with quartz (white) and the
lead sulphide galena (bluey-grey, most obvious towards the bottom of
the image). The only weathering it displays is a slight tarnish.
Compare that to some of the images just above!
The fact that such fresh, unweathered sulphides outcrop at surface in
places can only mean one thing: they have been exposed at the surface
recently (geologically speaking), for the pervasive weathering that
accompanies the manganese oxides can take millions of years to
To expose unweathered sulphides at surface, we need a powerful agent of
mechanical erosion to remove most of the weathered rocks above, and one
was last active only a few thousand years ago: and indeed we have just
such a candidate - the glaciers of the last ice-age.
The last glacial maximum was only 18,000 years ago (a blink in terms of
geological time). The Welsh mountains had ice-caps hundreds of metres
thick and glaciers flowed outwards from them, carving out the valleys.
Under the centres of the ice-caps, ice-movement was minimal, but
countless billions of tonnes of rock were removed by the mechanical
erosion of the ice along their flanks. It seemed reasonable to cite
this erosional activity as responsible for stripping away the upper,
weathered parts of lodes in many places to the point that fresh
sulphides were once again exposed at surface, a point that I made in a
paper published in 2004 (reference 5).
But what about the
weathering itself? Deep weathering, that pervasively turns dark grey
mudstones into yellows and pinks, has certainly not occurred in the
cool temperate climate we have had since the last ice-age - the rocks
are just as grey now as they were when the ice retreated!
Such weathering is more normally found in the tropics, where it
penetrates hundreds of feet down into the rocks. So you need the
environment to be one of dry land, with a tropical climate. When did
Wales last see such conditions?
Palaeogeography involves taking numerous lines of evidence to
reconstruct how the planet would have looked in the past. Now, we know
that the primary mineral deposits were forming into the Triassic
Period, so we have narrowed down the field, between when they formed
and when the glaciers stripped away a lot of the weathered material,
just leaving a few remnants here and there,
Wales was at least partly land during the Jurassic Period and the
uplands may even have escaped the almost total inundation of the Upper
Cretaceous sea from which the Chalk was formed. Here's how things
looked back then (globes - see reference 6):
By the following Cenozoic
era, which began 65 million years ago, the continents were drifting
towards their present position, although the Atlantic was much narrower
than it is today:
Here's a sketch-map of Eocene Britain. The sea covers the eastern half
of the country - this is when the marine London Clay was deposited -
but western areas were emergent throughout the time. Now, this time,
beginning about 65 million years ago, is a candidate for the period of
deep-weathering. So what was the climate like?
It was indeed tropical
- and not just in the UK either: it was pretty warm well to the north,
too. There are numerous lines of evidence for this, one compelling one
being what the fossil record shows. Here's an example of some recent
research findings, compiled from Reference 7:
Island - which is adjacent to Greenland - is one of the coldest,
driest environments on Earth and features tundra, permafrost, ice
sheets, sparse vegetation and few mammals. The temperatures range from
roughly minus 37 degrees F in winter (minus 38 C) to 48 degrees F (8
degrees C) in summer. During the Eocene, Ellesmere Island was probably
similar in nature and climate to swampy cypress forests in the
southeastern United States today. Eocene fossil evidence collected
there in recent decades by various teams indicate the lush landscape
hosted giant tortoises, aquatic turtles, large snakes, alligators,
flying lemurs, tapirs, and hippo-like and rhino-like mammals. The
average temperatures of the warmest month on Ellesmere Island during
the early Eocene were from 66 to 68 degrees Fahrenheit (19-20 degrees
C), while the coldest month temperature was about 32 to 38 degrees F
(0-3.5 degrees C).
The diagram below plots the fluctuations of climate through the whole
The diagram shows how warm conditions persisted right through the
Eocene, only falling off sharply at its boundary with the succeeding
Oligocene (Ol). Really cool conditions did not come along until the
late Miocene, when the plunge downwards towards the ice-ages and their
glacial-interglacial cycles commenced.
Truly tropical conditions (reference 8) are today found close to the
equator, where temperatures rarely exceed 35 degrees C by day and
rarely fall below 22 degrees C by night. Well, at a time when Ellesmere
Island way up in the north was seeing 19-20 degrees C in summer, it
seems quite possible that what is now the UK could have had
temperatures at least within the lower levels of the tropical span of
22-35 degrees C during at least parts of the Palaeocene and Eocene, so
that tropical and sub-tropical weathering took place. Reference 9
covers another of many cited cases of such deep, tropical weathering
nearby South-West England - it's just another of very many examples.
The old metal mines of Central Wales and elsewhere are of great
interest to the historians of industry, but the minerals that their
spoil-heaps contain provide an extremely important Earth Sciences
most important old mines of Central Wales are now protected as Sites of
Special Scientific Interest, to prevent them from being levelled out of
existence, as has happened to other sites in the past. Reference 10 is
comprehensive guide to the mineralogical SSSIs of Central Wales and
elsewhere, and extensive sample suites from the mines (including my
thesis collection) are preserved at the National Museum of Wales in
The story told
by the minerals in this post is just one of many themes: others include
evolution through time of crustal, fluid and tectonic processes, how
metals are mobilised and locked-up again in the natural environment and
what concentrates metals into deposits that may economically be
mined. But who would have thought that they might tell us about
climates of the past, too?
J.S. 1997. Regional polyphase and polymetallic vein mineralisation in
the Caledonides of the Central Wales Orefield. Transactions of the
Institution of Mining and Metallurgy (Section B: Applied Earth
Science), 106, B135-B144.
J.S. 1998. Tucekite, a mineral new to Britain, and other rare ore
minerals from the Central Wales Orefield. U.K. Journal of Mines and
Minerals, 19, 30-36.
J.S., and Green, D.I. 1995. Supergene minerals including exceptional
ramsbeckite from Penrhiw Mine, Ystumtuen, Dyfed. U.K. Journal of Mines
& Minerals, 15, 21-27.
Mineralogy of Wales: http://www.museumwales.ac.uk/en/791/
J.S. 2004. The development and preservation of supergene lead
mineralisation in Central Wales. UK Journal of Mines and Minerals, 24,
K.P. 1983: Tertiary lateritic weathering in Devon, England, and the
Palaeogene continental environment of South West England. Proceedings
of the Geologists' Association,
Volume 94, Issue 2, 105-114;
R.E., Young, B., Mason, J.S., Manning, D.A.C. and Symes, R.F. (2010)
References and further reading may be available for this article. To
view references and further reading you must purchase
England and Wales. Geological Conservation Review Series, No. 36, Joint
Nature Conservation Committee, Peterborough.
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