The petrographic and compositional character of variably K-enriched magmatic suites associated with Ordovician porphyry Cu-Au mineralisation in the Lachlan Fold Belt, Australia

Abstract

Mineralised igneous complexes of Ordovician age from New South Wales range in composition from quartz-rich medium-K dacites (e.g. Copper Hill), to quartz-poor, high-K to 'shoshonitic' monzodioritic to monzonitic complexes (Goonumbla and Cadia). Despite Ordovician igneous suites being considered mostly 'shoshonitic' by some authors, only the Cadia Igneous Complex is potassic in the sense of having molecular K/ Na greater than unity. A feature of the intrusive complexes is that each is spatially associated with volcanic rocks of similar compositional character, and that these relationships may occur over long periods of time within local regions (e.g. Goonumbla Volcanic Complex), or show progressive compositional evolution over probably shorter time periods as in the case of the Cadia District. The igneous suites are variably enriched in K and LILE elements, with uniformly high K/Rb ratios, low mantlecompatible element abundances and marked depletions in Ti, Nb and Ta, which is consistent with a subductionrelated tectonic setting. Patterns of enrichment and depletion between each suite are similar, and are consistent with the magmas having been derived from sources variably enriched by slab derived materials prior to fusion. Recent dating results from the Goonumbla Volcanic Complex suggest that this enrichment was not progressive over time, and that the oldest intrusions in the Ordovician are not necessarily the least compositionally evolved. These results confirm the overall 'calcalkaline' nature of Ordovician magmatism and do not support models that argue against contemporaneous subduction on the grounds that Ordovician magmatism was almost exclusively shoshonitic in character. Economic size and significance of associated deposits increase with the degree of K-enrichment (but not total alkali contents) observed in the associated igneous complexes, and supports conceptual models that link mineralisation potential and fertility with processes related to the production of K-enriched protoliths.

Dwarf copper-gold porphyry deposits of the Buchim-Damjan-Borov Dol ore district, Republic of Macedonia (FYROM)

“…(I) Late Permian-Early Triassic intracontinental rifting along the northern boundary of Gondwana and/or its already detached fragments; (2) Jurassic intracontinental rifting marked by ophiolitic complex; (3) suprasubduction magmatism; and (4) postcollision (orogenic) continental setting and related formation of various base-and noble-metal deposits (Pb-Zn-Ag, Cu-Au, Au-As-Sb-TI, Au Ag).…”

“…It should be noted that the rocks of shoshonitic series are typical of districts with porphyry copper mineralization at continental margins, e.g., Bajo Alumbrera, Gunumba, Bingham, Ok-Tedi, Grasberg, Cadia, etc. (Miller and Groves, 2000;Blevin, 2002 16. Trace element contents (Table 2) indicate magma fractionation in the course ofits evolution.…”

Geochemistry and age of magmatic rocks in the unexposed Narromine, Cowal and Fairholme Igneous Complexes in the Ordovician Macquarie Arc, New South Wales

“…Although most voluminous in the Junee -Narromine Volcanic Belt, especially in the Cowal Igneous Complex, high-level intrusive dacites and associated dioritic to granodioritic rocks are known from the Molong Volcanic Belt from the type locality at Copper Hill (Blevin 2002) Data presented by Blevin (2002) and demonstrate that the Copper Hill Suite has medium-K calc-alkaline affinities, with compositions extending from cumulate hornblende gabbros to granodiorite, although the average composition is probably dioritic. The abundance of hornblende in the cumulate mafic rocks suggests that they probably crystallised from andesitic rather than basaltic magmas.…”

Magmatic characteristics and geochronology of Ordovician igneous rocks from the Cadia – Neville region, New South Wales: implications for tectonic evolution

“…The shoshonitic compositions of these Ordovician volcanic and intrusive rocks have been used to support tectonic models that involve magmatism either unrelated to subduction (Wyborn 1992) or accompanied by coeval subduction Blevin 2002;Holliday et al 2002). Understanding the significance of the shoshonitic magmatism is, therefore, critical for interpretation of the Ordovician tectonic setting.…”