Archaean Gold Mineralization in an Extensional Setting: The Structural History of the Kukuluma and Matandani Deposits, Geita Greenstone Belt, Tanzania

Three major gold deposits, Matandani, Kukuluma, and Area 3, host several million ouncez (Moz) of gold, along a ~5 km long, WNW trend in the E part of the Geita Greenstone Belt, NW Tanzania. The deposits are hosted in Archaean volcanoclastic sediment and intrusive diorite. The geological evolution of the deposits involved three separate stages: (1) an early stage of syn-sedimentary extensional deformation (D1) around 2715 Ma; (2) a second stage involving overprinting ductile folding (D2–4) and shearing (D5–6) events during N-S compression between 2700 and 2665 Ma, coeval with the emplacement of the Kukuluma Intrusive Complex; and (3) a final stage of extensional deformation (D7) accommodated by minor, broadly east-trending normal faults, preceded by the intrusion of felsic porphyritic dykes at ~2650 Ma. The geometry of the ore bodies at Kukuluma and Matandani is controlled by the distribution of magnetite-rich meta-ironstone, near the margins of monzonite-diorite bodies of the Kukuluma Intrusive Complex. The lithological contacts acted as redox boundaries, where high-grade mineralization was enhanced in damage zones with higher permeability, including syn-D3 hydrothermal breccia, D2–D3 fold hinges, and D6 shears. The actual mineralizing event was syn-D7, and occurred in an extensional setting that facilitated the infiltration of mineralizing fluids. Thus, whilst gold mineralization is late-tectonic, ore zone geometries are linked to older structures and lithological boundaries that formed before gold was introduced. The deformation-intrusive history of the Kukuluma and Matandani deposits is near identical to the geological history of the world-class Nyankanga and Geita Hill deposits in the central part of the Geita Greenstone Belt. This similarity suggests that the geological history of much of the greenstone belt is similar. All major gold deposits in the Geita Greenstone Belt lack close proximity to crustal-scale shear zones; they are associated with intrusive complexes and volcanics that formed in an oceanic plateau rather than subduction setting, and formed late-tectonically during an extensional phase. They are not characteristic of typical orogenic gold deposits

The petrogenesis of the Neoarchean Kukuluma Intrusive Complex, NW Tanzania

The Kukuluma Intrusive Complex (KIC) is a late Archean igneous complex, dominated by monzonite and diorite with subordinated granodiorite. The monzonite and the diorite suites have low silica content (SiO2 ≤ 62 wt%), moderate Mg# (Mg#average = 49), high Sr/Y (Sr/Yaverage = 79) and high La/Yb (La/Ybaverage = 56) ratios, and strongly fractionated (Lan/Ybn = 9–69) REE patterns. Their moderate Ni (Niaverage = 50 ppm), Cr (Craverage = 85 ppm), variable Cr/Ni ratio (0.65–3.56) and low TiO2 (TiO2average = 0.5 wt%) indicate little to no interaction with the peridotitic mantle. For most major elements (Al2O3, FeOt, Na2O, TiO2 and P2O5) the monzonite and the diorite suites display subparallel trends for the same SiO2 content indicating they represent distinct melts. Intrusions belonging to the diorite suite have high Na2O (Na2Oaverage = 4.2 wt%), Dy/Ybn (Dy/Ybn-average = 1.6), a positive Sr anomaly and uncorrelated Nb/La and Zr/Sm ratios suggesting derivation from partial melting of garnet-bearing amphibolite. Intrusions belonging to the monzonite suite have higher Na2O (Na2Oaverage = 5.61 wt%), Dy/Ybn (Dy/Ybn-average = 2.21), a negative Sr anomaly and correlated Nb/La and Zr/Sm ratios consistent with derivation from partial melting of eclogite with residual rutile. Small variations in the Th/U ratio and near chondritic/MORB average values (Th/Umonzonite = 3.65; Th/Udiorite = 2.92) are inconsistent with a subducting slab signature, and it is proposed that the monzonite and the diorite suites of the KIC formed by partial melting of garnet-bearing, lower mafic crust of an oceanic plateau. The granodiorite suite has lower Mg# (Mg#average = 41), moderately fractionated REE, low Sr/Y (Sr/Yaverage = 20), La/Yb (La/Ybaverage = 15), Dy/Ybn (DyYbn-average = 1.24) and small negative Eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation. Near-MORB Th/U (Th/Uaverage = 2.92) and Zr/Sm (Zr/Smaverage = 30.21) ratios are consistent with intracrustal melting of amphibolite. Archean rocks with an “adakitic” geochemical signature have been used to argue in favour of a plate tectonics regime in the Archean. The results presented here suggest that tectonic models for the Tanzania Craton, which invoke a subduction-related setting for all greenstone belts may need revision

Zircon U-Pb ages and Hf isotope data from the Kukuluma Terrain of the Geita Greenstone Belt, Tanzania Craton: Implications for stratigraphy, crustal growth and timing of gold mineralization

