Porphyry deposits and Beer having something in common. Porphyry ore deposit form in large intrusions that acted as magma chambers feeding active volcanoes. 
You will remember that as the magma chamber cools from the outside inwards, barium, feldspar and quartz crystalize out so that the remaining melts becomes enriched in metals and volatiles. The volatiles migrate to the top of the chamber like the “foam in a glass of beer” analogy I used in the first ore deposits course. 
Feldspar crystals take up more space than feldspar melt, and this combined with the accumulating steam and gas builds up pressure inside the chamber until the surrounding rocks can no longer contain it. It’s cool solid brine to the magma chamber and country rock about that begins to rupture and the volatiles escape upwards carrying the metals with them. The metals then drop out a solution as the escaping fluids cool. Eventually those fluids reach the surface as hot springs. In this course we are interested in what happens to those fluids and how they drop their load of the metals to form epithermal deposits between escaping from the porphyry and reaching the surface. Epithermal so both genetically and spatially associated with porphyries.
