MAGMATIC AND HYDROTHERMAL HISTORY OF THE PORPHYRY-STYLE DEPOSITS OF THE ARDLETHAN TIN FIELD, NEW-SOUTH-WALES, AUSTRALIA

The cassiterite-sulfide deposits of the Ardlethan tin field are classi fied as porphyry-style Sn deposits on the basis of the intimate spatia l and temporal relationships of the breccia pipes and Sn deposits with the Ardlethan Granite, and in particular, the Mine porphyry; the cont inuum of salinity and temperature conditions from high-temperature and high-salinity fluids in the Ardlethan Granite to lower temperature an d less saline fluids in the breccia pipes in the Mine granite; and the isotopic character of the ore fluids which is consistent with a magma tic fluid source characterized by primitive isotopic signatures for ca rbon and sulfur. The deposits occur in breccia pipes in the Siluro-Dev onian Mine granite (417 +/- 2.5 Ma) close to its contacts with the Ard lethan Granite (410 +/- 2.5 Ma) and Ordovician metasediments. The Ardl ethan Granite and related phases are moderately to strongly fractionat ed and enriched in Rb, Th, U, Nb, Y, Li, As, Ta, W, F, and Sn. Tourmal ine nodules occur within the Ardlethan Granite and halite-bearing flui d 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 brecc ia pipe of the Mine granite following the onset of brecciation. Brecci ation and mineralization are linked to the release of magmatic volatil es and saline brines from the Ardlethan Granite. The vug-filling miner als in all the deposits show a paragenetic sequence from cassiterite, wolframite, arsenopyrite, milky quartz, and tourmaline to pyrite, chal copyrite, sphalerite, galena, tourmaline, and clear quartz to late-sta ge toothy quartz, fluorite, and cookeite. Fluid inclusion data from qu artz show that the deposition of milky quartz and cassiterite occurred between 300 degrees and 370 degrees C, sulfide and clear quartz betwe en 210 degrees and 280 degrees C, and toothy quartz, fluorite, and coo keite between 105 degrees and 245 degrees C. Fluid salinities ranged b etween about 13 and 18 wt percent NaCl equiv for the main stages of mi neralization. Coexisting CO2-and H2O-rich fluid inclusions permit a mi nimum pressure estimate of about 500 bars. Temperatures calculated fro m the compositions of hydrothermal biotite, muscovite, and chlorite ar e in good agreement with the results of fluid inclusion microthermomet ry and indicate a paleotemperature gradient in the Mine granite brecci a pipe. Cooling of the hydrothermal fluids contributed to the depositi on of cassiterite together with increases in DPI and fo(2) resulting f rom boiling of reduced fluids and wall-rock reactions. The delta(18)O values of milky quartz, clear quartz, and toothy quartz are mostly bet ween 11.5 and 13.5 per mil in all deposits, and the variance of delta( 18)O(fluid) with temperature approximates the theoretical curve for ro ck-dominated conditions in granitic systems. Sulfur isotope compositio ns of the sulfides formed during the main-stage mineralization range b etween -2.0 and 2.0 per mil and the value of delta(34)S(fluid) is esti mated at 0 +/- 1 per mil. Carbon isotope values of siderite are -4.3 /- 1.0 per mil.