the point is if metal prices increase and a well managing mining company is making money, then they will distribute profits to their shareholders in the form of dividends. so the PTB can manipulate prices down but eventually the profits stand out like a sore thumb. For example, newmont is paying a 3% dividend and Gold Fields is at about 5%. (Gold Fields is a bit less reliable, hence the higher yield, but the point is the dividend yield becomes a limit on downward stock prices.)
Lancers: Motives are two-fold: (1) many juniors expand by issuing new stock, so a depressed stock price cuts off their growth, and (2) keeps momentum traders and the general public out of the sector.