Franco Pirajno

Hydrothermal Processes and Mineral Systems

Foreword by Peter A. Cawood Acknowledgements. 1. Water and hydrothermal fluids on Earth 2. Hydrothermal processes and wall rock alteration 3. Tectonic settings, geodynamics and temporal evolution of hydrothermal mineral systems 4. Intrusion-related hydrothermal mineral systems 5. Porphyry systems fossil and active epithermal systems 6. Skarn systems 7. Submarine hydrothermal mineral systems 8. Metalliferous sediments and sedimentary rock-hosted stratiform and/or stratabound hydrothermal mineral systems 9. Orogenic, amagmatic and hydrothermal mineral systems of uncertain origin 10. Hydrothermal systems and the biosphere 11. Hydrothermal processes associated with meteorite impacts 12. Hydrothermal processes and systems on other planets and satellites 13. Uranium hydrothermal mineral systems References. Index

Warakurna large igneous province: A new Mesoproterozoic large igneous province in west-central Australia

Coeval mafic igneous rocks emplaced rapidly over;1.5 3 10 6 km 2 in western and central Australia represent the erosional rem- nants of a late Mesoproterozoic large igneous province, named here the Warakurna large igneous province. SHRIMP U-Pb dating of rocks separated by as much as 1500 km indicates that the main episode of magmatism occurred between 1078 and ca. 1070 Ma. The Warakurna large igneous province includes layered mafic- ultramafic intrusions and mafic to felsic volcanic rocks and dikes in central Australia, a 1000-km-long mafic sill province in Western Australia, and several swarms of mafic dikes. The large areal ex- tent and short duration imply emplacement above a mantle-plume head. Despite their wide separation, the mafic rocks have similar mid-oceanic-ridge basalt-normalized trace element patterns and rare earth element characteristics. West-directed paleocurrents, westward-radiating dike swarms, and the occurrence of high-Mg rocks indicate that the center of the plume head was located be- neath central Australia. Other late Mesoproterozoic large igneous provinces, in the Laurentia and Kalahari cratons, appear to be significantly older than the Warakurna large igneous province in Australia and thus are unlikely to be related to the same mantle- plume head.

Late Jurassic–Early Cretaceous granitoid magmatism in Eastern Qinling, central-eastern China: SHRIMP zircon U–Pb ages and tectonic implications

The Eastern Qinling region, located in the southern margin of the North China Craton, is characterised by widespread granitoid intrusions. Precise geochronological constraints on the age and timing of these intrusions are lacking. In this paper, we report SHRIMP zircon U–Pb ages of 12 representative granitoid plutons, together with one syenite stock, dolerite, diorite and granitic dykes in Eastern Qinling. The results revealed two main magmatic events, which occurred in the Late Jurassic–Early Cretaceous (158 ± 3 to 136 ± 2 Ma) and the Early Cretaceous (134 ± 1 to 108 ± 2 Ma), respectively. The granitoids formed in the early magmatic event are similar in composition and are characterised by a mantle source mixed with variable amounts of crustal components, whereas those resulting from the late-stage magmatism show characters of I-, S- and A-types granite and coexist with coeval dolerite dykes and syenite stocks. A similar magmatic age distribution is also recognised in other belts or regions in East China, and even in other parts of the East Eurasian continental margin. This suggests that the Jurassic–Early Cretaceous magmatism was associated with the subduction of the Izanagi plate at a shallow angle or flat-slab subduction beneath the eastern China continent, whereas the Early Cretaceous magmatism was related to lithospheric thinning, asthenospheric upwelling and partial melting of the lower crust, induced by a change in Izanagi plate motion parallel to the continent margin.

Ore deposits and mantle plumes

Preface. Acknowledgments. Introduction. Part One. 1. The Earths Internal Structure and Convection in the Mantle. 2. Mantle Plumes and Superplumes, Continental Breakups, Supercontinent Cycles and Ore Deposits. 3. Oceanic Islands, Large Igneous Provinces, Mafic Dyke Swarms, and Intracontinental Alkaline Magmatism. 4. Rifting Processes, Volcano-Sedimentary Basins and the Role of Mantle Plumes. 5. The Planetary and Meteorite Impact Context of Mantle Plumes. Part Two. 6. Intra-Continental Magmatism, Anorogenic Metamorphism, Ore Systems and Mantle Plumes. 7. Direct Links: Magmatic Ore Deposits - Fundamental Features and Concepts. 8. Magmatic Ore Deposits. 9. Indirect Links: Hydrothermal Mineral Deposits. 10. Indirect Links: Sedimentary Rock-Hosted Ore Deposits. Epilogue. Appendix. Index.

Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits: implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia

Relatively unaltered metabasalts of the Archaean Coonterunah and Warrawoona Groups, Pilbara Craton are compared with altered metabasalts from immediately beneath bedded cherts of these groups to provide evidence for the depositional environment and hydrothermal alteration processes of crust formation. The geochemistry of relatively unaltered basalt, stratigraphy, and inherited zircon data indicate that the lower Warrawoona Group (3.53–3.43 Ga) formed as an oceanic plateau complex built on a sialic basement to 3.724 Ga, following an analogue with the Phanerozoic Kerguelen oceanic plateau, and not as a mid-ocean ridge or convergent volcanic-arc complex as previously proposed. Advanced argillic, argillic, phyllic, and propylitic alteration zones in footwall basalts of this succession are products of repeated episodes of seafloor hydrothermal circulation, syngenetic with bedded chert deposition, in the distal parts of high-sulphidation epithermal systems. The upper part of the Warrawoona Group (3.350–3.315 Ga Euro Basalt) represents a continental flood basalt event, up to 8 km thick, that erupted onto the older succession across a regional unconformity on which the Strelley Pool Chert was previously deposited. Widespread silica–alunite alteration of dolomitic chert protoliths and phyllic and propylitic alteration of footwall basalts are interpreted as products of fluid circulation driven by heat from the overlying, newly erupted lavas.

Origin of gold metallogeny and sources of ore-forming fluids, Jiaodong Province, eastern China

In this paper we use published isotopic ages for gold deposits and related rocks in the Jiaodong Peninsula (East Shandong Province) to investigate the origin of the large-scale gold metallogeny in the region, which contains world-class lode gold deposits. According to this database, metallogenic processes in this area occurred in the Mesozoic, with peak activities between 110 Ma and 130 Ma. In the Jiaodong gold province (JGP), the mineralizing events are coeval with or postdate Mesozoic granitoid intrusions. Both the Rb-Sr isochron ages and zircon SHRIMP age dating results suggest that Mesozoic granitoids were emplaced during several thermal events. The identification of inherited zircons coupled with ISr ratios (>0.709) indicate that these granitoids were mainly sourced from the continental crust, by remelting or partial melting. I Sr values obtained from ores and fluid inclusions are generally higher than 0.709, and slightly higher than those for Mesozoic granitoids. This also indicates that both ore fluids and metals were mainly sourced from the crust. Synthesis of the available data suggests that collision between the South and North China continents was probably the dominant factor responsible for the gold metallogeny in the JGP. Granitoid emplacement and large-scale gold metallogenesis can be related to three important stages in the geodynamic evolution of a collisional orogen (compression—crustal thickening—uplift, lithospheric delamination and transition to extension, and a final extension phase). The most important metallogenic phase occurred at the transition from collisional compression to extension tectonics.

Evidence for early life in Earth’s oldest hydrothermal vent precipitates

Although it is not known when or where life on Earth began, some of the earliest habitable environments may have been submarine-hydrothermal vents. Here we describe putative fossilized microorganisms that are at least 3,770 million and possibly 4,280 million years old in ferruginous sedimentary rocks, interpreted as seafloor-hydrothermal vent-related precipitates, from the Nuvvuagittuq belt in Quebec, Canada. These structures occur as micrometre-scale haematite tubes and filaments with morphologies and mineral assemblages similar to those of filamentous microorganisms from modern hydrothermal vent precipitates and analogous microfossils in younger rocks. The Nuvvuagittuq rocks contain isotopically light carbon in carbonate and carbonaceous material, which occurs as graphitic inclusions in diagenetic carbonate rosettes, apatite blades intergrown among carbonate rosettes and magnetite–haematite granules, and is associated with carbonate in direct contact with the putative microfossils. Collectively, these observations are consistent with an oxidized biomass and provide evidence for biological activity in submarine-hydrothermal environments more than 3,770 million years ago.

Gold and silver metallogeny of the South China Fold Belt: a consequence of multiple mineralizing events?

