Mineralium Deposita (v.47, #4)
A unified model for gold mineralisation in accretionary orogens and implications for regional-scale exploration targeting methods by Jon M. A. Hronsky; David I. Groves; Robert R. Loucks; Graham C. Begg (339-358).
Accretionary orogens are the sites of long-lived convergent margin tectonics, both compressional and extensional. They are also the hosts to the majority of the world’s important gold deposits. A very diverse range of deposit types occurs within accretionary orogens, commonly in close proximity in space and time to each other. These include porphyry and associated high-sulphidation Au–Cu–Ag deposits, classic low-sulphidation Au–Ag deposits, low-sulphidation Au deposits centred on alkalic intrusive complexes, Carlin-type Au deposits, Au-rich volcanic-hosted massive sulphide deposits, orogenic Au deposits, intrusion-related Au deposits and iron oxide Cu–Au deposits. Empirical patterns of spatial distribution of these deposits suggest there must be fundamental generic controls on gold metallogeny. Various lines of evidence lead to the proposal that the underlying key generic factor controlling accretionary orogen gold metallogeny is regional-scale, long-term, pre- and syn-subduction heterogeneous fertilisation of the lithospheric mantle that becomes a source of mineralisation-associated arc magma or hydrothermal fluid components. This process provides a gold-enriched reservoir that can be accessed later in a diverse range of tectonomagmatic settings. Based on this concept, a unified model is proposed in which the formation of a major gold deposit of any type requires the conjunction in time and space of three essential factors: a fertile upper-mantle source region, a favourable transient remobilisation event, and favourable lithospheric-scale plumbing structure. This framework provides the basis for a practical regional-scale targeting methodology that is applicable to data-poor regions.
The evolution of the hydrothermal IOCG system in the Mantoverde district, northern Chile: new evidence from microthermometry and stable isotope geochemistry by Ana A. Rieger; Robert Marschik; Manuel Díaz (359-369).
New microthermometric data combined with stable isotope geochemistry and paragenetic relationships support a previously suggested cooling–mixing model for the iron oxide–copper–gold mineralization in the Mantoverde district. Fluid inclusions show characteristics of a CO2-bearing aqueous NaCl ± CaCl2 salt system. The evolution of the Mantoverde hydrothermal system is characterized by (1) an early hypersaline, high to moderate temperature fluid; (2) a moderate saline, moderate temperature fluid; and (3) a low saline, moderate to low temperature fluid. Early magnetite formation took place at median temperatures of 435.0°C, whereas hematite formed at median temperatures of 334.4°C. The main sulfide mineralization texturally post-dates the iron oxides and occurred before late-stage calcite, which developed at a median temperature of 244.8°C. Boiling occurs only locally and is of no relevance for the ore formation. The microthermometric and stable isotope data are supportive for a fluid cooling and mixing model, and suggestive for a predominantly magmatic–hydrothermal fluid component during the iron oxide and main sulfide mineralization. Thereafter, the incursion of a nonmagmatic fluid of ultimately meteoric or seawater gains more importance.
Keywords: Fluid inclusions; Copper–gold; Fluid mixing; C–O isotopes; Coastal Range
The masquerade of alkaline–carbonatitic tuffs by zeolites: a new global pathfinder hypothesis by L. S. Campbell; A. Dyer; C. Williams; P. R. Lythgoe (371-382).
Rapid and progressive reaction of alkaline–carbonatitic tuffs with magmatic and crustal fluids disguises their initial character and origin. This is collectively indicated from (a) the extensive literature on zeolite formation from volcanic glass precursors and alkaline fluids, (b) mineralogical characteristics of specific zeolite species, (c) a comparative review of global distributions of alkaline–carbonatite suites and of zeolite minerals, and (d) new trace element data from zeolite samples. A unifying conceptual model based on tectonic and geological settings, hydrological regime and mineralogy is presented that helps to explain the global distributions and current understanding of occurrences. The model will assist in resource exploration by contributing deeper understanding of the economically important bedded zeolite deposits and further, serve as a guide to the discovery of new alkaline–carbonatitic suites, potentially of economic significance (metallic ores and rare earth elements). It follows that future testing of the hypothesis will impact on models of natural carbon cycling as volcanic contributions of CO2 are reviewed.
Keywords: Natural zeolites; Alkaline magmatism; Carbonatites; Mineral reactivity; Trace elements
Metallogenic features of Miocene porphyry Cu and porphyry-related mineral deposits in Ecuador revealed by Re-Os, 40Ar/39Ar, and U-Pb geochronology by Philip Schütte; Massimo Chiaradia; Fernando Barra; Diego Villagómez; Bernardo Beate (383-410).
