Mesozoic gold deposits of Eastern Zabaykalye: relationship to magmatism, isotopic composition

It is noted that the formation of Mesozoic gold mineralization in Eastern Zabaykalye is associated with magmatic formations of the Amudzhikansky (J2-3), Shakhtaminsky (J2-3), and Sokhondinsky (J1) complexes formed during collisional and post–collisional processes. The nature of these magmatic formations has both mantle (87Sr/86Sr < 0.0706) and crustal components (87Sr/86Sr < 0.0706).Calculation of isotopic composition of oxygen from the ore veins of Aleksandrovsky and Lyubavinsky gold deposits revealed that fluids of magmatic nature were involved in their formation. These data indicate a magmatic source of the Mesozoic gold mineralization in Eastern Zabaykalye.

Synsedimentary to Diagenetic Cu±Co Mineralization in Mesoproterozoic Pyritic Shale Driven by Magmatic-Hydrothermal Activity on the Edge of the Great Falls Tectonic Zone–Black Butte, Helena Embayment, Belt-Purcell Basin, USA: Evidence from Sulfide Re-Os Isotope Geochemistry

The ca. 1,500 to 1,325 Ma Mesoproterozoic Belt-Purcell Basin is an exceptionally preserved archive of Mesoproterozoic Earth and its paleoenvironmental conditions. The Belt-Purcell Basin is also host to world-class base metal sediment-hosted mineralization produced in a variety of settings from the rift stage of basin evolution through the subsequent influence of East Kootenay and Grenvillian orogenies. The mineral potential of this basin has not been fully realized yet. New rhenium-osmium (Re-Os) data presented here for chalcopyrite, pyrite, and black shale contribute to refine a robust genetic model for the origin of the Black Butte copper±cobalt±silver (Cu±Co±Ag) deposit hosted by the ca. &gt;1,475 Ma Newland Formation in the Helena Embayment of the Belt-Purcell Basin in Montana, USA. Chalcopyrite Re-Os data yield an isochron age (1,488±34 Ma, unradiogenic initial 187Os/188Os composition Osi-chalcopyrite=0.13±0.11) that overlaps with the geological age of the Newland Formation. Further, the Re-Os data of synsedimentary to diagenetic massive pyrite yield evidence of resetting with an isochron age (1,358±42 Ma) coincident with the timing of the East Kootenay orogeny. The unradiogenic Osi-chalcopyrite at ca. 1,488 Ma (0.13±0.11) argues for derivation of Os from a magmatic source with a 187Os/188Os isotopic composition inherited from the upper mantle in the Mesoproterozoic (Osmantle 1,475 Ma=0.12±0.02). The unradiogenic Osi-chalcopyrite also suggests limited contamination from a continental crustal source. This source of Os and our new sulfur isotopic signatures of chalcopyrite (–4.1 to +2.1‰-VCDT) implies a dominantly magmatic source for metals. We integrate our new results and previously published geological and geochemical evidence to conceptualize a genetic model in which Cu and metals were largely contributed by moderate-temperature, reduced magmatic-hydrothermal fluids carrying reduced sulfur species with a magmatic origin and flowing as highly metalliferous fluids within the shale sequence. A subsidiary derivation of metals during thermally forced shale diagenesis is possible. Chalcopyrite mineralization replaced locally massive synsedimentary to diagenetic pyrite units close to the sediment-water interface, i.e., an ideal locus where magmatic-hydrothermal fluids could cool and the solubility of chalcopyrite would fall. We suggest that Cu mineralization was coeval with the timing of an enhanced thermal gradient in the Helena Embayment triggered until ca. 1,455 Ma by tholeiitic dike swarm that intruded into Archean basement rocks and intersected the NE-SW-trending Great Falls Tectonic Zone.

Crustal rejuvenation stabilised Earth’s first cratons

The formation of stable, evolved (silica-rich) crust was essential in constructing Earth’s first cratons, the ancient nuclei of continents. Eoarchaean (4000–3600 million years ago, Ma) evolved crust occurs on most continents, yet evidence for older, Hadean evolved crust is mostly limited to rare Hadean zircons recycled into younger rocks. Resolving why the preserved volume of evolved crust increased in the Eoarchaean is key to understanding how the first cratons stabilised. Here we report new zircon uranium-lead and hafnium isotope data from the Yilgarn Craton, Australia, which provides an extensive record of Hadean–Eoarchaean evolved magmatism. These data reveal that the first stable, evolved rocks in the Yilgarn Craton formed during an influx of juvenile (recently extracted from the mantle) magmatic source material into the craton. The concurrent shift to juvenile sources and onset of crustal preservation links craton stabilisation to the accumulation of enduring rafts of buoyant, melt-depleted mantle.

