Hydrothermal Mineral Deposits

A presentation by ST, which was accompanied by slides and rock samples as well as a short video on gold deposition in America

On our field trips, Doug has pointed out numerous Calcite Veins running through the rock exposures. Whilst a brief explanation has been given I think there has been some misunderstanding about their formation, so here I hope to fill in the gaps (bad Pun).

What are we dealing with, the process comes under the broad heading Hydrothermal Mineral Deposits. As you may guess from the title our prime suspects are Water and Heat. The process itself is fairly simple but there are 1000s of variations and complications along the way, but hopefully we can keep it simple.

Starting with Water, this can be Rain falling on to the rock surface, or once on the rock it can pool or run across the surface, then depending on the porosity of the rock it can be absorbed or dissolve the rock, once inside it becomes ground water. In some cases the processes leading to mineralisation begin here. We have dealt mostly with Limestones and Calcite and this is our starting point. Limestone contains Calcium Carbonate and when the Ph of the water is on the acidic side, the Calcium can be dissolved fairly readily.

We have seen the results of this on our field trips and come across Swallets where the water disappears below ground and follows channels that have been cut by the water. These channels can be narrow or open out into cave systems; they can also travel deep or over long distances. These systems have the potential to hold a lot of water and that water will hold a lot of Calcium in solution.

This brings us to another aspect of the process, Pressure. Water is relatively heavy, which you will have noticed when picking up a bucket of water, what you may not have noticed of that bucket full, is that the water exerts a pressure of 7 lbs per square inch onto the base of the bucket. Taking that a bit further a litre of water weighs 1 Kilo and a small hole about 35mm diameter and 1 meter long holds 1 litre so if that hole goes down 1 kilometre that is one tonne of weight pressing upon that 35 mm diameter. Other ---- can increase that weight.

The rock outcrops we look at on the surface today were once at least that depth if the Mendips were as guesstimated as having been 3 – 4000ft high originally.

The second part of the process is Heat, on the surface we feel rock as cold to the touch, unless it has been exposed to the sun for a length of time, but in general to us it feels cold. For those of you who have visited Mines and especially deep mines you will know that with depth the rock gets warmer which is to be expected as you are getting closer to the underlying magma. In regions where there is Volcanic activity then the source of heat can be even closer to the surface. In the past we have had limited volcanic activity and even today there must be something below us to provide the hot waters in Bath and Bristol.

The basic process so far requires Water and Heat, with the water comes Pressure which can also be increased by heating. The water can be further complicated by its Ph value and when the water source is from the Sea the degree of Salinity can play a part. When the heat is provided by Volcanic action water can also come from the magma (very complicated!!)

Once we have the three basic ingredients then we can move onto the next stage which is the minerals to be held in solution, these can be washed into the ground water from the surface, dissolved from the rocks as the water passes through them by chemical reaction and when magma is involved the minerals can be absorbed from the molten rock.

This may need some clarification and revolves around the water being under pressure, we understand the boiling of water at sea level, it boils at 100 °C and some ----- will be absorbed by the water at that temperature, but above 100°C the water turns to steam. But at depth and under pressure it will not turn to steam but becomes super-heated water and will remain liquid even at some very high temperatures. It may help to know that all of the elements in the Periodic table will at some combination of temperature and pressure be soluble in a liquid solution and can be deposited when the temperature and pressure drop below their particular limits.

Now we have our water full to the brim with minerals what do we do with them, the conditions for hydrothermal mineral deposition are,

1/ Hot water to dissolve and transport the minerals.
2/ Interconnected openings in the rock to allow the solutions to move.
3/ availabity of sites for deposition.
4/ Chemical reaction that will result in deposition. Deposition can be caused by Boiling, a drop in temperature or pressure, mixing with a cooler solution or chemical reaction between the solution and a reactive rock.

Here may I add the additional ingredient the circulation of water, normally we say what goes up comes down, but here it is what goes down comes up. Under gravity the water takes the easiest route down until it meets the heat source, once heated it wants to rise as we see when a pan of water boils on the stove, so some holes in the rock take water down and others bring it back up. As it travels up the cracks the temperature or pressure changes or the solution reacts to the minerals held in the host rock and deposition takes place.

As we heard in the last DVD lecture if the rock is cooler than the solution then the deposition will be fast and the crystals will be small, but if the surrounding rock is hot then the solution cools slowly and large crystals will form.

That is the basic process behind minerals being deposited, but depending upon which mineral is being deposited the parameters will change between heat, and pressure also other factors come into play that trigger the deposition. Here we are straying into Quantum Physics which we can avoid if you wish.

But Calcite deposition is our main interest for the moment, earlier I mentioned that Water with an Acidic Ph value will readily dissolve the calcium carbonate from the limestone. The water can hold quite a lot of calcium which you will know from the Lime scale that builds up around taps and household water systems. Factors that can affect the rate of both dissolving and precipitation are the Ph level, with low acid levels it will be slow and increased levels will speed it up, then with temperature again cold will slow or stop the absorption and Calcium/Calcite has an unusual characteristic called Retrograde Solubility which makes it less soluble as temperature increases. Though the actual range of low and high temperatures was not noted.

Once we have this Calcium soup being warmed up at the bottom of the labyrinth of cracks the solution starts to circulate around the cracks until the hot and therefore lighter in weight solution can begin to force its way to the surface. Whilst still in the warmer rocks the solution holds onto its mineral load but on contact with cooler rock the crack sides begin to receive a coating of Calcite. This coating will build up and up until the crack is completely full. If the crack is larger and the heat transfer into the rock is sufficient then the calcite can slowly grow into crystals. When we see the veins of Calcite this is usually easy to identify.

The largest single Calcite crystal so far found is in Iceland it is 7m x 7m x 2m and weighs 250 tonnes. The largest Hydrothermal crystal so far found is in the Giant Crystal cave in Mexico it is 12m x 4m and made of Selenite Gypsum.

Back to the Mendips, at around the same time as most of the Calcite was being deposited, around what is now the core of the Mendips Lead was also being deposited. The melting point of Lead is 327.5°C so to be soluble in water it would have been higher still. When we visited Sand bay in Weston we came across Volcanic activity within the Limestones beds so could this have been the source of extra heat needed.