Mineralization and alteration events at ten Miocene porphyry Cu and porphyry-related epithermal mineral deposits in southern, central, and northern Ecuador were dated by means of molybdenite Re-Os, biotite and alunite 40Ar/39Ar, and titanite U-Pb geochronology. Most of these hydrothermal events show a spatio-temporal correlation with porphyry intrusion emplacement as constrained by zircon U-Pb ages. The total age range for these events spans the 23.5–6.1 Ma period, without displaying systematic along- or across-arc age distribution trends. While epithermal deposits tend to be spatially associated with volcanic rocks of a similar age, porphyry Cu deposits in Ecuador are frequently spatially associated with deeper-seated basement units and batholith-scale precursor intrusive systems assembled over ≥5 m.y. time periods. In most cases, formation of the porphyry Cu deposits is related to the youngest magmatic (-hydrothermal) event in a given area, postdating batholith construction at a regional scale. The majority of Miocene deposits occurs in southern Ecuador where areally extensive, post-mineralization (late Miocene to recent) volcanic sequences with the potential to conceal mineralization at depth are lacking. Only few Miocene deposits occur in northern-central Ecuador, where they mainly crop out in the Western Cordillera, west of the productive present-day volcanic arc. The surface distribution of post-mineralization arc volcanism reflects along-arc variations in subducting slab geometry. Porphyry Cu and epithermal deposits in Ecuador define a Miocene metallogenic belt broadly continuous with its coeval counterpart in northern-central Peru. Although both belt segments were formed in an overall similar tectonomagmatic and metallogenic setting, their respective metal endowments differ significantly.
Keywords: Mineral deposits; Porphyry Cu; Miocene metallogenic belt; Ecuador; Mineralization; Alteration; Geochronology
Talc mineralisation associated with soft hematite ore, Gongo Soco deposit, Minas Gerais, Brazil: petrography, mineral chemistry and boron-isotope composition of tourmaline by Alexandre Raphael Cabral; Michael Wiedenbeck; Francisco Javier Rios; Antônio Augusto Seabra Gomes Jr; Orlando Garcia Rocha Filho; Richard David Jones (411-424).
Talc mineralisation occurs as hematite–talc schist between soft hematite ore and dolomitic itabirite at Gongo Soco, Quadrilátero Ferrífero of Minas Gerais, Brazil. The hematite–talc schist and soft hematite have a prominent tectonic foliation of tabular hematite. Tabular hematite without preferential orientation is superimposed on the tectonic foliation. The talcose schist is enriched in F and has a constant Fe/S ratio. Electron-microprobe analyses indicate trace amounts of S in different generations of hematite. The whole-rock Fe/S ratio possibly represents sulfate S from hematite-hosted fluid inclusions. Fluid inclusions in foliation-overprinting hematite and chlorite geothermometry from talcose rocks suggest, respectively, temperatures from <200°C to ~300°C. Tourmaline, a rarely observed mineral in the hematite–talc schist, belongs to the alkali group and falls in the dravite compositional field. Boron-isotope determinations of tourmaline crystals, using secondary ion mass spectrometry, vary from −20‰ to −12‰ δ11B. This compositional isotopic range and the tourmaline chemical composition suggest a meta-evaporitic origin. A non-marine evaporitic setting is the most likely source of acidic, highly oxidising fluids, which resulted in the abundant F-bearing talc and the presence of otherwise immobile Ti in hematite. Oxidising brines were channelled along shear zones and converted dolomitic itabirite into the Gongo Soco soft hematite and the talc mineralisation. The latter is envisaged as the hydrothermal wall-rock alteration of dolomitic itabirite, which gave rise to the soft hematite ore.
Keywords: Talc; Hematite; Mineral chemistry; SIMS; Boron isotopes; Gongo Soco; Quadrilátero Ferrífero; Minas Gerais; Brazil
Geological setting and timing of the Chah Zard breccia-hosted epithermal gold–silver deposit in the Tethyan belt of Iran by Hossein Kouhestani; Majid Ghaderi; Khin Zaw; Sebastien Meffre; Mohammad Hashem Emami (425-440).
