Maybe a separate mine, maybe another name for one of the nearby mines. CaMoA says:

Possible drainage adit for Rhös-mynach Mine

but that’s probably unlikely.

MJ 1835 p 63

“Newly Discovered Copper Mines. We learn from an intelligent correspondent that there has lately been discovered on the property of Lord Dinorben in the parish of Llanwenllwofo, [sic] Anglesey, and much purer than even the copper coinage of 1799: consequently, a question will arise for the consideration of geologists and others who feel pleasure in investigating these matters, whether the secondary stratum in which it is found must at some remote period have been acted upon by great and powerful heat, so as to dislodge the ore from the stone, and run it in a state of fusion in the form in which it is now found. The discovery is very reasonable as the Parys and Mona mines, which have long been a source of immense wealth to the proprietors, and of profitable employment to many hundreds of poor families, were becoming nearly exhausted, at least so far as they had been explored.”