The paper presents the evidence supporting ternary phase diagrams proposed in earlier articles and builds a quaternary diagram making use of observed mineral associations to supplement available chemical data. Phase relations explain the rarity or absence of a number of associations such as magnetite-covellite, pyrrhotite-hematite and, in the presence of magnetite, bornite-pyrite. Although most commonly observed assemblages are apparently stable over a wide range of temperatures there is a strong suggestion of shifting of tie-lines with change of temperature within the field of composition bounded by chalcocite, covellite, chalco-pyrite, and pyrite. The observed sequence in ore deposition is not so much a sequence of individual minerals as a sequence of assemblages, each higher than the preceding one in ratio of S to O and usually of Cu to Fe. However, it seldom reaches the ultimate high-S assemblage, covellite-pyrite. Late stage reversals of the trend (e.g., chalcocite replacing covellite) could be occasioned by waning of S supply before cooling was far advanced. Certain minerals such as pyrrhotite and magnetite are early members of the sequence, not because they are in themselves "high temperature" minerals, but because they are stable in low-S high-Fe assemblages.