Linking Gold Systems to the Crust-Mantle Evolution of Archean Crust in Central Brazil

The Goiás Archean Block (GAB) in central Brazil is an important gold district that hosts several world-class orogenic gold deposits. A better comprehension of the crustal, tectono-magmatic, and metallogenic settings of the GAB is essential to accurately define its geological evolution, evaluate Archean crustal growth models, and target gold deposits. We present an overview of gold systems, regional whole-rock Sm-Nd analyses that have been used to constrain the geological evolution of the GAB, and augment this with new in situ zircon U-Pb and Hf-O isotope data. The orogenic gold deposits show variable host rocks, structural settings, hydrothermal alteration, and ore mineralogy, but they represent epigenetic deposits formed during the same regional hydrothermal event. The overprinting of metamorphic assemblages by ore mineralogy suggests the hydrothermal event is post-peak metamorphism. The metamorphic grade of the host rocks is predominantly greenschist, locally reaching amphibolite facies. Isotope-time trends support a Mesoarchean origin of the GAB, with ocean opening at 3000–2900 Ma, and reworking at 2800–2700 Ma. Crustal growth was dominated by subduction processes via in situ magmatic additions along lithospheric discontinuities and craton margins. This promoted a crustal architecture composed of young, juvenile intra-cratonic terranes and old, long-lived reworked crustal margins. This framework provided pathways for magmatism and fluids that drove the gold endowment of the GAB.

Mineralogy of a High-Temperature Skarn, in High CO2 Activity Conditions: The Occurrence from Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania)

A shallow-level monzodioritic to quartz-monzodioritic pluton of the Upper Cretaceous age caused contact metamorphism of Tithonic–Kimmeridgian reef limestones at Măgureaua Vaţei (Metaliferi Massif, Apuseni Mountains, Romania). The preserved peak metamorphic assemblage includes gehlenite (Ak 33.64–38.13), monticellite, wollastonite-2M, Ti-poor calcic garnet, and Ca-Tschermak diopside (with up to 11.15 mol.% Ca-Tschermak molecule). From the monzodioritic body to the calcitic marble, the periplutonic zoning can be described as: monzodiorite/agpaitic syenite-like inner endoskarn/wollastonite + perovskite + Ti-poor grossular + Al-rich diopside/wollastonite + Ti-poor grossular + diopside + vesuvianite/gehlenite + wollastonite + Ti poor grossular + Ti-rich grossular (outer endoskarn)/wollastonite + vesuvianite + garnet (inner exoskarn)/wollastonite + Ti-rich garnet + vesuvianite + diopside (outer exoskarn)/calcitic marble. Three generations of Ca garnets could be identified, as follows: (1) Ti-poor grossular (Grs 53.51–81.03 mol.%) in equilibrium with gehlenite; (2) Ti-rich grossular (Grs 51.13–53.47 mol.%, with up to 19.97 mol.% morimotoite in solid solution); and (3) titanian andradite (Grs 32.70–45.85 mol.%), with up to 29.15 mol.% morimotoite in solid solution. An early hydrothermal stage produced retrogression of the peak paragenesis toward vesuvianite, hydroxylellestadite (or Si-substituted apatite), clinochlore, “hibschite” (H4O4-substituted grossular). A late hydrothermal event induced the formation of lizardite, chrysotile, dickite, thaumasite, okenite and tobermorite. A weathering paragenesis includes allophane, C-S-H gels and probably portlandite, unpreserved because of its transformation in aragonite then calcite. Overprints of these late events on the primary zoning are quite limited.

Composition and Structure of Zircon from Hydrothermal Uranium Occurrences of the Litsa Ore Area (Kola Region, Russia)

Zircon grains from various metagranitoids (plagio- and monzo-granites, gneisses, metasomatic rocks, and pegmatoid veins) from the Skal’noe and Dikoe sites of the Litsa uranium ore area (Kola Region, Russia) were studied in order to reconstruct the sequence and timing of events in the area and to observe effects of hydrothermal process related to uranium mineralization on structure and composition of zircon. Individual zircon grains were studied by means of laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS), ion microprobe and Raman spectroscopy. Isotopic LA–ICPMS data for the Skal’noye and Dikoe ore occurrences suggest the following age sequence of events in the area: intrusion of plagiogranites—2829 ± 12 Ma, formation of magmatic protolith of gneisses—2781 ± 17 Ma, metamorphism of plagiogranites—2636 ± 34 Ma; intrusion of monzogranites and pegmatoid veins—2549–2526 Ma, hydrothermal event with uranium input—2276 ± 21 Ma, last metamorphism of plagio- and monzo-granites—1892–1696 Ma. Ore-bearing rocks in the area are pegmatoid veins and quartz–feldspar metasomatites which contain uraninite. During a 2.3 Ga hydrothermal process, newly formed zircon rims grew simultaneously with the precipitation of uraninite in the veins and metasomatites. These rims are characterized by high U and rare earth elements (REE) contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U ratios (0.1–0.007) and a flat LREE-enriched pattern, in some cases inherited from minerals, dissolved during a hydrothermal event (magmatic plagioclase and probably monazite). Hydrothermal zircon rims grew with partial dissolution of the magmatic zircon, as evidenced by the rounded and curved shapes of zircon cores. The degree of alteration caused by hydrothermal events depends on the uranium content in the pre-existing zircon. The effects of zircon alteration and newly formed zircon composition reflect the redistribution of uranium in rocks.