The breccia-hosted epithermal gold–silver deposit of Chah Zard is located within a high-K, calc-alkaline andesitic to rhyolitic volcanic complex in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA), west central Iran. The total measured resource for Chah Zard is ∼2.5 million tonnes of ore at 12.7 g/t Ag and 1.7 g/t Au (28.6 t Ag, 3.8 t Au), making it one of the largest epithermal gold deposits in Iran. Magmatic and hydrothermal activity was associated with local extensional tectonics in a strike-slip regime formed in transtensional structures of the Dehshir-Baft strike-slip fault system. The host rocks of the volcanic complex consist of Eocene sedimentary and volcanic rocks covered by Miocene sedimentary rocks. LA-ICP–MS U–Pb zircon geochronology yields a mean age of 6.2 ± 0.2 Ma for magmatic activity at Chah Zard. This age represents the maximum age of mineralization and may indicate a previously unrecognized mineralization event in the UDMA. Breccias and veins formed during and after the waning stages of explosive brecciation events due to shallow emplacement of rhyolite porphyry. Detailed systematic mapping leads to the recognition of three distinct breccia bodies: volcaniclastic breccia with a dominantly clastic matrix; gray polymict breccia with a greater proportion of hydrothermal cement; and mixed monomict to polymict breccia with clay matrix. The polymictic breccias generated bulk-mineable ore, whereas the volcaniclastic breccia is relatively impermeable and largely barren. Precious metals occur with sulfide and sulfosalt minerals as disseminations, as well as in the veins and breccia cements. There is a progression from pyrite-dominated (stage 1) to pyrite-base metal sulfide and sulfosalt-dominated (stages 2 and 3) to base metal sulfide-dominated (stage 4) breccias and veins. Hydrothermal alteration and deposition of gangue minerals progressed from illite-quartz to quartz-adularia, carbonate, and finally gypsum-dominated assemblages. Free gold occurs in stages 2 and 4, principally intergrown with pyrite, quartz, chalcopyrite, galena, sphalerite, and Ag-rich tennantite–tetrahedrite, and also as inclusions in pyrite. High Rb/Sr ratios in ore-grade zones are closely related to sericite and adularia alteration. Positive correlations of Au and Ag with Cu, As, Pb, Zn, Sb, and Cd in epithermal veins and breccias suggest that all these elements are related to the same mineralization event.
Keywords: Epithermal gold; Breccia; U–Pb geochronology; Hydrothermal alteration; Chah Zard; Iran
The roles of magmatic and hydrothermal processes in PGE mineralization, Ferguson Lake deposit, Nunavut, Canada by Nelson O. Campos-Alvarez; Iain M. Samson; Brian J. Fryer (441-465).
The Ferguson Lake Ni–Cu–Co–platinum-group element (PGE) deposit in Nunavut, Canada, occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites. Massive to semi-massive sulfide occurs toward the structural hanging wall of the metagabbro, and a low-sulfide, high-PGE style of mineralization (sulfide veins and disseminations) locally occurs ~30–50 m below the main massive sulfide. Water–rock interaction in the Ferguson Lake Ni–Cu–Co–PGE deposit is manifested mostly as widespread, post-metamorphic, epidote–chlorite–calcite veins, and replacement assemblages that contain variable amounts of sulfides and platinum-group minerals (PGM). PGM occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high-PGE zones, at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide, in undeformed sulfide veins and adjacent chlorite and/or epidote halos, in hornblende adjacent to hydrothermal veins, and in plagioclase–chlorite aggregates replacing garnet cemented by sulfide. The PGM are mostly represented by the kotulskite (PdTe)–sobolevskite (PdBi) solid solution but also include michenerite (PdBiTe), froodite (PdBi2), merenskyite (PdTe2), mertieite II (Pd8[Sb,As]3), and sperrylite (PtAs2) and occur in variety of textural settings. Those that occur in massive and interstitial sulfides, interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures <600°C, are dominantly Bi rich (i.e., Te-bearing sobolevskite), whereas those that occur in late-stage hydrothermal sulfide/silicate veins and their epidote–chlorite alteration halos tend to be more Te rich (i.e., Bi-bearing kotulskite). The chemistry and textural setting of the various PGM supports a genetic model that links the magmatic and hydrothermal end-members of the sulfide–PGM mineralization. The association of PGM with magmatic sulfides in the massive sulfide and high-PGE zones has been interpreted to indicate that PGE mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization. However, the occurrence of PGM in undeformed sulfide-bearing veins and in their chlorite–epidote halos and differences in PGM chemistry indicate that hydrothermal fluids were responsible for post-metamorphic redistribution and dispersion of PGE.
Keywords: Ferguson Lake; Nunavut (Canada); Hydrothermal remobilization; Magmatic sulfides; Ni–Cu–Co–PGE deposit