Volcano dynamics vs tectonics on Mars: evidence from Pavonis Mons

&lt;p&gt;Volcanic activity is widespread within the inner Solar system and it can be commonly observed on rocky planets.&lt;br&gt;In this work, we analyse the structures of Pavonis Mons, which is one of the three large volcanoes in the Tharsis volcanic province of Mars, by performing structural mapping, azimuth, and topographic distribution of linear features on the flanks of Pavonis, such as grabens and pit chains. We tested whether their formation is to be ascribed to the internal volcano dynamics and magmatic activity or the tectonics related to the Tharsis volcanic province activity.&lt;br&gt;Through the length size distribution and fractal clustering analyses of the structural features, we found that large grabens are vertically confined in the upper mechanical layers of the brittle crust whereas pit chains penetrate the whole crust up to the magmatic source, indicating that they can be considered the main feeders of Pavonis Mons. We inverted the topography with dykes and faults models to test whether grabens at the surface are the expression of intrusions at depth and we suggest that thin dykes inducing normal faulting is the most likely mechanism. Furthermore, two azimuthal distribution of the grabens are identified: concentric grabens occur on the volcano summit while linear grabens at its base show NE-SW trend as the Tharsis Montes volcanoes alignment. The analyses show that faults related to large grabens are confined in a mechanical layering in the upper layers of the brittle crust, whereas deeper structures such as pit chains are most likely associated to magma injection/dykes and therefore, connected to the subcrustal magma source at a depth of ~80&amp;#8211;100 km.&lt;br&gt;Therefore, based on our results, we infer that Pavonis Mons recorded active rifting at the initial stages of development with the formation of the large linear graben and faults at its base followed by a phase of volcano growth and concentric magma intrusions when volcano and magma chamber dynamics prevailed&lt;/p&gt;

Geochronology and Lu-Hf isotopic study of the granodioritic pulse in the Qaradagh batholith (NW Iran)

&lt;p&gt;The Qaradagh batholith in northwest Iran mainly comprises granodioritic rocks, which makes more than 50% of the batholith. This lithology is the first intrusive pulse within this batholith and the oldest Tertiary magmatism in the region, though other younger pulses of granite, diorite, quartz-diorite, syenite, quartz-syenite, monzonite, quartz-monzonite, quartz monzodiorite, monzogranite and gabbro intruded the main body. These magmatic rocks have intruded the Upper Cretaceous and Paleogene sedimentary, volcano-sedimentary and igneous rocks.&lt;/p&gt;&lt;p&gt;The Qaradagh batholith hosts vein-type and some local stock-work type Cu&amp;#8211;Au&amp;#8211;Mo mineralization, especially in its central parts, while skarn-type deposits have been formed at its contacts with peripheral carbonate rocks. Its extension towards the north into the neighboring south Armenia (which is part of the South Armenian Block) is known as the Meghri&amp;#8211;Ordubad pluton (MOP), which hosts several large porphyry Cu&amp;#8211;Mo deposits and other precious and base metal mineralizations. U&amp;#8211;Pb geochronology on the zircons separated from the granodioritic unit yielded a weighted &lt;sup&gt;206&lt;/sup&gt;Pb/&lt;sup&gt;238&lt;/sup&gt;U mean age of 43.81 &amp;#177; 0.18 (MSWD=1.38) and a Pb*/U concordia age of 44.04 &amp;#177; 1.00 Ma (MSWD= 24), which correspond to Middle Eocene.&lt;/p&gt;&lt;p&gt;Since the Qaradagh batholith and especially its earliest magmatic phase are considered as the oldest plutonic event of the Cenozoic age in northwest Iran, thus this investigation testifies to the fact that intrusive activities of Tertiary in this region has commenced in Middle Eocene, contrary to the opinion of the majority of authors who believe that plutonism in this region occurred during Oligocene.&lt;/p&gt;&lt;p&gt;However, this age is much older than the molybdenite Re&amp;#8211;Os ages of quartz-sulfide veins hosted by granodioritic rocks (25.19 &amp;#177; 0.19 to 31.22 &amp;#177; 0.28 Ma), indicating that mineralization in this batholith is related to another much younger intrusive phase, and even to several phases, as the published ages of molybdenites from various veins and mineralized zones show a large interval. Comparing the obtained age with those from the MOP in southern Armenia indicate that southern part of the MOP is almost coeval with the emplacement of the granodioritic rocks in Qaradagh batholith.&lt;/p&gt;&lt;p&gt;The U and Th contents of the zircons range from 17.1 to 1534.0 and from 4.9 to 641.0 ppm, respectively, with Th/U ratios between 0.66 and 5.82 (mean of 1.26), indicating a magmatic source. Meanwhile, the &amp;#949;Hf&lt;sub&gt;(t) &lt;/sub&gt;values of the zircons range from 8.7 to 11.1 with the mean of 9.5, which are plotted between the CHUR and the Depleted Mantle evolution lines, indicating a juvenile and homogeneous magmatic source and the predominance of mantle-derived magmas with limited crustal assimilation.&lt;/p&gt;