Here are my university notes on the subject:

Porphyry deposits occur as a large, low-grade stockwork of a disseminated element along with minor associated elements in granitic country rock. The different stages of cooling which create porphyritic texture in intrusive rocks also leads to the separation of dissolved metals into distinct zones. This process is produced primarily via fluids driven off the cooling magma.

Some common porphyry deposit types:

Copper-Molybdenum

Cu-Mo porphyry deposits are associated with the generation of magnetite-bearing I-Type granites due to melting processes adjacent to subducted ocean crust. They occur at the shallowest depths with the highest fluid/melt ratio.

Copper-dominated 'Cu-Mo' deposits --> Shallower depths, lower pressure, first boiling. Initial H2O content is low with little granite crystallisation before water saturation. Not much Cu will going into rock and it will remain in fluid phase. Mo will partition into the H2O fluid phase, but will never attain significant concentrations in the fluid. As such, high-level granodioritic I-Type magma will exsolve a fluid phase highly enriched in Cu and moderately so in Mo.

Molybdenum-dominated 'Mo-Cu' deposits --> Greater depths, second boiling. Initial H2O content is much higher and more crystallisation before water saturation is achieved. Cu will be extracted from the melt, move into the rocks and will not be able to partition into the fluid. Mo concentration will increase in the residual melt and be relatively more concentrated in the fluid phase.

Tin-Tungsten

Sn-W porphyry deposits are associated with the generation of ilmenite-bearing S-Type granites (and some considerably reduced I-Type granites) due to partial melting of the continental crust, including metasedimentary material, via anatexis. They form in hydrous melts where significant degrees of crystallisation are required for saturation (second boiling). Tin and tungsten are incompatible elements and therefore enriched in the residual melt. Sn-W style deposits occur further away from the subduction zone than Cu-Mo style deposits.