This is the 100th daily episode of the History of the Earth. I appreciate all my listeners and readers! Thanks! If you are listening on iTunes and have a suggestion for ways I can improve the program, please leave a review on iTunes. You can also post comments and questions on the blog.
I’m going to try hard to keep the daily episodes going, but once it becomes summer here in Butte, Montana, my life gets a lot fuller. I do historic walking tours and drive the Chamber of Commerce tourist trolley, and they take up a lot of my time. I’ll try, though, to keep the podcasts coming on a daily basis. Thanks again for your interest.
Today, we focus on a mineral deposit that’s hosted in Silurian rocks. As with many mineral deposits, it’s often not entirely clear exactly when mineralization happened, and often enough it can take place over many millions of years. But we do know the age of the rocks that the mineral resources are in.
In this case, we are talking about the largest single deposit of mercury ore anywhere on earth. It’s at Almadén, Spain, where more than 250,000 tons of mercury have been produced over the past 2,000 years or so. It’s one of the longest operating mine complexes anywhere in the world. The name of the location, Almadén, comes from Arabic, meaning, “The Mine.”
The Romans ground the red mineral to use as pigment. Alchemists in the Middle Ages were fascinated by mercury – it is a liquid metal, after all – and by the 16th century, the discovery was made that mercury could combine with gold and silver, which were otherwise relatively inert, in a process called amalgamation. That made mercury important in extraction of gold from rock – and the Spanish were discovering tremendous deposits of gold and silver in the Americas in the 1500s. The mines at Almaden were owned by German bankers in the 1500s and 1600s, in return for loans they gave to the Spanish government. Beginning about 1566 convict labor was used to mine the ores. They were fed at least moderately well, but about a quarter of the convicts died, usually from mercury poisoning.
The cinnabar ore at Almadén is in cracks and pores in Silurian quartzites. Quartzite can be a metamorphic rock – basically, pure quartz – or it can be a sedimentary rock that’s really densely compacted. At Almadén, the rock was most likely originally a pretty clean quartz sand laid down in a delta, near a large river mouth. The rock is pretty brittle, easy to fracture, and the mercury ores at Almadén are in fractures within the quartzite.
There’s speculation that the mercury ultimately came from older black shales of Ordovician age. Something heated the water in these rocks, which percolated through the shale, picking up mercury, which was deposited in the fractures in the overlying quartzite. Alternatively, the mercury came from the earth’s mantle, much deeper than any of the near-surface sedimentary rocks.
Wherever the mercury ultimately came from, it seems that magmatic activity – moving molten rock – probably was the driving force that mobilized the mercury, or the hot waters that moved it around. The richest deposits are quite close to an explosive volcanic vent whose rocks are more or less basaltic in composition. The volcanic activity is almost certainly the most important factor in concentrating the mercury ore.
But why is there so much, right here in this one small district? By some estimates, a third of all the mercury on earth is at Almadén, Spain. You can talk about modes of concentration, volcanoes and fractures, but to my mind the question of why so much is here is unanswered. What I tell tourists about the incredibly rich deposit of copper, silver, and many more minerals here at my home in Butte, Montana, is that I think it’s the luck of the draw – that there were inhomogeneities in the early earth, essentially blobs of the good stuff, that have been altered, changed, and moved around by geologic processes, but that must have been persistent concentrations from way back in geologic time. That’s my guess for the mercury at Almadén. But trust me, I definitely don’t know for sure.
Because mercury is so toxic, its use has declined precipitously in the past couple decades. The last time primary mercury was produced in the United States was in 1992, from a mine in Nevada. Mercury does have a lot of uses, however, and the biggest consumer in the U.S. is the production of caustic soda. Electronics and fluorescent light bulbs also use mercury, but all uses are decreasing because of mercury’s toxicity and persistence in the environment.
The European Union ended mercury production in 2003, and the mines at Almadén were closed. Almost 2,000 tons per year are still mined around the world – mostly in China, which produces almost three-quarters of the world’s mercury, with Kyrgyzstan in second with about 14% of the total.
—Richard I. Gibson
Cinnabar photo by Rob Lavinsky via Wikipedia, under Creative Commons license.
More on mercury uses can be found in my other book, What Things Are Made Of (2011)
Further reading about the Almaden mine's geology: