Summary:
The Braúna kimberlite deposits occur as discrete intrusions crosscutting the 2155 to 2132Ma Nordestina granodiorite batholith of the Paleoproterozoic Rio Itapicurú greenstone belt. The kimberlite occurrences are controlled by a NW-SE fracture system, which hosts three kimberlite pipes (Braúna 03, 04 and 07) and twenty kimberlite occurrences in the form of two parallel dyke systems, oriented approximately N30W. The pipe-like deposit morphologies range from circular to elliptical and are strongly controlled by local joints and faults with commonly irregular and locally outward dipping contacts. The dyke-like deposit morphologies range from single to en-echelon subvertical sheets. The dykes form segments up to 1km long and are typically <1m wide at surface. At depth, the dykes are frequently branched, with drill core intersection lengths ranging from 20 cm to 1.3m. The dyke system can be traced for about 17km along strike. Dykes are commonly observed between the pipes and in some cases, they appear to represent connections between pipes, suggesting they acted as feeders during pipe growth.
Owing to the tropical weathering, surface exposure of the pipes and dykes is scarce, and fresh kimberlite is available only through drilling. Macrocrystic and aphanitic coherent and volcaniclastic kimberlite has been recovered to date, including hypabyssal, possible extrusive coherent and probable pyroclastic kimberlite varieties. Primary mineralogy comprises olivine, phlogopite, clinopyroxene, perovskite, spinel, ilmenite, apatite, carbonate, and serpentine. The kimberlites are notably phlogopite-rich and compositionally the phenocrystic and groundmass phlogopite is tetraferriphlogopite comparable to that from South African orangeites, transitional kimberlites and olivine lamproites. Mantle-derived minerals include olivine, garnet, chrome diopside, spinel and ilmenite. The most common crustal xenolith type is wall-rock granodiorite, with lesser amphibolite and gneisses. These xenoliths can be highly altered and exterisive reaction with the host kimberlite and replacement by kimberlitic minerals is observed. Mantle xenoliths are relatively rare and highly altered in the Brauna kimberlites and include garnet-bearing peridotite and eclogite. The volumetrically most significant body is the Brauna 03 pipe, which is described in more detail below.
The Braúna kimberlites represent the first known primary diamond source of Precambrian age in Brazil.
Braúna 03 Geology
The Braúna 03 pipe consists of three adjacent irregular-shaped, pipe-like lobes referred to as the North, Central and South Lobes with interconnecting neck areas that have a combined surface area of approximately 2 ha. The lobes are circular to elliptical in plan view and are elongated and aligned in a northwest-southeast direction. They are vertically continuous and delineated by drilling to maximum depths below surface of 470m, 150m and 565m (North, Central and South Lobes, respectively).
The nature of the kimberlite infilling each lobe is broadly comparable, comprising hypabyssal kimberlite in root zone conditions, also presenting multiple pulse hypabyssal units, however subtle distinctions are apparent in the textural and clastic component characteristics, proportions of internal country rock dilution, and the degree of alteration which overprints and obscures the primary features of the kimberlite. Country rock xenolith-rich kimberlite (breccias) is observed discontinuously along the pipe margins and less commonly in the central portions of the lobes.
Brauna 03 Kimberlite Domain
K1
K1 is the geological domain that includes the following faciological unit variations: MHPK, MHPKAB, MGHPK.
MHPK: This petrographic facies is the most common of the Braúna kimberlites. It has an equigranular to inequigranular texture (aphanitic to porphyritic), composed mainly of macro/megacrysts of olivine pseudomorphs, immersed in a phlogopite-rich groundmass phlogopite. MHPK groundmass is often cut by secondary/late stage veins of calcite (syn- to late stage magmatic), serpentine and, more rarely, hematite.
Olivine (phenocryst and xenocryst) has an ovoidal to ellipsoidal appearance, and frequently appears as pseudomorphs substituted by serpentine, calcite and more rarely hematite. Olivine macrocrysts represent around 5-35% of this particular petrographic facies.
Garnet xenocrysts on MHPK are generally represented by the following suites: pyroxenitic low Cr203 (garnets G1, G4, G5 and G9); eclogitic low Cr203 and high CaO (G3); harzburgite high Cr203 (G10 and G12). These vary in abundance from around 0.1 to 2% of the volume of the interval of the analyzed rock.
Pyroxenitic garnets are the most abundant gamet phase. Xenocrysts of phlogopite are commonly observed, with granulometry varying from medium to coarse, and size ranging from 0.2cm to 6.0cm. They also occur in vein form, and as cumulates in the kimberlitic matrix.
MHPKAB: MHPKAB generally shows abrupt contact with its adjacent kimberlitic facies, however, transitional contacts associated with kimberlite breccia facies (MHPKB) also occur, though rarely. The facies is defined by the quantity of autoliths (autoclasts) that it contains (>15% vol.).
The matrix is clast-supported, being generally composed of MHPK, HPK and SHK clasts.
Secondary veins of calcite and serpentine (0.1-2cm) are frequently observed in these facies, as well as veins of hematite, associated with late magmatic stages. Polygonal serpentine (i.e. serphophite) and amorphous serpentine also occur in the form of centimetric veins.
Garnet xenocrysts are less common in this facies, representing less than 1% of the interval volume. When present they are composed of rounded pyroxenitic - low Cr203 (garnets G1, G4, G5 and G9) and harzburgitic - high Cr203 (G10 and G12) (<2mm).
MGHPK: The MGHPK (Megacryst Hypabyssal Phlogopite Kimberlite) facies is the most unusual in the Brauna kimberlite in volumetric terms. This facies is dark brown, very fresh, rich in olivine, serpentine and phlogopite, intersected by veins of calcite and serpentine. Cumulates of calcite and phlogopite are present in this facies. The fractures are filled by calcite and serpentine and late stage calcite veins cut crustal and mantle xenoliths. Typically, this unit contains up to 15% olivine megacrystals (>1cm), and are represented by small intervals kimberlites not exceeding 3m in length.
Garnet xenocrystals are very common in this facies, representing about 1% of range volume. When present, they are composed of pyroxenites rounded - low Cr203 (garnets G1, G4, G5 and G9) and harzburgite - high Cr203 (G10 and G12). The most common gamet macrocrystals are related to the peridotitic suite. Eclogitic garnets also occur - low Cr2O3 and high CaO (G3).
Garnet megacrystals are also common in MGHPK and are represented by peridotitic and eclogitic suites.
Crustal xenoliths are rare in this facies and when present are rounded, completely serpentinized and represent less than 1% of its total volume. The set of crustal xenoliths is composed of deep crustal xenoliths and fragments of rock walls less than 10 cm long.
K2: K2 is the geological domain that includes: HPK facies (Hypabyssal Phlogopite Kimberlite), SHK facies.
K3: The K3 facies is defined as a hypabyssal breccia (MHPKB – K3) and is composed of macrocrystalline K1 units containing between 15-25% of crustal xenolith.
K4: The K4 facies is defined as a hypabyssal breccia (MHPKB – K4) and is composed of macrocrystalline K1 units containing >25% of crustal xenolith.
CG: The CG unit was defined based on the granulometry of the kimberlitic matrix of the K1 unit, where it varies from medium to coarse grained restricted to the north lobe.
FG: The FG unit was defined based on the granulometry of the kimberlitic matrix of the K2 and K1 units, where it varies from aphanitic to fine grained restricted to the north lobe.