TOWARDS AN UNDERSTANDING OF THE AGE AND ORIGIN OF MESOTHERMAL GOLD MINERALIZATION IN THE ETHERIDGE GOLDFIELD, GEORGETOWN REGION, NORTH QUEENSLAND

Small mesothermal vein quam-gold-base-metal sulfide deposits from whic h some 20 t of Au-Ag bullion have been extracted, are the most common gold deposits in the Georgetown region of north Queensland-several hun dred were mined or prospected between 1870 and 1950. These deposits ar e mostly hosted by Proterozoic granitic and metamorphic rocks and are similar to the much larger Charters Towers deposits such as Day Dawn a nd Brilliant, and in some respects to the Motherlode deposits of Calif ornia. The largest deposit in the region-Kidston (> 138 t of Au and Ag since 1985)- is substantially different. It is hosted by sheeted quar tz veins and cavities in brecciated Silurian granite and Proterozoic m etamorphics above nested high-level Carboniferous intrusives associate d with a nearby cauldron subsidence structure. This paper provides new information (K-Ar and Rb-Sr isotopic ages, preliminary oxygen isotope and fluid-inclusion data) from some of the mesothermal deposits and c ompares it with the Kidston deposit. All six dated mesothermal deposit s have Siluro-Devonian (about 425 to 400 Ma) ages. All nine of such de posits analysed have delta(18)O quartz values in the range 8.4 to 15.7 parts per thousand, Fluid-inclusion data indicate homogenisation temp eratures in the range 230-350 degrees C. This information, and a re-in terpretation of the spatial relationships of the deposits with various elements of the updated regional geology, is used to develop a prelim inary metallogenic model of the mesothermal Etheridge Goldfield. The m odel indicates how the majority of deposits may have formed from hydro thermal systems initiated during the emplacement of granitic batholith s that were possibly, but not clearly, associated with Early Palaeozoi c subduction, and that these fluid systems were dominated by substanti ally modified meteoric and/or magmatic fluids. The large Kidston depos it and a few small relatives are of Carboniferous age and formed more directly from magmatic systems much closer to the surface.