Evolution of magmatic fertility for porphyry Au & Cu deposits through the prism of zircon chrono-chemistry, Balkan Peninsula, SE Europe

Ireland, Timothy1; Bilyarska Teodora2, Bilyarski, Stoimen1, Protic, Nenad1, and Stefanova, Elitsa3

1First Quantum Minerals Ltd,, Vancouver, Canada, 2St Kliment Ohridski University of Sofia, Sofia, Bulgaria, 3Bulgarian Academy of Science – Geological Institute, Sofia, Bulgaria

The accreted arc terranes of the Balkan western Tethyan host a complex metallogeny that includes almost all Cu and Au mineralisation styles typical of subduction-related arcs and post-collisional settings. Broadly, magmatism in the belt began as a series of submarine arcs constructed on thinned continental basement, and evolved to subaerial post-collisional magmatism during and after arc-continent collisions in the Late Cretaceous and Eocene. Each collisional event is associated with a phase of metallogenesis that changes in terms of metal budget and deposit style as collision occurs and then collapses, e.g. in the Timok and Panagjurishte districts, three phases of magmatism, each lasting 5-10 my. record a transition in space and time from sub-volcanic diorites associated with porphyry Cu-Au deposits, to more voluminous effusive andesites associated with epithermal deposit styles, and then monzonitic intrusions with a Au-dominant metal budget. The rocks from all of these three phases are the products of hydrous, intermediate magmas that yield equivalent ‘fertile-looking’ results in terms of whole rock magma chemistry proxies such as Sr/Y that explorers may use for area selection in arc environments.

A similar metallogenic progression occurs among less well-documented magmatism that occurred subsequent to the Alpine orogeny. In this case the main, early metallogenic phase is represented by porphyry and epithermal Cu-Au deposits associated with trachyandesite and monzonitic volcano-plutonic complexes. As post-collisional extension comes to dominate the regional tectonic environment the metallogeny becomes dominated by Pb-Zn in a series of ‘ore fields’ that we infer to be genetically related to syn-extensional granitoids, and thereafter by epithermal gold deposits in terrestrial grabens.

In this study we compiled all the available published whole rock chemistry and zircon chronology and chemistry for magmatic rock units associated with each of these metallogenic stages, and collected new complementary data from rock samples and stream sediments. The result is a coherent record of the temporal evolution of magmas associated with each of these metallogenic stages. We observe that the detrital zircon populations are closely related to the results from rock samples, however, we contend that the detrital record is a superior medium as it avoids sampling bias and undersampling. These detrital results were interpreted as a holisitic record of the magmatism in each district. There are patterns among this zircon chrono-chemical evolution that correspond to consistent aspects of regional metallogeny. Short periods of zircon crystallisation (i.e. <3 my) in which zircons span a wide range of compositions suggesting both primitive and enriched or evolved source contributions tend to characterise the metallogenic events in which Au is the primary economic commodity. In contrast, more protracted zircon crystallisation history and slower evolution to fertility implied by proxies such as Eu/Eu* characterises the major porphyry Cu camps. This approach may be applicable in other terranes wherein the magmatic-metallogenic history is not well constrained.


Biography

I’m a jack-of-all-trades explorer with experience in porphyry-epithermal systems, carlin-type deposits, sed-hosted Cu and sed-hosted Zn. I’m currently the Principal Exploration Geologist – and informal resident geochemist – at FQM. I did my postgrad research at CODES, and have spent much of my career working in E Europe and the central African copperbelt.  

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