ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The Karouni orogenic gold deposits are located in north-central Guyana 35 km to the west of the 5 Moz Omai gold deposit. They are hosted in a 2200 to 2100 Ma greenstone belt within the Paleo- to Neoproterozoic Guiana Shield. Karouni consists of two deposits, Smarts and Hicks, located 2 km apart along the NW-striking Smarts-Hicks shear zone. Both deposits are hosted within a sequence of greenschist facies mafic volcanic rocks and felsic intrusions. The hydrothermal alteration mineral assemblages vary according to lithology and are characterized by narrow selvages (〈1–4 m in width). A chlorite-talc-calcite assemblage dominates in high MgO basalt, whereas in high TiO〈sub〉2〈/sub〉 dolerite a progression toward the vein is seen from chlorite-calcite-rutile- to albite-dominated mineralogy. Karouni is anomalous among orogenic gold deposits for its dominant sodic alteration and distinct lack of potassic alteration. Gold is located within inclusions in coarse, disseminated pyrite associated with the proximal alteration zones and as coarse native gold within the quartz-carbonate veins. Minor gold is also located within Au-bearing telluride minerals. The high TiO〈sub〉2〈/sub〉 dolerites formed a favorable chemical trap due to their high magnetite content, suggesting sulfidation via redox reaction as a possible mechanism of gold deposition. Mass balance modeling of the hydrothermal alteration indicates a wall rock-dominated system with limited addition or subtraction of major elements with the exception of C, S, and Na. Modeling of the proximal alteration has also shown strong trace element enrichment of W-Bi-Ag-Te-Mo-Pb, all of which are correlative with gold. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element geochemistry and secondary ion mass spectrometry (SIMS) S isotope analyses of pyrite from the gold-bearing hydrothermal system within the deposit indicate a geochemically and isotopically homogeneous system with only minor trace element variation due to differences in host rock, suggesting a single hydrothermal pulse correlative with the late stages of the Trans-Amazonian orogeny.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Karouni orogenic gold deposit is located in north-central Guyana, 35 km to the west of the 5 Moz Omai gold mine. The deposit is hosted within 2.2 to 2.1 Ga volcano-sedimentary rocks of the Barama-Mazaruni Supergroup, part of the Paleo- to Neoproterozoic Guiana Shield. Karouni consists of two zones, Smarts and Hicks, located 2 km apart along the NW-striking Smarts-Hicks shear zone, a second-order splay of the regional-scale Makapa-Kuribrong shear zone. The Karouni camp is composed of a lower sequence of mafic volcanic rocks, overlain by a lower sequence of immature sandstone and conglomerate, and an upper sequence of sandstone and laminated carbonaceous siltstone, intruded by several generations of felsic plutons and dikes. Whole-rock geochemical analysis indicates their formation in oceanic island-arc environment, and mantle-like characteristics of the high MgO basalts may indicate the presence of deep-seated structures during the early history of the camp. Regional-scale deformation during the Trans-Amazonian orogeny led to tectonic inversion of the volcano-sedimentary basins, greenschist facies metamorphism, and the development of strike-slip shear zones. Late movement on these shear zones is interpreted to be responsible for hydrothermal fluid flow, alteration, and gold mineralization within the Karouni gold camp. The Smarts and Hicks orebodies are localized within dilatational bends formed at changes in strike of the Smarts-Hicks shear zone during late dextral transcurrent movement. Rheological contrast played a dominant role in the formation of the deposits with shear-hosted, NW-striking, and steeply dipping quartz-carbonate-chlorite ± tourmaline-pyrite-gold (V〈sub〉2a〈/sub〉) veins preferentially hosted in ductilely deformed, high MgO basalts, whereas mineralized N-S, quartz-carbonate-chlorite ± tourma-line-pyrite-gold (V〈sub〉2b〈/sub〉) veins are hosted within rheologically competent high TiO〈sub〉2〈/sub〉 dolerite sills and granodiorite dikes. The interaction of these structures with favorable lithology is key for localizing high-grade orebodies.〈/span〉

Description: Deciphering the tectono-metamorphic evolution of Precambrian terranes can be difficult due to reworking by later superimposed events. Whole-rock elemental and isotopic geochemistry and zircon U–Pb geochronology are often employed in those studies, but these approaches are often not sensitive to the presence of multiple events and medium-grade metamorphic episodes. The Rio Apa Terrane (RAT), an allochthonous fragment of the Amazonian Craton, is a crustal block with a well-characterized crustal evolution but with no detailed thermal constraints for its tectono-metamorphic evolution. In contrast to previous studies, we show the existence of four tectono-metamorphic events at c. 1,780, c. 1,625, c. 1,420–1,340, and c. 1,300–1,200 Ma on the basis of apatite, titanite, and rutile U–Pb, in situ white-mica Rb–Sr, and in situ garnet Lu–Hf geochronology combined with mineral chemistry and phase-equilibria modelling. The c. 1,780 Ma event is recorded in the basement of the Western domain, representing an extensional event coeval with the development of its Eastern domain in response to the retreat stage of the accretionary system. This is followed by juxtaposition of the Western and Eastern domains along a major crustal boundary at c. 1,625 Ma, which is defined by the magnetic profiles and zircon U–Pb–Hf data across the boundary. The third and fourth events correspond to progressive high-pressure/medium-temperature (HP/MT) metamorphism, characterized by an anticlockwise P–T path, suggesting a convergent-to-collisional tectonic setting. The RAT was accreted to the adjoining Paraguá Terrane at c. 1,420–1,340 Ma under an isobaric P–T evolution spanning ~530°C to 600°C and ~10.0 kbar. Subsequently, the combined Rio Apa and Paraguá terranes collided with the SW Amazonian Craton at c. 1,300–1,200 Ma, reaching P–T conditions of ~560–580°C and ~10.9–11.7 kbar during crustal thickening. This study reveals for the first time the existence of a HP/MT metamorphic evolution related to the growth of the SW Amazonian Craton as part of an accretionary orogenic system during Rodinia assembly in the Palaeoproterozoic to Mesoproterozoic.

Description: The investigation of key minerals including zircon, apatite, titanite, rutile, monazite, xenotime, allanite, baddeleyite and garnet can retain critical information about petrogenetic and geodynamic processes and may be utilized to understand complex geological histories and the dynamic evolution of the continental crust. They act as small but often robust petrochronological capsules and provide information about crustal evolution, from local processes to plate tectonics and supercontinent cycles. They offer us insights into processes of magmatism, sedimentation, metamorphism and alteration, even when the original protolith is not preserved. In situ techniques have enabled a more in-depth understanding of trace element behaviour in these minerals within their textural context. This has led to more meaningful ages for many stages of geological events. New developments of analytical procedures have further allowed us to expand our petrochronological toolbox while improving precision and accuracy. Combining multiple proxies with multiple minerals has contributed to new interpretations of the crustal history of our planet.