Evolution of the Munali Intrusive Complex: host to a carbonate-rich Ni-(Cu-PGE) sulfide deposit

The Munali Intrusive Complex is hosted within supracrustal metasedimentary rocks located along a major structural lineament within the Zambezi Belt in southern Zambia. The complex comprises unmineralised gabbro surrounded by a marginal heterogeneous mafic–ultramafic breccia unit that is host to Ni-Fe sulfide. This marginal unit comprises a range of variably evolved brecciated mafic–ultramafic rocks that include gabbro, olivine-gabbro and dolerite, alongside younger, pegmatitic, apatite-magnetite-bearing clinopyroxenite, wehrlite and dunite. The magmatic evolution is most consistent with a model whereby early mafic rocks interact with hot, MgO- and volatile-rich melts along gabbro contacts, causing localised metasomatism of gabbro and pyroxenites, and progressively replacing pyroxene-rich rocks with olivine, forming pegmatitic ‘replacive dunites’. Sulfide mineralisation is characterised by a carbonate-rich apatite-magnetite-bearing assemblage predominately present as lenses of semi-massive to massive sulfide ore. The complex is enveloped almost entirely within a unit of marble, yet C and O isotope signatures of carbonate at Munali have revealed a clear mantle signature for some of the carbonate associated with sulfide, alongside a more dominant, crustally derived component. The carbonate occurring alongside sulfide displays micro to macro textures signifying the presence of carbonate melts formed from anatectic melting of the country rocks. The presence of fracture sets that define coarse breccia clasts (>1 m) indicate that the host rock was significantly crystallised and brittly deformed prior to carbonate and sulfide melt infiltration. Both carbonate and sulfide melts appear to have independently utilised these pre-existing weaknesses producing a pseudobreccia, and accounting for the seemingly chaotic nature of the orebody. The indication of sulfide being a significantly later phase suggests that the sulfide did not form in situ and was mobilised from elsewhere to be subsequently emplaced late within the Munali system

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

The nature and genesis of the Munali nickel sulfide deposit, Zambia

The Munali Ni-(Cu-Co-PGE) deposit is located within the Zambezi Belt in southern Zambia, and appears to represent a conduit-hosted sulfide ‘breccia’ deposit, situated along the Munali Fault. However, Munali displays many atypical features including abundant carbonate that question the applicability of traditional models of magmatic sulfide genesis. This work highlights how carbon, either as a primary mantle-sourced component, or a crustally-derived contaminant, can play an important role in the transport, generation and style of magmatic sulfides.The Munali complex comprises a dynamic suite of mafic-ultramafic rocks emplaced at ~860 Ma, with sequential emplacement of magmas with increasing MgO and FeO contents that include gabbro and (Cr-poor apatite-bearing) clinopyroxenite, wehrlite and metasomatic ‘replacive’ dunite, emplaced during the initiation of Zambezi rifting. Thin S- and evaporite-rich limestone units adjacent to the Munali Fault provided favorable pathways for ascending mantle-derived magmas. Pervasive interaction and assimilation of S and CO2 were critical in promoting magnetite crystallisation and the formation of carbonate and sulfide melts.Mineralisation is characterised by an unusually Ni-Fe-dominant massive sulfide, comprised of pyrrhotite>>pentlandite ± chalcopyrite ± pyrite, associated with abundant magmatic-hydrothermal carbonate-apatite-magnetite. Sulfide represents one of the youngest intrusive phases, present as a series of elongate lenses (0.5-30 m thick) that infiltrated the Munali Complex as a density-driven slurry, causing localised brecciation of the host mafic-ultramafic rocks. Sulfide displays extremely variable Ni/Cu ratios (0.1-71.5), moderate to high Pd/IrN ratios (~150-3500) and negative Cu and Au anomalies, primarily as a consequence of fractional crystallisation of the sulfide liquid during mobilisation, resulting in the segregation and crystallisation of Ni-Fe sulfide and the continued migration of a residual Cu-rich sulfide liquid.The Munali Complex shows indications of significant post-emplacement metamorphism during the Pan African Orogeny, with deformation of host rocks and potential for late-stage remobilisation of Cu and Au. As such, Munali represents a complex igneous system resulting from a range of syn- and post-magmatic processes.</div

