Red Mountain Number 2. The red color is caused by iron oxide (nature's rust). The original rock had iron sulfide which is gold colored, not rust colored. Hot waters from below oxidized the pyrite to hematite (red). This natural process can cause acid waters or natural acid rock drainage. The human process of mining also brings unaltered pyrite to the surface where it can oxidize and creates acid mine drainage. It is important when studying acidic waters in Colorado to determine whether they are a result of mining or natural processes. There are 24 other topographic features in Colorado named "Red Mountain" which gives you some idea of the extent of the problem.