Spain's Minas de Almaden, once the world's biggest mercury producer, has no plans to restart its idle mine but has "tens of thousands of tonnes" of mercury mineral stored, the firm's commercial director said.
The mine still contains mercury but falling demand and environmental and regulatory issues led to the decision to close it. Labour costs were also a factor.
"We're not going to produce more mineral," Commercial Director Manuel Ramos told Reuters. He added that the firm was working on a plan to turn the mine into a tourist attraction but said no decision was irreversible.
He declined to be more precise about the company's stockpile but confirmed it was less than 100,000 tonnes and that it would last four or five years.
Mineral from state-owned Almaden yields 3.5 kilogrammes of metal per 100 kg of mineral, Mr Ramos said.
The company, which is still converting stockpiled mercury mineral into metal, is supplying its clients only some 60 per cent of what they order, and has been since last summer.
"The demands... have led us to, let's say, ration, not supply the total quantities they ask for but rather keep our regular clients supplied."
"We're not giving them material so they can store it." Mr Ramos also said he saw mercury, already at 20-year highs of $600-$700 a flask after soaring from around $200 in 12 months, hitting $1,000 in the short-term.
"I don't know where the ceiling could be. I do see in the short-term we'll be dealing with $1,000."
However, he did not expect current tightness in the market - sharpened by below capacity production in Kyrgyzstan and an output halt in Algeria - to continue long-term.
"I personally do not have the impression that this extraordinary tension and these price levels... are going to be permanent," Mr Ramos said, adding he expected it to last at least a couple of years.
A metals trader said Almaden's decision was not a surprise. "The mine has been closed for some time, but this is a political decision, and it does confirm what we expected."
Mercury, which is mostly used in the production of caustic soda, batteries, thermometers and other products, has been linked to neurological problems and is especially harmful to young children.
It enters the environment when toxic waste is burned and then builds up in the bodies of animals and fish that ingest contaminated plants and water.
It will probably not be used in five years' time in western Europe and the European Union has mandated that the chlor-alkali industry - which produces chlorine through electrolysis - converts plants away from mercury.
Ramos said demand has been falling for 20 years and he saw no sign of that being reversed. Traders estimate the world market at around 100,000-120,000 flasks per year.
The company says the mine in Almaden, central Spain, has produced since Roman times and has provided a third of the world's mercury. It has produced an annual average of some 35,000 flasks in recent years.
Now the company focuses on transforming the mineral into metal and rescuing mercury from disused industrial installations which it then treats and resells.