Abstract

The Pb isotope signatures of sulfide mineralization within the Lachlan fold belt can be divided into those containing almost exclusively mantle-derived Pb, those containing Pb which has had a long crustal residence time, and those containing Pb of mixed mantle and crust parentage. In the Early Paleozoic (Ordovician to Silurian) all the major and the majority of other deposits have either crust or mantle signatures with very little evidence of mixing. Porphyry to epithermal Cu and/or Au mineralization hosted within Ordovician shoshonitic rocks (e.g., the Goonumbla deposits) have a wide range of 206 Pb/ 204 Pb ratios (17.68-18.21) and very low 207 Pb/ 204 Pb (15.40-15.49) and 208 Pb/ 204 Pb (37.21-37.83) ratios. Ores and their corresponding host rocks have very similar isotopic compositions suggesting that Pb is magmatic in origin On 208 Pb/ 204 Pb vs. 206 Pb? 204 Pb and 207 Pb/ 204 Pb vs. 208 Pb/ 204 Pb diagrams, these data plot on very precise lines (mantle mixing lines) 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 magmatic cycle and possibly also the degree of mixing of melts derived from a more primitive asthenosphere and a more enriched lithosphere, potentially fertile for Cu and Au. Exploration samples which plot on the mantle mixing lines and have high 206 Pb/ 204 Pb ratios are considered to have the best chance of representing a large metallogenic event. Deposits 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 recycled Pb from the Ordovician rocks and mixed it with crustal Pb...

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