Tara Mine

 geology  4 Responses »
Sep 162009
 

Tara Mine, near Navan, Co. Meath, Ireland is the largest zinc mine in Europe. It is now owned by the Swedish based Boliden Group and they generously allowed a group of us geophysicists down the mine and around the rock crushing mill following the EAGE’s Near Surface 2009 conference last week. I’ll have a second post on the processing plant later but this post concentrates on the mine visit.

Here is what they are after – the grey mineral on the left is galena (lead ore) the brown one on the right is sphalerite (zinc ore).
lead and zinc ore

The ore is hosted as a number of lenses in Dinantian (Lower Carboniferous) limestone. The ore formation must have started very early as one of the units is a submarine debris flow unit with already mineralised sulphide clasts. The ore body reaches the surface near the town of Navan and was originally detected by a soil geochemistry survey in the 1960s by Rio-Tinto Zinc. However, RTZ management failed to follow up on their geologists’ recommendation to drill the geochemistry anomaly and they surrendered the exploration licence and walked away from what is now the fifth largest zinc mine in the world.

The exploration licence was taken up in 1969 by the Tara company who did a bit more geochemistry and some geophysics followed up by a test drill hole. With the luck of the Irish, they hit the ore body – but by just 50m from its edge and if their hole had been vertical rather than inclined, they would have missed it. But hitting a 12m zone of 6.4% zinc and 2.5% lead gave their backers the reasons to pay for more drilling and the mine started production in 1977.

The orebody dips moderately near the surface and then flattens out with the deepest part of the workings now 950m below the surface at the southwest end. Two adits from the surface follow the orebody downwards and own visit was by landrover simply driving into the mine.

First it was necessary to kit up in protective clothing.
Suited and booted

The journey underground took us to one of the drill faces. The lighter coloured rock is one of the ore lenses, the lines indicate the driller to drill the next shot holes, with the shot holes from the previous blasting round circled for the driller to avoid in case there is any unexploded charge remaining.
Drill face

After blasting the loose rock is removed by large excavator which is remote controlled.
remote controlled excavator

Here we have a diamond drill further in the mine drilling core samples in the roof rock.
Diamond drill

Finally, for now, here is a borehole draining water from an upper level.
Drain hole

Wrong Rock II

 Staffordshire  1 Response »
Sep 052009
 

I’ve just finished preparing a presentation I’m giving at the 15th European Meeting of Environmental and Engineering Geophysics of the Near Surface Geoscience Division of EAGE (or Near Surface 2009). I’m so used to talking for a micro-century (~50 minutes) in lectures, it is hard to discipline one’s self to a 15 minute slot.

But, before I head of to Dublin, there is just time for the latest instalment of “Wrong Rock“. Last Time I discussed a church that had a new extension being built in a rock type that didn’t match the original. I also noted that the disused quarry that had supplied the original building was less that a kilometre away and it would have been much better from both an aesthetic and ‘stone miles’ point of view if a way could have been found to re-open that quarry to supply the stone for the church. This week I have uncovered what I consider to be an even bigger crime against stone.

Another Staffordshire Church

Like most old churches it was originally built from the local rock, in this case the Lower Triassic Kidderminster Formation (in old nomenclature Bunter Pebble Beds) which as the old name suggests is a coarse, pebbly sandstone. Again, the original quarry is close by, less than 500 metres from the church. This time it isn’t the modern extension (which is round the back) that I’m railing against, but the repair work on the original. These repairs have been done with Triassic Hollington Stone (formerly Lower Keuper Sandstone) which I’ve discussed before. It might not look too bad from a distance, but close up it really clashes.

Repair work

What is worse, is the nature of the repair. Rather than being flush with the existing stones, the steps on the top of the replaced blocks will collect water and cause the old stone block above it to weather even faster. And this is on a Grade One Listed Building, “a building of outstanding architectural or historic interest”. Special permission is required for repairs and particular materials are usually specified (although churches with current worship do have some exemptions). How much better the repairs would have been if the local stone had been specified and the replacement stone fitted flush with the original.

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