A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32–35, Ti/Zr = 107–147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9–1.3], with abundances of 3–10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6–0.8] and Th [(Th/La)pm = 0.6–0.9]. Initial isotope compositions (ɛNd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ∼50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ~2830 and ~2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition

Orebody geometry in lode gold deposits from Zimbabwe: implications for fluid flow, deformation and mineralization

The geometry of some orebodies can be described simply and accurately by three orthogonal axes, U≥V≥W. The ratios between these axes can be expressed as a parameter j=(U/V−1)/(V/W−1), and represented by a graph of U/V plotted against V/W, analogous to the treatment of strain ellipsoids. The orientations of orebodies can be plotted simply on projections using the UVW axes. Measurements of ore bodies from two examples of lode gold deposits from the Zimbabwe craton show that most of these orebodies are oblate. However, orebodies can have significant U/V ratios, implying a component of pipe-like fluid flow during mineralization. Pipe flow is demonstrated to be orders of magnitude more conductive than flow in planar veins and faults. There are significant variations in orebody geometry between deposits and within different sections of a single deposit. W values appear to be influenced by host rock: more permeable rocks have higher W. A negative trend of j value with orebody volume indicates that orebodies do not evolve in a self-similar way, but tend to more oblate shapes with increasing volume

Archean magmatic granulites, diapirism, and Proterozoic reworking in the Northern Marginal Zone of the Limpopo Belt

The Northern Marginal Zone (NMZ) of the Limpopo Belt, southern Africa, is a high-grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite, which intruded minor mafic-ultramafic and metasedimentary rocks between 2.74 and 2.57 Ga. The intrusive rocks have crustal and mantle components, and occur as elliptical bodies interpreted as diapirs. Peak metamorphism (P ≤800 MPa, T = 800–850 °C) occurred at ca. 2.59 Ga. The highly radiogenic nature of the rocks in the NMZ, supplemented by heat from mantle melts, led to heating and diapirism, culminating in the intrusion of distinctive porphyritic charnockites and granites. Horizontal shortening and steep extrusion of the NMZ, during which crustal thickening was limited by high geothermal gradients, contrast with overthickening and gravitational collapse observed particularly in more recent orogens. The granulites were exhumed by the end of the Archean. The pervasive late Archean shortening over the whole of the NMZ contrasts with limited deformation on the Zimbabwe Craton, possibly owing to the strengthening effect of early crust in the craton. In the southeast of the NMZ, strike-slip kinematic indicators occur within the Transition Zone and the Triangle Shear Zone, where dextral shearing reworked the Archean crust at ca. 1.97 Ga

Visualizing structural geology: from Excel to Google Earth

[Extract] Virtual globes are outstandingly useful for visualizing structural geology (e.g. McCaffrey et al., 2008, Simpson and De Paor, 2009, Whitmeyer et al., 2009, Whitmeyer et al., 2010 and De Paor and Whitmeyer, 2011). Virtual globes can combine displays of geology and topography, allow rapid changes of scale and view point, and can easily integrate other digital data such as outcrop photographs (Whitmeyer et al., 2010). Virtual globes are also very effective for teaching structural geology (Tewksbury and Tewksbury, 2009), partly because they give more intuitive views of geology than traditional maps. Keyhole Markup Language (KML) allows users of virtual globes to visualize and manipulate spatial data without having to learn or purchase more sophisticated GIS packages (Bailey and Chen, 2011)

