Itabira District Research Articles

LATE CAMBRIAN Au-Pd MINERALIZATION AND Fe ENRICHMENT IN THE ITABIRA DISTRICT, MINAS GERAIS, BRAZIL, AT 496 Ma: CONSTRAINTS FROM U-Pb MONAZITE DATING OF A JACUTINGA LODE

Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in the province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilatero Ferrifero of Minas Gerais, the ~2.0 Ga Transamazonian event and the ~0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceicao Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaco, along the platiniferous Au-Pd belt of Minas Gerais.

JACUTINGAITE, Pt2HgSe3, A NEW PLATINUM-GROUP MINERAL SPECIES FROM THE CAUE IRON-ORE DEPOSIT, ITABIRA DISTRICT, MINAS GERAIS, BRAZIL

Jacutingaite, Pt 2 HgSe 3 , is a new species of platinum-group mineral discovered at the Caue iron-ore deposit, Itabira district, Minas Gerais, Brazil. Observed in a polished section, the mineral is about 50 μm across and occurs on an aggregate of atheneite, potarite and hematite. The new mineral is partly altered to a Pt–O phase. Synthetic jacutingaite is megascopically grey in color, has a metallic luster and a grey streak. The mineral is brittle, with a very good {001} cleavage. Values of VHN 10 (15 indentations on five grains of synthetic Pt 2 HgSe 3 ) are between 119 and 245 kg/mm 2 , with a mean value of 169 kg/mm 2 , corresponding to a Mohs hardness of approximately 3½. Under plane-polarized light, jacutingaite is light grey, has a moderate to distinct bireflectance, a bluish grey to rusty brown pleochroism, and a weak to distinct anisotropy; it does not exhibit internal reflections. Reflectance values of synthetic jacutingaite in air (R max , R min , in %) are: 51.1, 47.4 at 470 nm, 50.5, 48.2 at 546 nm, 49.6, 48.0 at 589 nm, and 47.8, 47.1 at 650 nm. Three electron-microprobe analyses of natural jacutingaite gave an average composition: Pt 37.30, Pd 5.91, Hg 25.72, Ag 0.16, Cu 0.82, and Se 31.48, total 101.39 wt.%, corresponding to the empirical formula (Pt 1.46 Pd 0.42 Cu 0.10 Ag 0.01 ) ∑1.99 Hg 0.98 Se 3.04 based on six atoms; the average result of nine analyses of synthetic jacutingaite is: Pt 47.14, Hg 24.24, and Se 28.62, for a total of 100.00 wt.%, corresponding to Pt 2.00 Hg 1.00 Se 3.00 . The mineral is trigonal, space group P 3 m 1, with a 7.3477(2), c 5.2955(1) A, V 247.59(1) A 3 and Z = 2. The crystal structure was solved and refined from the powder X-ray-diffraction data on synthetic Pt 2 HgSe 3 . Jacutingaite is isostructural with Pt 4 Tl 2 X 6 ( X = S, Se, or Te); no structural analogue is known as a mineral. Jacutingaite is structurally related to sudovikovite. The strongest lines in the X-ray powder-diffraction pattern of synthetic jacutingaite [ d in A(I) hkl ] are: 5.2917(100)001, 2.7273(16)201, 2.4443(10)012, 2.0349(18)022, 1.7653(37)003, 1.3240(11)004 and 1.0449(11)025. The mineral is named after the specular-hematite-rich vein-type mineralization locally known as “jacutinga”, in which it occurs.

