The Jales gold district is located in Vila Pouca de Aguiar County, northern Portugal, approximately 100 km ENE of the city of Porto. The district encompasses three areas with epigenetic mineralization of Variscan age. Mineralization is hosted in syn- to late-tectonic Variscan granites and in greenschist-facies metasedimentary rocks, ranging in age from Cambrian to Lower Devonian. Sulfide paragenesis proceeds from early Fe-As sulfides to late base-metal sulfides. The deposits proximal to the intrusion (Campo and Gralheira) are relatively sulfide rich and contain paragenetically late electrum associated with significant amounts of base-metal sulfides, whereas the deposit distal to the intrusion (Três Minas) is sulfide poor and contains high-fineness gold that is paragenetically early. Petrographic studies, coupled with arsenopyrite geothermometry and chlorite solid solution modeling indicate that fluid evolution was different at Três Minas than in the two other areas, explaining the distinct mineral assemblages and metal associations. At Três Minas, gold was precipitated early from sulfide complexes due to oxidation. During the early stage at high temperatures, silver and base metals were transported as chloride complexes and therefore were not affected by oxidation. At Campo and Gralheira, early oxidation was not as extensive as at Três Minas, and gold precipitated later due to decrease in H2S activity and cooling. During the lower-temperature late stage, silver was transported as a sulfide complex, so that decrease in H2S activity and cooling resulted in the precipitation of electrum, rather than high-fineness gold. Additionally, cooling destabilized chloride complexes carrying base metals, explaining deposition of significant amounts of base-metal sulfides at Campo and Gralheira.
Abstract This work investigates the relationships between partial melting, melt extraction, pluton growth and silicic volcanism in garnet-bearing felsic volcanic rocks that were extruded in the Iberian Pyrite Belt, at ca . 345 Ma. The garnets are of peritectic origin, displaying textural and chemical features of disequilibrium crystallization during partial melting reactions involving biotite at high temperatures (up to 870 °C) in the middle-lower crust. Major element composition suggests compositional equilibrium with the entrained and pinitized peritectic cordierite, but reveals some subsequent homogenization by diffusion. Trace element maps and spot analyses of garnet show, nonetheless, significant trace element variations, reflecting biotite and Y-REE-P-rich accessory phase breakdown during partial melting reactions. Peritectic garnet and cordierite growth resulted in the preservation of Th- and Y-rich prograde suprasolidus monazite, which constrains the timing of partial melting of the metapelitic protolith at ca. 356.8 ± 2.4 Ma. The zircon cargo further shows that a significant amount of zircon crystals from previously crystallized felsic melts were also remobilized and erupted. These were likely stored in an upper crustal pluton that grew episodically since ca . 390 Ma during voluminous melt generation periods within the middle to lower crust, which also resulted in voluminous volcanism. The geochemical trends of the felsic volcanic rocks reflect the entrainment of xenoliths of peritectic garnet, cordierite and feldspar, and as such, the garnet-bearing felsic volcanic rocks represent an erupted mixture of a lower-temperature ( ca. 770 °C) silicic melt and autocrysts, and peritectic phases and zircon crystals from previously crystallized and stored felsic melts.
The Jales Au district encom passes three areas with epigenetic m ineralization o f Variscan age.M ineralization is hosted in syn-to late-tectonic Variscan granites and in greenschist facies m etasedim entary rocks, ranging in age from Cambrian to Lower Devonian.Sulfide paragenesis proceeds from early Fe-As sulfides to late base-m etal sulfides.The deposits proxim al to the intrusion (Campo and Gralheira) are relatively sulfide-rich and contain paragenetically late electrum associated with significant amounts o f base-metal sulfides, whereas the deposit distal to the intrusion (Três M inas) is sulfide-poor and contains high-fineness Au that is paragenetically early.In spite o f displaying characteristics related to Orogenic Au deposits, the re g io n a l a s s o c ia tio n w ith W -Sn d e p o sits, clo se tem p o ral re la tio n sh ip o f m ineralizatio n w ith granitic intru sio n s, m etal associations characterized by significant Bi contents or Bi-Au correlation, a paragenesis defining a low fCVlow f$ 2 environm ent and S (and Pb) isotope studies suggest that the m ineralization is related to syn-to late-tectonic reduced granites and could also be classified as intrusion-related.
Geologic samples are extremely diverse and share a tendency for both heterogeneity and complexity. This is especially true for ores, which commonly result from a complex interplay of processes in highly dynamic environments. In recent years, a number of tools allowing the chemical mapping of major (e.g., mineral liberation analysis, MLA), minor (e.g., electron microprobe, EPMA), and trace (e.g., laser ablation-inductively coupled plasma-mass spectrometry, LA-ICP-MS) elements in geologic samples at ~1- to 50- μ m resolution and over mm2 areas have seen rapid development and have become readily available. To date, the application of synchrotron-based X-ray fluorescence (SXRF) mapping has been limited to addressing key questions because of low availability and high cost. This paper demonstrates how recent advances in X-ray fluorescence detector technology are bringing new possibilities to ore petrology. Millisecond dwell times allow collection of thin section size maps at resolutions of a few μ m in hours, while improvements in data analysis software simplify the production of quantitative elemental maps.
