Session 5 Epigenetic gold systems Chapter 5-1 5-1 Geochemical and isotopic constraints on Palaeozoic orogenic gold endowment and crustal evolution of the south central Andes, NW Argentina Frank P. Bierlein School of Geoscience, Monash University, Melbourne, VIC 3800, Australia, and Tectonics Special Research Centre, The Uni- versity of Western Australia, Crawley, WA 6009, Australia Beatriz Coira CONICET and Instituto de Geología y Minería, Universidad Nacional de Jujuy, 4600 San Salvador de Jujuy, Argentina Holly Stein AIRIE Program, Department of Earth Resources, Colorado State University, Fort Collins, CO 80523-1482 USA Abstract. Major and trace element concentrations, and Nd isotope spreading ridges and/or mantle plumes, play important compositions of Ordovician volcano-intrusive rocks in the Puna of roles in the ore-forming process (Bierlein and Crowe 2000). northwestern Argentina point to their formation in a pericratonic Thus, it can be hypothesised that variations in the vol- setting above thickened and evolved continental crust. Input from juvenile sources was limited and there is no evidence for oceanic ume and composition of mafic rocks underlying a meta- crust or the presence of a terrane-bounding suture in the region. In sedimentary succession represent key elements in the eco- contrast to analogous terrains elsewhere in the Central Andes (e.g., nomic evaluation of a given fold belt. in Peru and Bolivia), relatively minor orogenic gold mineralisation In this study, we report results of petrogenetic and iso- is known to occur in the Palaeozoic succession in northwestern Ar- topic investigations aimed at characterising and finger- gentina. Possible explanations for the lack of more substantial oro- genic gold endowment include the absence of hydrated oceanic printing mafic intrusives, related volcanic rocks, and substrate in the Puna, lack of 1st-order conduits and the limited ex- metasedimentary rocks in the Puna Argentina. In addi- tent of crustal shortening and thickening during Ordovician oro- tion to providing insight into the tectonic evolution of genic events. Palaeozoic northwestern Argentina, these findings may allow for a more accurate assessment of gold endowment, Keywords. Orogenic gold, Puna Argentina, Re-Os, endowment, petro- genesis fertility and future potential of this region, and contrib- ute to an improved understanding of the processes that control the genesis of orogenic gold deposits. 1 Introduction 2 Geological framework of orogenic gold deposits Orogenic gold mineralisation is developed within virtu- in the Puna Argentina ally all Palaeozoic metamorphic belts of the Pacific Rim (e.g., Lachlan Orogen, SE Australia; Hodgkinson-Broken Gold deposits in the Sierra de Rinconada in the Puna of River Province, NE Australia; Mongol-Zabaikal belt, Rus- northwestern Argentina are hosted by low-grade meta- sian Far East; Buller Terrane, New Zealand; Eastern morphosed and upright folded turbidites of mid-Ordovi- Andean Cordillera in Peru, Bolivia and Argentina; cian age in the Ocloyic fold belt (Rodriguez and Bierlein Goldfarb et al. 2001). In each of these terrains, orogenic 2002). Wallrock alteration associated with the predominantly gold mineralisation formed in response to crustal short- laminated- to massive vein-hosted deposits is characterised ening and thickening, and from hydrothermal fluid sys- by chloritisation, sericitisation, (de)silicification, and tems that operated (intermittently) throughout uplift and sulphidation. Notably, carbonatisation is only very weakly exhumation of the orogenic belts. However, within this developed. A number of hard-rock deposits are known in geological framework significant variations exist to the the Palaeozoic of northern Argentina, with gold produc- overall endowment in orogenic gold deposits and prov- tion typically dating back to Incaic times, but production inces. These variations are likely to be influenced and con- figures are not recorded. Since 1890 only minor produc- trolled by geological, structural and physico-chemical fac- tion has come from several deposits characterised by rich tors that operate on the province scale. The spatial asso- accumulations of very coarse, vein-hosted gold, with the ciation of major orogenic gold deposits with terrane- vast majority of gold produced from about 20 deposits in bounding sutures, and meta-sedimentary rocks that are the Sierra de Rinconada having come from placer work- typically interbedded with ‘primitive’ oceanic crust, im- ings. The total production of about 2 tonnes from this plies that these rock sequences, as well the subduction of part of Eastern Andean Cordillera is in stark contrast to 522 Frank P. Bierlein · Beatriz Coira · Holly Stein the > 160 tonnes of turbidite- and granite-hosted