THE PLATINIFEROUS Au-Pd BELT OF MINAS GERAIS, BRAZIL, AND GENESIS OF ITS BOTRYOIDAL Pt-Pd AGGREGATES

We delineate an approximately 240-km-long, north-trending Au-Pd-Pt belt in Minas Gerais, Brazil, which hosts hydrothermal Pd-Pt-bearing gold mineralization in its southern part (Quadrilatero Ferrifero and Itabira district) and alluvial Pd-Pt-Au mineralization in its northern part (southern Serra do Espinhaco). The hydrothermal Pd-Pt-bearing gold mineralization, locally known as jacutinga, occurs as late-orogenic quartz-hematite-(talc-kaolinite) veins, which truncate the Brasiliano regional tectonic foliation of the itabiritic host rocks of the Paleoproterozoic Itabira Iron Formation. The alluvial Pd-Pt-Au mineralization occurs as botryoidal Pd-Pt nuggets and delicate palladiferous gold aggregates in clastic and organic-rich valley infill on quartzite and hematitic rocks of the Paleo- to Mesoproterozoic Espinhaco Supergroup. The Au-Pd-Pt belt follows the regional trend of major thrust faults related to the ~0.6 Ga Brasiliano orogeny, and the hydrothermal jacutinga mineralization could be regarded as an oxidized variant of the orogenic gold deposit spectrum. Secondary botryoidal and arborescent Pt-Pd aggegates and primary (detrital) Au-Pd grains are peculiar to the alluvial platiniferous deposits of the northern part of the belt in the Serra do Espinhaco. We suggest that their source was jacutinga-style vein mineralization, as deduced from abundant detrital hematite and rutile, and the discovery of detrital hongshiite, a Pt-Cu alloy that forms bonanza pockets within jacutinga at Itabira. Reconnaissance laser ablation-inductively coupled plasma-mass spectrometry analyses for trace metals in the botryoidal Pt-Pd grains show that they have elevated Tl and Se contents of 0.9 to 354 and 54 to 2,700 ppm, respectively. These elements correlate positively, with a Tl/Se ratio of about 0.08, a value that contrasts with Tl/Se >1 for hongshiite from the hydrothermal jacutinga system at Itabira. The low and roughly constant Tl/Se ratios of the alluvial samples are close to the average value of 0.12 for fluvial waters. We suggest that the botryoidal Pt-Pd aggregates grew in situ by electrochemical metal accretion from dilute solutions carrying Pt and Pd from the dissolution of detrital Pt-Pd minerals from jacutinga. The connection between jacutinga and platiniferous alluvia indicates that there is a prospective trend of about 240 km for Pd-Pt-bearing gold mineralization reaching from Ouro Preto to Diamantina. The very high oxidation state required for efficient low-temperature aqueous mobilization of Pt, Pd, and Au was apparently controlled by the quartzitic and hematitic rocks in the Serra do Espinhaco, but not in the southern belt where greenstone rocks buffered fluids to moderate pH and lower oxidation states.

Structural control of Au–Pd mineralization (Jacutinga): An example from the Cauê Mine, Quadrilátero Ferrífero, Brazil

Precambrian banded iron formations (BIFs) in the Quadrilátero Ferrífero host a special kind of Au–Pd mineralization known as Jacutinga. The main orebodies are hosted within the Cauê Syncline, a SW-verging fold that involves Paleoproterozoic metasedimentary rocks in the Itabira District, a regional synclinorium with BIFs in the core of synclinal folds in the northeastern part of Quadrilatéro Ferrífero, Minas Gerais. Structural analysis reveals two important features of the district: the polydeformed character of the rocks and the importance of brittle structures in the control of the orebodies. Two deformational events are recognized in this area. The first event developed the main foliation, S1, that is the enveloping surface of the Cauê Syncline. The second event is better defined in the northern boundary of the structure where it is represented by a right-lateral wrench fault zone that has developed a foliation, S2, that truncates S1. This wrench fault was also responsible for the development of a system of fractures (Frm) that host the Au–Pd mineralization. The auriferous bodies of Cauê Syncline (Y, X, Área Central, Aba Norte, Noroeste and Aba Leste/Aba Leste Inferior) were generated during this second event. Shear fractures (R, R′ and P) and tension fractures (T) developed in response to the wrench fault system under brittle conditions. The best-developed, and most commonly mineralized fractures are R and T in all auriferous bodies. Elsewhere, the best mineralization occurs in the contacts of hematite bodies (soft/hard) and intrusive rocks with fractured itabirites. Other mineralization (Aba Norte, Área Central and X) is hosted on the contacts of other units. A system of fractures, as well as their intersections, thus represents the structural control on Jacutinga bodies and is responsible for the geometry of the orebodies. Of importance, there is no control by mineral/stretching lineations, fold axes and other ductile structure on the geometry and plunge of the orebodies.

