Roberto Perez Xavier

Tourmaline B-isotopes fingerprint marine evaporites as the source of high-salinity ore fluids in iron oxide copper-gold deposits, Carajás Mineral Province (Brazil)

The Carajas Mineral Province in northern Brazil contains a variety of world-class (>100 Mt ore) iron oxide copper-gold (IOCG) deposits, including the only Archean examples of this deposit class (e.g., the Igarape Bahia/Alemao and Salobo deposits). Tourmaline of schorldravite composition, a common gangue mineral in these deposits, precipitated shortly prior to and after the ore assemblage. A boron isotope study of texturally different tourmaline from three IOCG deposits (Igarape Bahia, Salobo, and Sossego) using secondary ion mass spectrometry (SIMS) provides new evidence in the long-standing debate of magmatic versus non-magmatic sources for the high salinity (up to 50 wt% NaCl equiv.) of ore fl uids in these deposits. Values of δ 11 B from 14‰ to 26.5‰ for the Igarape Bahia and Salobo deposits confi rm marine evaporite–derived brines in the ore fl uids, whereas lower δ 11 B values for the Igarape Bahia deposit (5.8‰ to 8.8‰) suggest that these fl uids may have mixed with an isotopically different hydrothermal fl uid, or one that had a mixed boron source. More variable and isotopically lighter boron in tourmaline from the Sossego deposit (–8‰ to 11‰) is attributed to mixed sources, including light boron leached from felsic intrusive and volcanic host rocks, and heavy boron derived from marine evaporites. The boron isotope data indicate that the characteristic high salinity of the ore fl uids in the Carajas Mineral Province was acquired by the interaction of hydrothermal fl uids with marine evaporites. For IOCG deposits that contain tourmaline as a common gangue mineral, boron isotopes offer a valuable tool to constrain the high-salinity source problem, which is a critical issue in metallogenesis of IOCG deposits worldwide.

Ore-Forming Fluids of Vein-Type Fluorite Deposits of the Cerro Aspero Batholith, Southern Cordoba Province, Argentina

Vein-type fluorite deposits in the southern part of the Sierras Pampeanas, Córdoba Province, Argentina, occur mainly hosted by calc-alkaline porphyritic biotite granites, which belong to the Paleozoic, post-tectonic Cerro Aspero batholith. The fluorite veins, of Cretaceous age, occupy steeply dipping, strike-slip regional fault zones, and are composed of fluorite and chalcedony, locally with subordinate amounts of pyrite and, in some cases, coffinite and pitchblende. These veins show typical open-space-filling textures and are closely related to pervasive silicic and argillic alteration of the host granite. Three successive stages of mineralization were distinguished on the basis of vein chronology, REE data, and fluid-inclusion study in fluorite ores. These stages generally display slightly fractionated REE patterns (La/Yb = 1.4 to 14), with REE behavior given by a relatively stronger LREE fractionation with respect to HREE. The REE composition of the fluids responsible for fluorite deposition was largely controlled by differential mobility of the REE during the silicic or argillic alteration of the host granite. Preferential leaching of HREE over LREE occurred during both alteration types, but in the argillic alteration the LREE were practically not removed. The total homogenization of primary-like aqueous inclusions took place invariably in the liquid phase at temperatures ranging from 187°C to 103°C, with concentrations of values around 160°C, 136°C, and 116°C (stages I, II, and III, respectively), defining a clear trend of fluid cooling. This cooling is accompanied by large changes in the fO2 of the fluid, from oxidizing to reducing, as inferred from the Eu/Eu∗ ratios and the mineral assemblage (pyrite, pitchblende, and coffinite). The three stages of fluorite deposition exhibit temperatures of ice melting within the interval from −0.3°C to +0.4° C, indicating that the mineralizing fluids were exclusively aqueous and highly dilute. No evidence of fluid mixing or boiling was found. The fluid-inclusion data suggest that the proposed three stages of mineralization probably were the result of a single hydrothermal event, and strongly support a single, uniform fluid reservoir for the ore-forming solutions; evidently, the latter were heated meteoric waters rather than fluids generated in deep-seated environments within the crust.

The Gold-Carbonaceous Matter Association in Lode Deposits of the Rio Itapicuru Greenstone Belt, Northeastern Brazil

Carbonaceous units commonly host or are closely related to lode-gold mineralization in the mesothermal Fazenda Maria Preta (FMP) and Fazenda Canto (FC) deposits of the Paleoproterozoic Rio Itapicuru greenstone belt of northeastern Brazil. In these deposits, the carbonaceous matter occurs mainly as: (1) straight to anastomosing seams (Type I) along or transecting the rock fabric, or as stylolitic structures in quartz veins; (2) single grains composed of an agglomerate of highly anisotropic subgrains (Type II); or (3) single grains with a homogeneous internal texture (Type III), which are either enclosed in Type-I carbonaceous seams or disseminated in the rock matrix. Type-I carbonaceous matter commonly hosts or is overgrown by the gold-related sulfide paragenesis, particularly arsenopyrite, whereas both Type I and Type II enclose crystals of arsenopyrite or occur as inclusions and in sharp contact with the sulfide phases. The three morphological types of carbonaceous matter exhibit similar Raman spectral c...