Abstract The South China Fold Belt is part of the South China Block that is interpreted to be the result of multiple tectonic and magmatic events that formed a collage of accreted Proterozoic and Phanerozoic terranes. The Jurassic to early Cretaceous Yanshanian period (180–90 Ma), a time of major tectono-thermal events that affected much of eastern and southeastern China, is of great metallogenic importance in the fold belt. This period is linked to subduction of the Pacific plate beneath the Eurasian continent, and is manifested by voluminous volcano-plutonic activity of predominantly calc-alkaline affinity. The distribution of gold and silver deposits in the South China Fold Belt indicates the presence of two distinct metallogenic provinces. A region of basement uplifts, which are controlled by shear zones and form Neoproterozoic inliers of metamorphosed iron-rich rock types, defines the first province. In this province, orogenic lodes and volcanic-related epithermal deposits represent the more significant precious-metal mineralization. The second province is essentially confined to a belt of Yanshanian felsic–intermediate volcanic and subvolcanic rocks that extends along most of the southeastern China coast in an area known as the Coastal Volcanic Belt. Deposits in the Coastal Volcanic Belt are silver- and/or copper-rich, volcanic-hosted and epithermal in character. The precious-metal metallogeny of the South China Fold Belt is interpreted to have developed in at least three stages: one as a result of collision events, during the Caledonian Orogeny (ca. 400 Ma), the second during the Indosinian Orogeny (ca. 200 Ma) and the third during or soon after the formation of the Yanshanian magmatic belt (Yanshanian Orogeny; 180–90 Ma). The latter was responsible for a hydrothermal event that affected large sections of the belt and its Proterozoic substrate. This may have resulted in the redistribution and enrichment of precious metals from preexisting orogenic gold lodes in Neoproterozoic basement rocks, which are now exposed as windows in zones of tectonic uplift. The Yanshanian hydrothermal activity was particularly widespread in the Coastal Volcanic Belt and resulted in the formation of both low- and high-sulfidation epithermal gold and silver, and locally copper and other base-metal mineralization. It is suggested that the Coastal Volcanic Belt has greater potential for world-class epithermal and porphyry deposits than previously realised.

Woodleigh, Carnarvon Basin, Western Australia: a new 120 km diameter impact structure

Abstract The Woodleigh multi-ring structure, buried by Cretaceous and, at its centre, Lower Jurassic lacustrine sediments, east of Hamelin Pool, Carnarvon Basin, Western Australia, is identified as an impact structure, the largest discovered to date on the Australian continent. An impact origin is indicated by: a central core of uplifted granitoid basement probably less than 25 km in diameter, which displays shock-induced planar deformation features in quartz, pervasive diaplectic vitrification of feldspar and penetrative pseudotachylite veining; and an inner ring syncline containing a ∼70 m thick thermally modified diamictite overlain by ∼380 m of Lower Jurassic lacustrine deposits. An outermost diameter of 120 km, defined by gravity, magnetic and surface drainage, indicates a ring fault that sharply intersects the NS-striking regional structure. At the centre of the basement uplift shock metamorphosed granitoid was intersected at a depth of 171 m, at least 1800 m higher than the gravity-modelled level of regional basement. Pseudotachylite vein systems within the shocked granitoid are strongly enriched in Al, Ca, Mg, Ni, Co, Cr, V and S, and depleted in K and Si, suggesting chemical fractionation attendant on shock volatilisation, enrichment by an injected and volatilised meteoritic component, and potentially of sulfide mineralisation. The impact age is constrained by overlying Lower Jurassic strata, reworked Early Permian palynomorphs in the Jurassic lacustrine section, and deformed Lower Devonian and older units. A regional thermal event identified by apatite fission track at 280–250 Ma hints at a possible Permian–Triassic boundary age for the impact, although the lack of Triassic fossils in the crater fill favours a late Triassic age.

Pseudomorphs after evaporitic minerals interbedded with 2.2 Ga stromatolites of the Yerrida basin, Western Australia: Origin and significance

Widespread quartz pseudomorphs after evaporitic minerals are interbedded with stromatolites in 2.2 Ga sedimentary rocks in the Yerrida rift basin of Western Australia. These deposits preserve diverse original crystal morphologies that grew displacively either as individuals or as clusters within stromatolitic horizons and associated fine-grained siliciclastic beds. The stromatolites were deposited in shallow-marine, restricted environments. Although evaporitic minerals are largely replaced by quartz, their crystal shapes include lenticular, rosette, needle, and nodular forms, suggesting that they formed from calcium sulfate–rich brines. Textures suggest that replacement by quartz occurred during early alteration of the sediments. Original calcium sulfate minerals such as gypsum or anhydrite are indicated by the morphologies of the pseudomorphs; this concept is supported by anhydrite inclusions in the quartz crystals that form the pseudomorphs, and this finding indicates that calcium sulfate existed in Early Proterozoic sedimentary environments.