Evidence for metre-scale variations in hematite composition within the Palaeoproterozoic Itabira Iron Formation, Minas Gerais, Brazil

AbstractHematite is a mineral the chemical composition of which rarely differs significantly from stoichiometric Fe2O3. As such, little attention has been paid to the mineral chemistry of hematite in Precambrian iron formations, where hematite forms monomineralic high-grade orebodies. Electron microprobe analysis of hematite from two iron-ore deposits, Cauê (Itabira district) and Gongo Soco, in the Palaeoproterozoic Itabira Iron Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil, has revealed distinct variations in chemical composition with respect to Ti and Cr. Hematite containing Ti and/or Cr is of very local occurrence in the itabirite unit and shows a spatial relationship to hematitic, palladiferous gold-bearing veins (known as ‘jacutinga’), occurring either within the veins (adjacent to, or included in, palladiferous gold grains) or in their vicinity. Where present, titaniferous hematite (to ∼1.3 wt.% TiO2) is lepidoblastic and defines a pervasive tectonic foliation (S1). In contrast, Ti-free, chromiferous hematite (to ∼6.4 wt.% Cr2O3) characteristically occurs as inclusions in palladiferous gold within S1-truncating ‘jacutinga’. Replacement of granoblastic, Ti-free, chromiferous martite with relicts of magnetite by lepidoblastic, Cr-depleted, titaniferous hematite proves that Cr and Ti were mobile during metamorphism. Chromium was ultimately fractionated into the hematite found in auriferous aggregates within cross-cutting ‘jacutinga’. A positive correlation between Cr and Pt in bulk-rock samples from the Itabira district suggests that Cr is a potential prospective guide for Au-Pd-Pt-bearing hematitic veins (‘jacutinga’).

The Au-Pd Mineralization at the Conceicao Iron Mine, ItabiraDistrict, Southern Sao Francisco Craton, Brazil: An Example of aJacutinga-Type Deposit

The Conceicao mine, which is located in the Itabira district, southern Sao Francisco craton, is a rare example of gold and palladium mineralization hosted by jacutinga, an altered Lake Superior-type iron-formation. The mine sequence consists of three metamorphosed lithostratigraphic units, which are from the oldest to the youngest: (1) an Archean volcano-sedimentary sequence, (2) a Paleoproterozoic iron-formation, and (3) a Paleoproterozoic quartzite. These units are cut by mafic intrusive rocks and, with the exception of intrusive contacts, all contacts between the various units are structural and interpreted as thrust faults. The rocks were subjected to three phases of folding: (1) D1, characterized by tight asymmetric and isoclinal folds, which develop locally into sheath folds (the D1 fabrics are the S1 foliation and elongation lineation); (2) D2, represented by tight, large-scale folds and associated parasitic folds; and (3) D3, characterized by open folds with an associated crenulation cleavage. The D1 and D2 deformation are interpreted to be products of simple shear with a transport direction consistently east over west. These rocks were metamorphosed to typical mineralogy of amphibolite facies conditions during the D1 phase of folding. The Conceicao orebody consists of a single quartz vein surrounded by jacutinga. The vein is parallel to the S1 foliation, which is locally oriented west-southwest and dips 55° to 75° NW, and was folded by D2 deformation. It extends 100 m downdip in the direction of the Le elongation lineation and 400 m along strike. The jacutinga consists of centimeter-scale layers of muscovite, specular hematite (hematite I), and octahedral hematite grains (hematite II) with remnants of magnetite, and a finely laminated envelope around the quartz vein, which is composed mainly of Fe hydroxides, hematite (hematite III), kaolinite, muscovite, and tourmaline. The quartz vein is dismembered and quartz grains are partially corroded and filled by Fe hydroxides and kaolinite. The ore minerals, interpreted to have formed during the main hydrothermal event, are palladian gold (up to 3.2 wt % Pd) and an unknown Pd-Sb-As mineral (tentatively identified as mertierite II), which are typically included in hematite II octahedra. A few grains of palladian gold (up to 0.9 wt % Pd) were found mantling hematite II grains. Native gold is commonly associated with Fe hydroxide in the finely laminated alteration envelope around the quartz vein and is interpreted to have precipitated during weathering. The similarities between the Au-Pd orebodies at the Conceicao and Caue deposits (referred to as jacutinga-type deposits) suggest that they were formed by the same processes and their characteristics favor an epigenetic model for gold and palladium concentration in the iron-formation. Since the Cretaceous, these Archean and Proterozoic rocks have been weathered, and the hydrothermal silicates and oxides have been replaced by kaolinite and Fe(Mn) hydroxides; gold and palladium have been remobilized and reprecipitated as pure metals or Pd oxides.