Caracterização das Litofácies da Sequência Sedimentar do Grupo Igarapé Bahia

Resumo O grupo Igarapé Bahia insere-se na sequência metavulcanossedimentar do Supergrupo Itacaiúnas (~2.75 Ga) da Província Carajás e hospeda o depósito de óxido de ferro-cobre-ouro (IOCG) Igarapé Bahia. A parte superior do grupo concentra a Formação Grota do Vizinho e é dominada por uma sequência sedimentar que divide-se em 5 unidades que incluem Formação Ferrífera Bandada, Conglomerados, Grauvacas, Brechas e Ritmitos. São comumente observadas estruturas sedimentares primárias e estruturas secundárias por toda a sequência sedimentar. Nódulos e layers de calcopirita ocorrem de forma concordante ao acamamento sedimentar nos ritmitos.

COMPOSIÇÃO E EVOLUÇÃO DOS FLUIDOS NO DEPÓSITO AURÍFERO SÃO FRANCISCO, FAIXA SERIDÓ, PROVÍNCIA BORBOREMA, NORDESTE DO BRASIL

The Sao Francisco gold deposit is located within the Serido Belt, Borborema Province, Northeast Brazil. The deposit area consists predominantly of mica schists and minor calc-silicate lenses (Serido Formation), that are metamorphosed to mid-upper amphibolite facies conditions and cut by pegmatite intrusions. The primary mineralization is considered to have occurred synchronously with the peak metamorphism and is hosted by the sillimanite-muscovite zone confmed to a sin-S 3 medium to high angle shear zone, ali considered to be Neoproterozoic. Free gold, or associated with Fe ± Cu ± Pb sulfides (especially pyrite), occurs within the veins or host mica schists as interstitial fine grains (primary mineralization) or segmented in millimeter- to centimeter-sized fractures (secondary mineralization). This study, accomplished in auriferous and barren quartz veins, consisted of the investigation in the mode of occurrence of the fluid inclusion populations, their textural relationships and analyses by microtherrnometry and Raman microspectroscopy. Four types of fluid inclusions have been identified, in decreasing order of abundance: type l - CO 2 , with subordinate amounts of CH. and N 2 ; type 2 - H 2 O, could contain a complex mixture of solutes; type 3 - CH 4 -rich; and type 4 - H 2 O-CO 2 , with low-salinity. Most of the fluid inclusion populations occur along inter- and intragranular healed microfractures. Types l, 3 and 4 are texturally the earliest and fill low- to moderate-angle microfractures, whereas type 2 inclusions are later and delineate mostly moderate- to high-angle microfractures, ali referred to the XZ surface of finite strain ellipsoid. The microfractures host only one fluid inclusion type, except in one barren vein where carbonic (type 1) and aqueous-carbonic (type 4) inclusions were found along the same micro fracture. In general, the auriferous veins contain fluid inclusions of types l, 2 and 3, whereas the barren veins show types l and 2 or l, 2 and 4. Dissolved salts in the aqueous fluid inclusions (type 2) pf the barren veins are dominated by NaCl (± KC1), but in the auriferous veins NaCl, CaCL 2 and probably KC1, LiCl, MgCl 2 and FeCL 2 appear as important solutes. The results of this study, combined with structural, petrographic and metamorphic data, demonstrate that the circulation of aqueous-carbonic (low salinity) and aqueous fluids (varied salinity) occurred coevally during peak metamorphism and formation of the Sao Francisco gold deposit. Most of the inclusions of the aqueous-carbonic fluids suffered dehydration post-entrapment (the current carbonic inclusions) and the variable salinity of the aqueous fluids is consequence of the chemical mobility during the metamorphism. The primary mineralizing fluids were probably low-salinity aqueous- carbonic and both chlorine- (possibly dominant) and sulfur-complexes may have had an important role in the transport of the gold. It is possible that aqueous fluids with complex salts played a prominent role in the remobilization of the primary gold (sulfides) to the fractures. Although further studies are necessary for a better assessment of the processes responsible in the deposition of the primary gold, mixing of fluids, sulfidation and immiscibility within the fluid conduits (veins) appear to have been particularly important.