PRECISE LEAD-ISOTOPE FINGERPRINTING OF HYDROTHERMAL ACTIVITY ASSOCIATED WITH ORDOVICIAN TO CARBONIFEROUS METALLOGENIC EVENTS IN THE LACHLANFOLD BELT OF NEW-SOUTH-WALES

The Pb isotope signatures of sulfide mineralization within the Lachlan fold belt can be divided into those containing almost exclusively man tle-derived Pb, those containing Pb which has had a long crustal resid ence time, and those containing Pb of mixed mantle and crust parentage . In the Early Paleozoic (Ordovician to Silurian) all the major and th e majority of other deposits have either crust or mantle signatures wi th very little evidence of mixing. Porphyry to epithermal Cu and/or Au mineralization hosted within Ordovician shoshonitic rocks (eg., the G oonumbla deposits) have a wide range of Pb-206/Pb-204 ratios (17.68 - 18.21) and very low Pb-207/Pb-204 (15.40 - 15.49) and Pb-208/Pb-204 (3 7.21 - 37.83) ratios. Ores and their corresponding host rocks have ver y similar isotopic compositions suggesting that Pb is magmatic in orig in. On Pb-208/Pb-204 vs. Pb-206/Pb-204 and Pb-207/Pb-204 vs. Pb-206/Pb -204 diagrams, these data plot on very precise lines (mantle mixing li nes) which represent mixing of Pb from two or more mantle reservoirs. It is postulated that the position of each deposit along this line is an indication of the timing of metallogenesis within the Ordovician ma gmatic cycle and possibly also the degree of mixing of melts derived f rom a more primitive asthenosphere and a more enriched lithosphere, po tentially fertile for Cu and Au. Exploration samples which plot on the mantle mixing lines and have high Pb-206/Pb-204 ratios are considered to have the best chance of representing a large metallogenic event. D eposits which are spatially related to Ordovician volcanics but which have more crustlike Pb isotope signatures and are generally relatively small in size, are considered to have formed in response to younger ( Silurian? to Devonian) magmatic or metamorphic events which have recyc led Pb from the Ordovician rocks and mixed it with crustal Pb. Volcani c-hosted massive sulfide mineralization has a very restricted range of Pb isotope ratios (Pb-206/Pb-204 = 18.03 - 18.11; Pb-207/Pb-204 = 15. 56 - 15.63; Pb-208/Pb-204 = 37.95 - 38.22), with the largest deposits containing Pb with strong crustal affinities. With rare exceptions, bo th large and small deposits contain very homogeneous Pb isotope ratios , although in the large Woodlawn deposit there is evidence of more tha n one data population, indicative of superimposed hydrothermal activit y. The overall range of Pb-206/Pb-204 ratios for the volcanic-hosted m assive sulfide deposits results from mixing of a less radiogenic crust al Pb, represented by the Currawang deposit hosted in pillow basalts, and a more radiogenic component, represented by the Woodlawn deposit, hosted in more felsic volcanic and volcaniclastic rocks. The range of Pb-207/Pb-204 and Pb-208/Pb-204 ratios results from mixing of crustal- dominated Pb with Pb derived from rocks with a mantle heritage, princi pally Ordovician magmatic units. Evidence of such mixing, which may be due to hydrothermal or magmatic processes, is concentrated along the boundaries of the Silurian volcanic terranes adjacent to the Ordovicia n units. Deposits which show evidence of incorporation of mantle Pb al so tend to have higher Au concentrations. Metallogenesis in the Devoni an and Carboniferous is dominantly related to granitoid intrusion, alt hough some sediment-hosted base metal mineralization and epithermal Au mineralization is probably related to comagmatic volcanism. The Pb in the intrusion-related mineralization is isotopically very similar to magmatic Pb, with very little evidence of hydrothermal mixing with int ruded host or wall rocks. The Carboniferous mineralization shows great er evidence of incorporation of mantle Pb. A plumbo-tectonic model for the Lachlan fold belt based on mixing isochrons between a crustal end member growth curve and a mantle end-member growth curve yields miner alization model ages for the Paleozoic with a precision generally bett er than +/- 15 m.y. The Lachlan fold belt crustal growth curve is a mo dified last stage to the Cumming and Richards (1975) global crustal gr owth curve. It is defined by a mu of 13.3 and passes through the avera ge values for the Woodlawn Zn-Pb mineralization (Pb-206/Pb-204 = 18.09 0, Pb-207/Pb-204 = 15.610, Pb-208/Pb-204 = 38.118) at an age of 420 Ma . The mantle curve has a mu of 10 and passes through the Goonumbla ore body data (Pb-206/Pb-204 = 18.204, Pb-207/Pb-204 = 15.487, Pb-208/Pb-2 04 = 37.831) at an age of 440 Ma. The points of intersection of these curves with the respective global curves are as yet unclear due to the dearth of ore Pb isotope data in the Upper Proterozoic. The model als o appears to be valid for older (Kanmantoo fold belt) and younger (New England orogen) units of the Tasman fold belt system. The relatively high precision of the model is an indication of the presence of Paleoz oic and Upper Proterozoic source regions which have homogeneous Pb iso tope compositions and U/Pb and Th/U ratios.