Is Metal Mining Exclusively a Capitalist Activity?

Not unless you include the Soviet Union as a capitalist society. Joseph Stalin launched his "industrialization process" in 1928. Among the favoured industries were the basic mineral sectors: energy resources, iron and steel, non-ferrous metals, cement and fertilizers. During the second five-year plan (1933–37), the mining industry expanded extremely rapidly; by 1937 coal output was 127 million tons and pig iron 14.5 million tons. This was no workers paradise and the program was extremely harsh on industrial workers:

Quotas were difficult to fulfill, requiring that miners put in 16 to 18-hour workdays. Failure to fulfill the quotas could result in treason charges. Working conditions were poor, even hazardous. By some estimates, 127,000 workers died during the four years (from 1928 to 1932). Due to the allocation of resources for industry along with decreasing productivity since collectivization, a famine occurred. The use of forced labor must also not be overlooked. In the construction of the industrial complexes, inmates of labor camps were used as expendable resources. But conditions improved rapidly during the second plan. Throughout the 1930s, industrialization was combined with a rapid expansion of education at schools and in higher education. (source here).

During this time the massive Magnitogorsk iron and Norilsk Nickel mining complexes were constructed by slave labour in the Gulag system. Some CSR program! The environmental legacy of Soviet mining isn't something to be excited about either. With no land owners to compensate for damages, the Soviet mining industry expanded according to the needs of the Soviet, without regard for citizens or environment. While Norilsk is still suffering from the Soviet environmental legacy, market place pressure from shareholders of the new post-communist Norilsk entity may now bring about changes in environmental policy.

Well that is all fine and good. But surely the Soviets didn't delve into evil Capitalist elements and metals like gold, silver and diamonds? No, they liked those things too. The second largest open pit mine in the world is the Mir diamond mine located in Siberia and was dug for diamonds under Stalin who ordered construction of the mine to satisfy the Soviet Union’s need for industrial-grade diamonds. Jet engines were used on the permafrost in order to melt it. As for gold, Stalin ordered Alexander Serebrovskiy to create "Soviet" gold industry. the victory of socialism in our country, the gold industry played a major role...source: Alexander Serebrovskiy, 1936

During the Soviet era, the Magadan gold district, amongst others in the far east, were mined by political prisoners and slave labour. In fact it can be argued that the Gulags were purpose built by the Soviets for the gold mines.

So to return to the opening question, why do some groups who oppose mining associate the mineral industry with capitalism? Or any other political system for that matter? Mining has been with us since the Stone Age. In fact we measure human progress through the Copper, Bronze and Iron Ages by the metals we mined and refined. Mining is an essential activity to any form of civilization no matter how it is politically structured including pre-Industrial communal societies, Soviet communist society and modern western social democracies. Metal mining will remain an important contributor of recyclable metals to future human civilizations as well, no matter what their politics.