They will let you put down as a deposit anything up to around 1% of the total amount of shares you are buying (altho obviously if the share price goes 1% the wrong way you lose your deposit).
mining stocks have all got hit badly over the last couple of months, most are down 25-50%. Most of the ones I'm watching seemed to have hit their bottoms though, so I reckon now is a good time to top up on any of them. should do well holding for long term.