gold genetic characteristics of 15 volcano-intrusive rock units that have come from the Pataz Province in northern Peru, from the Puna Argentina in an attempt to i) constrain and the ~15 tonnes of placer gold from southeastern Peru the nature of the substrate beneath the Ordovician-De- and central-southern Bolivia (Haeberlin et al. 2002). In vonian sedimentary succession, ii) assess evidence for sharp contrast to these belts in the Eastern Andean Cor- crustal growth along the Pacific margin of Gondwana, dillera, as well as many other metamorphic terrains host- and iii) consider the presence of an obscured terrane ing orogenic gold mineralisation, there is no direct evi- boundary/suture in the south central Andes of northwest- dence of syn- or post-tectonic magmatism in the Sierra ern Argentina. Crustal growth would be indicated by a de Rinconada (e.g. Coira et al. 1999). pronounced contrast in petrogenetic character of sampled material across the Puna and mark the appearance of ju- 3 Crustal evolution of the Puna Argentina - venile or exotic sources across a lithospheric break. Con- insights from whole-rock and Sm-Nd data versely, homogenous data from volcano-intrusive rocks could indicate their derivation from a common magmatic The geodynamic setting and tectonic evolution of the source, with those of sedimentary rocks implying recy- Palaeozoic of northwestern Argentina is poorly con- cling of pre-existing material. If Ordovician volcano-in- strained, but formation of the Puna turbidite complex is trusive and sedimentary rocks in the Puna originated in inferred to have occurred in response to the change from a setting involving active subduction and arc magmatism, an extensional to a foreland basin setting in the Middle the juvenile nature of the material should be reflected by Ordovician times (Bahlburg 1990). Very little is known trace elements and radiogenic Nd isotope signatures. about the nature of the basement underlying the Puna Ordovician metasedimentary rocks from the Puna turbidite complex. Therefore, we determined the petro- Argentina are characterised by acidic arc signatures, with Chapter 5-1 · Geochemical and isotopic constraints on Palaeozoic orogenic gold endowment and crustal evolution of the south central Andes, NW Argentina 523 La/Th ratios close to the field of upper continental crust. provenance (as discussed above), a crustal initial ratio of They have Th/Sc ratios of about 1 and trace element char- ~0.9 can be assumed. Sample Py-03 from the Minas Azules acteristics that are consistent with upper crustal deriva- has Re (2.5 ppb) and Os (22.3 ppt) that can be well mea- tion. Rare earth element patterns of all samples are also sured. Its 187Re/188Os ratio is 1054 and a model age with similar to the pattern of upper crust with light rare earth an initial of 0.9 yields approximately 360 Ma. However, enrichment, pronounced negative Eu anomalies and rela- the error on this model age is rather large at ± 50 Ma. tively flat heavy rare earth patterns. On the basis of Hf- Sample Py-06 from the El Torno deposit has notably higher Th-Ta and Zr-Nb-Y relations, the volcano-intrusive rocks Re (5 ppb) with low Os (34 ppt). It yields a 187Re/188Os have implied sources that range from alkaline within- ratio of 2630 and a reasonably well-constrained calcu- plate-type to back-arc-like chemistry. The majority of the lated model age of 440 ± 18 Ma, corresponding to the samples are characterised by moderate to steep REE slopes, timing of the Ocloyic orogeny. Field evidence supports low La/Ta ratios and weakly elevated Th and U concen- the notion of multiple ore genesis across the Sierra trations. These patterns are consistent with the forma- Rinconada in the Palaeozoic (Rodriguez and Bierlein tion of Ordovician volcano-intrusive rocks in the Puna in- 2002), but further geochronological data are required to volving a transition from a waning arc regime to a setting ascertain the age(s) of orogenic gold formation. dominated by oblique convergence or strike-slip tectonics, as proposed by Coira et al. (1999). In such a setting, limited 5 Make-up of the orogen - a possible explanation crustal melting could occur along sub-parallel fault zones for low gold endowment? cutting attenuated continental crust, with mantle decom- pression giving rise to alkaline magmatism along short- Whole-rock data and Nd isotopic compositions of Or- lived pull-apart basins in the former back-arc. dovician volcano-intrusive and sedimentary rocks do not EpsilonNd (450 Ma) isotope compositions for Ordovi- support a tectonic setting that involved significant crustal cian volcano-intrusive rocks from the northern Puna growth during the Palaeozoic in the south central Andes range from 7.4 to -6.4, with samples of silicic units in the of northwestern