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By Dr L Haynes
Economic Geology Expert
Directorate General of Minerals

Earth scientists from the Universities of Cambridge and Southampton have been conducting a research project in the northern part of the Oman Mountains over the last few weeks, and it is hoped their findings will be of considerable economic benefit to the Sultanate.

The team consisted of Prof Michael Bickle, and his wife, Dr Hazel Chapman, and their research student, Amy Davis from the Department of Earth Sciences at Cambridge University. Also involved were Dr Damon Teagle and Dr Clive Boulter of Southampton University School of Ocean and Earth Sciences. Prof Bickle and Dr Teagle are jointly supervising Davis' research project into hydrothermal (hot water) systems in the Oman Ophiolite.

Both have considerable experience of the hot water systems which circulate through the volcanic rocks below the modern seafloor, and form black smoker-type mineral deposits.

Dr Chapman is an expert on the Troodos Ophiolite of Cyprus, where similar systems have been extensively mined for copper and gold, and she runs the strontium isotope laboratory at Cambridge. Dr Boulter is famous for working on the seafloor environments under which massive mineral deposits are formed, and his work at the huge deposits at RioTinto in Spain has revolutionised scientific opinions about these deposits, and led to a new extension of the 100-year-old mining operations there.

The current work has been sponsored by Oman Mining Company and the Directorate General for Minerals, and DGML geologists have assisted with the project. Dr Haynes of the DGML explains.

"Hot water systems circulating below the seafloor are mainly formed of seawater which has penetrated the cracks formed in the volcanic lavas as they cooled. These waters penetrate the rocks to variable depths before they become too hot to go further, and rise again to the surface forming black smokers.

''As they pass through the rocks, they leach out metals of economic importance, such as copper, zinc, and gold, and then deposit them in the black smoker on cooling. Which metals, and how much, depends on the volume of rock leached, the metals available in it, and the temperature of the water doing the leaching (the hotter the water the quicker is the chemical reaction and the more metal can be held in solution).

But even at low metal concentrations, if the hot water system continues for a long time, and is big enough, large mineral deposits can eventually be formed on the ocean floor.

However, it takes temperatures in excess of 200oC for zinc and gold to be leached and in excess of 300oC for copper. Systems below these temperatures will only contain iron and manganese.

Fortunately we trace these systems in the rocks by identifying how the volcanic rocks are altered by the chemical reaction. This alteration causes a new set of minerals to replace the original higher-temperature volcanic minerals in the rock. One of the most prominent of the alteration minerals is a bright pistachio-green mineral called epidote, which only forms when the reaction between the water and rock is at temperatures higher than 320oC. Therefore where the alteration systems contain this mineral, we know there is good potential for a copper deposit in the area.

Fortunately epidote is very resistant to further alteration once it has formed, and is very stable, so we can effectively map out hydrothermal zones containing epidote.

In this way, hopefully, new deposits can be found which contain copper, but not all of these will contain gold, because gold only occurs in some of the ophiolite volcanic rocks. Generally the copper deposits will not be economic at the present prices unless they contain gold. So how do we find the epidote-rich systems which contain both copper and gold?

The British research team have here come up with an ingenious answer. Epidote contains a lot of calcium, and strontium replaces some of the calcium in all calcium minerals. Strontium has several isotopes, and the relative amount of these isotopes (isotopic ratio) is characteristic of different types of lavas. Lavas which formed in an oceanic environment by melting inside the earth have the lowest ratios - (Sr87/Sr86 at about 0.701-0.703). Those which formed by melting in a subduction zone where the earth's crust is forced downwards below the ocean floor, have higher values at about 0.705-0.707, and those formed in areas on the earth's continents have higher ratios still, often up to 0.712.

Fortunately gold in Oman is only present in lavas of the second group, so epidotes formed by alteration of these lavas will have strontium isotope ratios around 0.706 while the epidotes formed from lavas of the first group (with no gold) have ratios around 0.703.

The important point here is, because the lavas are highly altered and deformed in Oman, it is often very difficult to tell which of the two types you are dealing with. Looking at the strontium isotopes not only does this, but indicates if the hydrothermal system has gold potential or not. Clearly because 0.703 and 0.706 are not very different, the apparatus used to measure the ratio has to be very accurate and sophisticated, and even the smallest contamination of the sample must be avoided. This is not a problem in these days, and already preliminary results from sampling last year have shown this to be a very viable exploration technique.

Why can we not simply find deposits visually? That is because often they are hidden below wadi gravels or terraces, or completely below the surface. Those at the surface were found long ago by the ancient miners.

For example, during this year's work we have identified a very large epidote system in one area, which must somewhere have one or several very large deposits associated with it, but when we map the system it disappears under a wadi and only terraces are found on the other side.

What the isotopes will tell us cheaply is whether this system is potentially gold-bearing and we should then use other more expensive techniques to try and locate the buried deposits.

Finally, why is this research cheap for Oman? It is because the Universities concerned are bearing the cost of all the analysis and maintenance of the expensive equipment used. Only logistic support is provided by the authorities. Clearly this is a good way to develop mineral exploration in the future."

Oman Observer 3rd March 2002