Based on the imaging of six samples representative of different commodities (Pt, U, Cu, Ge) and different geologic contexts (PGE deposit; sandstone-hosted U deposit; vein-type polymetallic hydrothermal deposit; iron oxide-copper-gold (IOCG) deposit), we demonstrate that megapixel SXRF (MSXRF) can efficiently provide the information necessary to understand metal speciation in the context of thin section-scale textural complexity. Image analysis revealed a number of new results for the studied deposits, for example, (1) the distribution of micrometer-sized Pt-rich grains and Ti mobility during the formation of schistosity at the Fifield Point prospect (New South Wales, Australia); (2) the presence of Ge contained in organic matter and of Hg minerals associated within quartzite clasts in the Lake Frome U ores (South Australia); (3) confirmation of the two-stage Ge enrichment in the Barrigao deposit, with demonstration of the presence of Ge in solid solution in the early chalcopyrite (Portuguese Iberian pyrite belt); and (4) the enrichment of U during late dissolution-reprecipitation reactions in the bornite ores of the Moonta and Wallaroo IOCG deposits (South Australia). These results illustrate that MSXRF is a powerful technique for locating nano- to microparticles of precious metals (Pt) and trace contaminants (e.g., Hg) that form distinct (micro) minerals. In addition, it is a powerful tool for understanding commodities with relatively low ore grades and complex distribution (100–1,000 ppm; e.g., U, Ge).
The Isortoq Giant Dykes in the Proterozoic Gardar Province, South Greenland, include the Isortoq South giant dyke and the Isortoq North giant dyke. The fine-grained Fe-Ti-V deposit hosted by the Isortoq South giant dyke, referred to as the Isortoq Fe-Ti-V deposit, is considered a good test site for the use of magnetic susceptibility for the mapping of ore grades. Here, we test this and show that the Fe, Ti and V distribution is controlled by titanomagnetite disseminated throughout fine-grained troctolite. The deposit displays a clear correlation between magnetic susceptibility and Fe, Ti and V grades in bulk samples of consecutive 2 m sections from 11 drill cores, totalling 2671 m in length. We observe that Fe, Ti and V are almost entirely hosted in titanomagnetite, which controls the magnetic susceptibility. Field measurements of the magnetic susceptibility can thus be considered as a reliable exploration tool for this type of mineralisation. We further consider the origins of the deposit by reconnaissance petrography, mineral and bulk rock chemistry of the large mass of aphanitic Fe-rich troctolite in the Isortoq South giant dyke. We suggest that the deposit may represent the base of a basanitic to trachybasaltic magma chamber, in which Fe-rich immiscible melts accumulated, crystallised and fractionated. The processes suggested here may apply to other giant dykes and intrusions of the Gardar Province.
Emerald is a dark-green chromium-vanadium rich variety of beryl, Be 3Al 2Si 6O18 . The rarity of emerald arises from the combination of two elements (Be and Cr), which are normally mutually incompatible in geochemical terms. Emeralds are more rare and often more valuable than diamonds of the same size and quality. Emerald s are gems that hold their color well and are easier to work with than diamonds. For these re asons emeralds are among the most treasured of all gemstones and have been mined sinc e ancient times in Egypt. Present day annual production reaches 15 million ca rats, with Colombia being the largest producer, followed by Brazil and Zambia and, with s ubstantially lower productions, by Russia, Madagascar, Mozambique, Zimbabwe, Pakistan and Afghanistan (Giuliani et al, 1998). Emerald deposits can be subdivided into two differe nt categories (Giuliani et al, 1997). The predominant deposit type is associated with pegmati tes and granites (also known as glimmerite type) while the rare but probably the mo st valuable deposit type is associated with thrusts and shear-zones (also known as telethermal type).
Trace amounts of alluvial xenotime (YPO4) were identified in black sands from pan concentrates collected during a rare earth reconnaissance survey, carried by the Instituto Geologico e Mineiro (IGM) in Central Eastern Portugal. The xenotime occurs as sub-rounded grains with an average size of ≈ 250 μm, and its identification was confirmed through XRD methods and EPMA analysis. It is believed that xenotime is more abundant than normally recognized and therefore frequently overlooked. The largest concentrations of this mineral occur in Nisa, S.to Antonio das Areias and Marvao. The regional geology and the accompanying mineral suite suggest xenotime originates from the granitic massifs of Nisa and Penamacor as well as the Beira Baixa Arkoses and levels of Plio-Pleistocene gravels interspersed with sandy clay. Although its economic interest is presently limited, its demand may increase as a result of the potential use of yttrium in the manufacturing of superconductors that are stable at ambient temperature.
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