Relationships between faults, extension fractures and veins, and stress

Faults are commonly related to extension fractures, defined here as including extension veins. Extension fracturing is integral to fault initiation and propagation, and extension fractures also form after fault slip. In these situations, fault planes contain the intermediate principal stress σ2, and slip is perpendicular to the line of intersection between the fault and the extension fractures. However, for reactivated faults, multiple fault sets, and faults formed according to the Healy theory, σ2 is not necessarily within the fault plane. A theoretical analysis shows that the trace of an extension fracture on a fault can make angles from 0° to 90° with the maximum resolved shear stress. The angle depends on the fault orientation relative to the principal stresses, and the ratio between the stresses. Extension fractures only intersect faults perpendicular to the maximum resolved shear stress on faults containing the maximum or intermediate principal stresses, or when their magnitudes are equal (σ1 = σ2). Field observations show that extension fractures can intersect faults along lines at oblique angles to slip directions, as predicted by the theory. Such angles may be indicators of fault reactivation, multiple sets of faults, or Healy theory faulting

Applications of fractal geometry to mineral exploration

Fractal methods are now standard analytical tools in the life sciences, physics and chemistry, and they are\ud commonly applied in the Earth sciences, For example, petroleum exploration and production make extensive\ud use of fractals. Comparison with pure and applied research in these areas suggests that there are many\ud problems in the minerals industry and ore deposit research that can be solved by fractal techniques. This\ud presentation reviews the use of fractals to analyse the spatial distribution of mineral deposits, the relation\ud between fault roughness and mineral endowment, and applications of fractals in economic geology, in order\ud to demonstrate some fractal methods and their potential in exploration

Strain in sulphide filled foliation boudinage structures at the Mount Isa Cu deposit, Australia

Foliation boudinage structures are vein-like boudins forming in homogeneous, anisotropic rocks that can be filled with significant volumes of mineral infilling. At Mount Isa, foliation boudinage structures are associated with and filled by ore-related sulphides, predominantly pyrrhotite. Deformation of syn-tectonic, ore-related sulphides at Mount Isa has generally not been documented, fuelling controversy about ore timing. EBSD techniques, combined with strain analyses show that the ore-related sulphide infill of foliation boudinage structures underwent significant intracrystalline deformation. Pyrrhotite grains show well-developed grain shape and crystallographic preferred orientations, indicating that the foliation boudinage structures have undergone layer normal shortening at a relatively late stage in the deformation history. Deformation of pyrrhotite grains by dislocation creep was dominated by basal slip leading to dynamic recrystallisation of pyrrhotite by subgrain rotation. A change from plane to flattening strain maintained the same shortening direction, and may therefore have occurred continuously, rather than as two separate deformation events. Results of this study are consistent with a current kinematic model for the Mount Isa system and may have implications for future exploration in the area

Local to regional scale structural controls on mineralisation and the importance of a major lineament in the eastern Mount Isa Inlier, Australia: review and analysis with autocorrelation and weights of evidence

Although major crustal lineaments may play an important role in mineralisation, the relationship between\ud lineaments and mineral deposits can be quite cryptic, and structural controls may vary as a function of scale along\ud lineaments.Major lineaments alone may be of limited use for detailed target generation. The Cloncurry Lineament\ud in the Eastern part of the Mount Isa Inlier is a crustal scale structure defined by potential field-derived ‘worms’.\ud Weights-of-evidence quantifies the association between mineral occurrences and this lineament. Autocorrelation\ud is used to recognise structural controls on mineralisation at different scales, by progressively limiting the lengths\ud of the vectors between mineral occurrence points in the autocorrelation plot. The weights-of-evidence analysis\ud shows that Au, Au–Cu, Cu–Au and Cu deposits have a positive spatial correlation to the Cloncurry Lineament,\ud which suggests it that acted as a primary crustal scale control on the localisation of Cu and Au through focussing\ud mineralisation systems on a broad scale. However, autocorrelation defines a variety of local structural controls, which can be interpreted as shear zones, variably oriented fault sets, en echelon fault arrays, and potentially the orientation of bedding and/or iron formations which localise fluid flow and mineral deposition at finer scales. The results suggest that major lineaments defined by geophysical contrasts can be used in conjunction with techniques of spatial analysis for targeting structurally controlled mineralisation in areas under thin cover adjacent to mineralised terrains such as the Mt Isa Inlier