The cassiterite-sulfide deposits of the Ardlethan tin field are classified as porphyry-style Sn deposits on the basis of the intimate spatial and temporal relationships of the breccia pipes and Sn deposits with the Ardlethan Granite, and in particular, the Mine porphyry; the continuum of salinity and temperature conditions from high-temperature and high-salinity fluids in the Ardlethan Granite to lower temperature and less saline fluids in the breccia pipes in the Mine granite; and the isotopic character of the ore fluids which is consistent with a magmatic fluid source characterized by primitive isotopic signatures for carbon and sulfur. The deposits occur in breccia pipes in the Siluro-Devonian Mine granite (417 + or - 2.5 Ma) close to its contacts with the Ardlethan Granite (410 + or - 2.5 Ma) and Ordovician metasediments. The Ardlethan Granite and related phases are moderately to strongly fractionated and enriched in Rb, Th, U, Nb, Y, Li, As, Ta, W, F, and Sn. Tourmaline nodules occur within the Ardlethan Granite and halite-bearing fluid inclusions found in quartz from the nodules homogenized between 250 degrees and 485 degrees C. One of the most fractionated phases of the Ardlethan Granite--the Mine porphyry-- was emplaced into the major breccia pipe of the Mine granite following the onset of brecciation. Brecciation and mineralization are linked to the release of magmatic volatiles and saline brines from the Ardlethan Granite. The vug-filling minerals in all the deposits show a paragenetic sequence from cassiterite, wolframite, arsenopyrite, milky quartz, and tourmaline to pyrite, chalcopyrite, sphalerite, galena, tourmaline, and clear quartz to late-stage toothy quartz, fluorite, and cookeite. Fluid inclusion data from quartz show that the deposition of milky quartz and cassiterite occurred between 300 degrees and 370 degrees C, sulfide and clear quartz between 210 degrees and 2800C, and toothy quartz, fluorite, and cookeite between 105 degrees and 245 degrees C. Fluid salinities ranged..