Transamazonian tectonism and Au-Pd mineralization at the Cauê Mine, Itabira District, Brazil: Pb isotopic evidence

Pb isotopic results are reported for mineral separates from iron formations that host Pd-Au mineralization at the Cauê Mine, in the Itabira District of the Southern São Francisco Craton, as well as for an amphibolite that has been tectonically imbricated with the iron formations during the earliest D1 tectonic event in this area. Pb isotopic results were obtained for hornblende and ilmenite that were rotated within the regional S1 foliation of an amphibolite. These minerals yielded a metamorphic age of 2.66 ± 0.27 Ga and a time-integrated μ1 value of 8.35. This shows, for the first time, the involvement of late Archean to Paleoproterozoic basement rocks in the tectonic history of the Itabira District; this unit is tentatively correlated with the Archean Rio das Velhas Supergroup in the Quadrilátero Ferrífero. Pb isotopic results were also obtained for quartz, gold and specular hematite that were formed during the transformation of the iron formations into itabirite and jacutinga which occurred during the D1 event. These minerals, which are stretched within the S1 foliation plane, yielded an age 1.83 ± 0.10 Ga which corresponds to the Tranzamazonian Orogenesis in the southern part of the São Francisco Craton. Other Pd-bearing gold deposits hosted in Minas Supergroup iron-formations of the southern São Francisco Craton have many similarities with the Cauê mine, suggesting that Pd-Au mineralization may have been a widespread process associated with the tectonic transport of the Minas sequence during the Transamazonian orogenesis.

Palladium-bearing gold deposit hosted by Proterozoic lake superior-type iron-formation at the Caue iron mine, Itabira District, southern Sao Francisco Craton, Brazil; geologic and structural controls; reply

Palladium-bearing gold deposit hosted by Proterozoic lake superior-type iron-formation at the Caue iron mine, Itabira District, southern Sao Francisco Craton, Brazil; geologic and structural controls; reply

Palladium-bearing gold deposit hosted by Proterozoic lake superior-type iron-formation at the Caue iron mine, Itabira District, southern Sao Francisco Craton, Brazil; geologic and structural controls; discussion

Palladium-bearing gold deposit hosted by Proterozoic lake superior-type iron-formation at the Caue iron mine, Itabira District, southern Sao Francisco Craton, Brazil; geologic and structural controls; discussion

Palladium minerals from the Cauê iron mine, Itabira District, Minas Gerais, Brazil

AbstractPalladium-bearing minerals from the Cauê iron mine, Itabira District, Minas Gerais, Brazil, are found in gold-rich jacutinga, a hydrothermally-altered Lake Superior-type carbonate-bearing oxide facies iron-formation. Palladium occurs as: native palladium with trace contents of Au, Fe and Cu; palladseite ((Pd,Cu,Hg)17Se15), which was found in the core of a grain of palladium; palladium–copper oxide ((Pd,Cu)O); and arsenopalladinite (Pd8(As,Sb)3), with inclusions of palladium–copper oxide. The palladium and palladium–copper oxide grains are coated with films of gold and commonly do not exceed 100 µm in width. These palladium minerals occur in hematite bands and in boudinaged bands of quartz and white phyllosilicate parallel to the S1 mylonitic foliation. Palladium-copper oxide also occurs as inclusions in gold grains which are strongly to weakly stretched parallel to S1.Palladium mineralization is interpreted as synchronous with intense D1 shearing and contemporaneous with the peak of thermal metamorphism. At high oxygen fugacities and high temperatures (up to 600°C), Pd may have been transported as chloride complexes and deposited following changes in pH caused by mineralizing fluids reacting with jacutinga. Deposition may also have been prompted by the formation of insoluble selenide and arsenide–antimonide minerals and by the dilution of C1 concentrations in the mineralizing fluid. Textural studies, and the zonation observed in palladium and other hydrothermal minerals, suggest that oscillations in the physico-chemical conditions of hydrothermal fluids occurred during the mineralizing event.

