According to the English section in the current Culture Express, there will be a guided tour to the Jinguashi (Kinkaseki) Gold Mine on June 28th (saturday).
It is sponsored by the Taipei Natural Education Council, price is NT$900. The trip cost includes bus ride, tour, insurance and informational material. I think you have to sign up beforehand.
Information under tnature at msa48.hinet.net or phone: 2366-1011.