GEMOC ARC National Key CentreA RE-EVALUATION OF MINERALISED ORDOVICIAN INTRUSIVES IN THE LACHLAN FOLD BELT: IMPLICATIONS FOR TECTONIC AND METALLOGENIC MODELS.
Phillip Blevin, GEMOC ANU
Ordovician porphyry-style Cu-Au mineralisation in the central Lachlan Fold Belt is related to intrusive complexes of diverse compositional character. The intrusive suite at Copper Hill is medium-K calcalkaline in character, those at Goonumbla and Rain Hill are transitional from high-K calcalkaline to "shoshonitic" (in terms of K2O versus SiO2), while the intrusive complex at Cadia is "shoshonitic". The intrusive suites at Cargo and Lake Cowal appear to be medium-K calcalkaline in character. Surprisingly, of all these suites examined in this study, only that at Cadia is actually "potassic" in the strict sense (i.e. molecular K/Na > 1). All the intrusive suites are silica near-saturated to saturated, with magmatic quartz being more abundant (and appearing paragenetically earlier) in the medium-K suites. Plagioclase phenocrysts in the more K-rich suites are commonly mantled by alkali feldspar, a result of peritectic reactions during crystallisation, and not high temperature K metasomatism. Pyroxene compositions are typical of calcalkaline arc rocks in terms of Ca-Fe-Mg end member components and Al-Ti relationships. Mineralogical evidence for alkaline and/or silica undersaturated magmatic affinities are absent from all the intrusives.
Incompatible trace element abundances are significantly lower than for other "shoshonitic" rock suites from various tectonic settings, and most closely match those from arc related medium- to high-K calcalkaline rocks. The relative abundances of incompatible trace elements are proportional to the K content of each suite with the exception of Rain Hill. K2O contents in the K-rich suites increase rapidly with increasing SiO2 compared to changes in K2O content in the medium-K suites. Internal differentiation within the suites is controlled by fractional crystallisation and cumulate processes dominantly involving plagioclase and clinopyroxene. Low to very low Ni contents in all suites are consistent with early olivine removal. Eu anomalies do not increase significantly despite plagioclase fractionation, presumably due to high magmatic fO2 conditions. The Ordovician intrusive suites (including similarly aged granitoids) are chemically distinct from granitoids of Silurian and Devonian age in having lower Rb, Y and HREE, and higher Sr and K/Rb at any given SiO2 value.
The systematic relationships between petrography, mineralogy, rock composition and inter-element ratios argue persuasively against any one of these features (e.g. quartz phenocrysts, graphic groundmass textures, low K contents etc) being due to post crystallisation alteration, metamorphism etc. The mineralised Ordovician intrusive suites show a systematic progression in petrographic and compositional character from rocks of indisputable medium-K calcalkaline affinity to high-K calcalkaline affinity, and finally to relatively more enriched rocks. Given their relationship to calcalkaline rocks, these enriched equivalents would be better termed as "very high-K calcalkaline" instead of "shoshonitic". The use of the terms "shoshonite" or "shoshonitic", even if correctly applied using IUGS classification criteria, obscures the genetic relationship between such rocks and their contemporaneous, less K- and incompatible element-enriched calcalkaline equivalents.
These results have implications for metallogenic and tectonic models for the LFB. Porphyry Cu-Au style mineralisation is commonly associated with arc-related calcalkaline magmatism, and the Ordovician of the LFB provides no exception in that regard if indeed contemporaneous subduction was present. The occurrence of Cu-Au mineralisation in suites of diverse compositional character suggests that models linking the metallogenic potential of these suites to incompatible element enrichment processes requires critical re-evaluation. Similarly, models arguing against the existence of contemporaneous subduction during the Ordovician on the grounds that "normal" calcalkaline rocks are absent need to be reassessed. Given the above results for the intrusive components of the Ordovician System in NSW, a contemporaneous subduction scenario cannot be discounted on these grounds.
Acknowledgments: This work was undertaken as part of AMIRA project P425. Project sponsors are thanked for their support and their generous permission to publish.