Palladium-bearing gold deposit hosted by Proterozoic lake superior-type iron-formation at the Caue iron mine, Itabira District, southern Sao Francisco Craton, Brazil; geologic and structural controls

The Caue iron mine is located in the Itabira district in the southern Sao Francisco craton. Three geologic units metamorphosed to amphibolite facies have been identified: (1) an Archean volcano- sedimentary sequence; (2) a Proterozoic iron-formation unit; and (3) a Proterozoic quartzite unit. The iron-formation unit is cut by amphibolite dikes and tectonically imbricated with talc schists and amphibolites of the volcano-sedimentary sequence. All the rocks were affected by three phases of folding and associated thrusting and boudinage. In the iron-formation unit, the D 1 and D 2 structures have been generated by a progressive simple shear regime with a transport direction consistently east-over-west. D 3 structures are characterized by open folds with an associated crenulation cleavage. The Caue palladium-bearing deposit comprises five orebodies hosted by jacutinga in the iron-formation unit. Jacutinga is a highly sheared, metamorphosed, hydrothermally altered Lake Superior-type iron-formation composed of hematite, quartz, talc, phlogopite, with minor amounts of tourmaline, apatite, and monazite. In the Corpo Y orebody, the highest gold concentrations occur in quartz veins, whereas in the Corpo X, Central, Aba Leste, and Aba Notre orebodies, the highest gold grades occur in hematite veins with local concentrations of quartz. The gold-rich veins are parallel to S 1 foliation in all orebodies except Aba Norte. In the Aba Norte, the gold-rich boudinaged hematite vein is parallel to S 2 . The ore minerals are palladian gold, gold, palladium, palladium oxide, and palladseite. The main mineralizating event was synchronous with the peak of thermal metamorphism (T = 600 degrees C) and with D 1 shearing and thrusting. The hydrothermal alteration associated with this event is characterized by the formation of tale, phlogopite, and tourmaline, and hematite and quartz veins, generating the jacutinga rock type. Weathering of the jacutinga resulted in the alteration of silicates to kaolin and oxides to goethite...

Palladian gold from the cauê iron mine, Itabira District, Minas Gerais, Brazil

AbstractPalladian gold from the Cauê iron mine, Itabira District, Minas Gerais, Brazil is found in horizons of hydrothermally-altered Lake Superior-type oxide-facies iron-formation locally called jacutinga. Palladium content of the gold grains varies from 1% to 20%, high palladium values being associated with palladium-copper-oxide inclusions and showing island-mainland and replacement (relict) textures in the Corpo Y and Corpo X orebodies. In the Aba Leste orebody, palladium is homogeneously distributed throughout the gold grains.Palladium and gold mineralization was synchronous with intense D1 shearing. Palladium was deposited early during the generation of S1-mylonitic foliation, and was replaced by gold during progressive deformation.At high oxygen fugacities (hematite stability field) and high temperatures (up to 600°C) Pd and Au may have been transported as chloride complexes, and their deposition may have occurred mainly as a result of changes in pH, but dilution of Cl concentrations may have also caused Pd and Au precipitation. Deposition of Pd as selenide may have taken place early during the creation of S1-fabrics.

Chemical and mechanical stability of hematite iron-ore in seawater

Hematite ore was considered as a potential high-density rock-fill material for the constuction of a major coastal protection work in The Netherlands (the Storm Surge Barrier in the Eastern Scheldt). Three groups of hematite ore from the Itabira District, Brazil, were investigated for their chemical and mechanical stability in a prefeasibility study. Chemical leaching tests showed only minor chemical activity in one of the three investigated groups of rock samples. The microstructure of the ore was shown by slake durability testing and a sand blast test to be a significant factor influencing mechanical resistance against abrasive processes. The iron ore contains an important volume percentage of cryptocrystalline limonite. The mechaical characteristics of the ore are influenced by the microstructural distribution of the limonite. Redistribution of limonite within the ore and release of limonite from the ore takes place in flowing water, which might lead to visual pollution of the environment if this material is used as rock-fill in coastal protection constructions.