Supergene enrichment of the Kitumba IOCG deposit, Zambia

Iron oxide copper-gold (IOCG) deposits host Cu and Au, as well as numerous other potential by-products such as U, Ag, Co, and rare earth elements. The Kitumba IOCG deposit in Zambia has undergone multiple stages of alteration and sulfide formation and subsequent supergene enrichment, with Cu reaching up to 30% in supergene zones down to depths of several hundred metres. Quantitative mineralogy characterised five styles of mineralisation. Stage 1 hypogene chalcopyrite and pyrite, is progressively replaced by Stage 2, a continuum of supergene stages: 2a chalcocite and minor covellite; 2b cuprite and native Cu, 2c malachite and 2d brochantite. In addition, a distinct zone of Au enrichment (up to 2 ppm) is found in Fe-oxide breccia near the upper parts of the deposit which appears to be hosted by a pocket of folded and heavily Fe-altered Katangan metasedimentary rocks. Replacement of hypogene sulfides by chalcocite occurred under acidic conditions (pH 5.5–6); with the precipitation of cuprite, native Cu under near neutral, more oxidising conditions (pH > 6) and precipitation of malachite under further oxidising conditions (pH 6.5–8), with localised zones of acidity (pH 5.9–7) that encouraged the precipitation of brochantite. Cobalt is notably depleted in the supergene zone relative to hypogene, whilst Ag is enriched. We present a genetic model for supergene mineralisation at the Kitumba IOCG deposit, and as such has implications for other supergene altered IOCG deposits with a similar supergene mineralogy. Importantly, the Cu mineralisation styles established each have distinctive geometallurgical characteristics, and as such, has implication for processing of ore. </p

The use of magnetite as a geochemical indicator in the exploration for magmatic Ni-Cu-PGE sulfide deposits: a case study from Munali, Zambia

Magmatic sulfide deposits hosted by mafic-ultramafic intrusions are the most important source of Ni and PGE on Earth. Exploration strategies rely on geophysics to identify the host intrusions, and surface geochemistry to identify anomalous concentrations of Cu, Ni, Co, Cr, As and other associated elements. The use of geochemical indicator minerals in overburden is used widely in diamond exploration and mineral chemistry in fresh rock is increasingly used to identify proxies for mineralisation in magmatic-hydrothermal systems. However, no indicator mineral techniques are routinely applied to magmatic sulfides. Magnetite represents an ideal indicator mineral for this mineralisation style due to its ubiquity in such deposits, its resistance to weathering, its recoverability from soil samples, and its chemical variability under differing conditions of formation. We use the Munali Ni sulfide deposit to test the use of magnetite as an indicator mineral. Magnetite from mafic, ultramafic, and magmatic sulfide lithologies in fresh rock at Munali show discernible differences in the most compatible elements (V, Ni, Cr). We propose a new Cr/V versus Ni discrimination diagram for magnetite that can be used to indicate fractionation of the parent magma (Cr/V increases from ultramafic to mafic), and the presence of co-existing sulfides (Ni contents >300ppm). The signatures of these three elements at Munali are comparable to sulfide-related magnetites from other deposits, supporting the broad applicability of the discrimination diagram. Samples taken from overburden directly on top of the Munali deposit replicate signatures in the fresh bedrock, strongly advocating the use of magnetite as an exploration indicator mineral. Samples from areas without any geophysical or geochemical anomalies show weak mineralisation signatures, whereas magnetite samples taken from prospects with such anomalies display mineralisation signatures. Magnetite is a thus a viable geochemical indicator mineral for magmatic sulfide mineralisation in early stage exploration

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.</p