The Geita Greenstone Belt is a late Archean greenstone belt located in the Tanzania Craton, trending approximately E-Wand can be subdivided into three NW-SE trending terrains: the Kukuluma Terrain to the east, the Central Terrain in the middle and the Nyamullilima Terrain in the west. The Kukuluma Terrain, forms a NW-SE trending zone of complexly deformed sediments, intruded by the Kukuluma Intrusive Complex which, contains an early-syntectonic diorite-monzonite suite and a late-syntectonic granodiorite suite. Three gold deposits (Matandani, Kukuluma and Area 3W) are found along the contact between the Kukuluma Intrusive Complex and the sediments. A crystal tuff layer from the Kukuluma deposits returned an age of 2717 ± 12 Ma which can be used to constrain maximum sedimentation age in the area. Two granodiorite dykes from the same deposit and a small granodiorite intrusion found along a road cut yielded zircon ages of 2667 ± 17 Ma, 2661 ± 16 Ma and 2663 ± 11 Ma respectively. One mineralized granodiorite dyke from the Matandani deposit has an age of 2651 ± 14 Ma which can be used to constrain the maximum age of the gold mineralization in the area. The 2717 Ma crystal tuff has zircon grains with suprachondritic 176Hf/177Hf ratios (0.28108e0.28111 at 2717 Ma) and positive (þ1.6 to þ2.6) εHf values indicating derivation from juvenile mafic crust. Two of the granodiorite samples have suprachondritic 176Hf/177Hf ratios (avg. 0.28106 and 0.28107 at 2663 and 2651 Ma respectively) and nearly chondritic εHf values (avg. -0.5 and -0.3 respectively). The other two granodiorite samples have chondritic 176Hf/177Hf ratios (avg. 0.28104 and 0.28103 at 2667 and 2661 Ma respectively) and slightly negative εHf values (avg. -1.1 and -1.5 respectively). The new zircon age and isotope data suggest that the igneous activity in the Kukuluma Terrain involves a significant juvenile component and occurred within the 2720 to 2620 Ma period which, is the main period of crustal growth in the northern half of the Tanzania Craton

Geological controls on gold mineralization in the Kukuluma Terrain, Geita Greenstone Belt, NW Tanzania

[Extract] This thesis applies a multidisciplinary approach to better understand the geology of the Kukuluma Terrain and the geological controls on gold mineralization. I used a combination of field, computer and laboratory based techniques to help me constrain a variety of factors influencing the gold mineralization. This thesis is subdivided into 6 chapters with appendices as follows: Chapter 1 is the introduction to the thesis study topic and the geological setting of the Tanzania craton and Geita Greenstone Belt. It also provides a brief history of the Geita Gold Mine. Chapter 2 presents the methodologies followed in compiling the data sets presented in chapters 3, 4 and 5. I report on data collected from internal mine reports, regional and pit scale geological mapping, drill core logging, cross-section interpretations, and 3D modelling using Leapfrog (software version 3.1.1), and provide an overview of the methodologies used in the structural (chapter 3), geochemical (chapters 4 and 5) and geochronological (chapter 5) studies. Chapter 3 presents the first detailed structural evolution history of the Kukuluma Terrain and discusses the structural controls on gold mineralization. The structural evolution of the area was developed based on overprinting relationships observed from detailed regional and deposit-scale geological mapping, drill core logging and the review of historical datasets such as maps and geophysical images, the details of which are included in chapter 2. I use the relationships between the distribution of gold mineralization and different structures to place spatial and temporal correlation between the structural evolution and the mineralizing event. I also constrained the relative timing of dykes and intrusive rocks emplacement in relation to deformation and mineralization. This chapter has been prepared for publication. Chapter 4 deals with the petrogenesis of the dykes and intrusive units within the Kukuluma Terrain. I present detailed petrographic descriptions of the igneous rocks and high quality major and trace element geochemical analyses. The igneous rocks that intruded the Kukuluma Terrain form three suites: a monzonite suite, a diorite suite, and a granodiorite suite. Because the three suites overlap in space and partly in time I grouped them into the Kukuluma Intrusive Complex. The monzonite and the diorite suites form a ~ northwestsoutheast oriented body that runs through the middle of the Kukuluma Terrain and they are virtually undistinguishable in the field while the granodiorite suite occurs mostly as dykes with various orientations and locally form small intrusive bodies. The petrogenetic evolution of the igneous rocks is of particular importance, because of their close spatial relationship with the gold mineralization. It is also possible that fluids related to the emplacement of the igneous rocks may have played a role in gold mineralization. For example, the timing of emplacement and the geochemical characteristics of sanukitoid-type igneous rocks have been linked to gold mineralization in many Archaean gold deposits (Mikkola et al., 2014). This chapter has been submitted for publication. Chapter 5 presents the first zircon ages and Hf isotope data from igneous rocks and sediments from the Kukuluma Terrain. The geochronological results are used to constrain the absolute timing of emplacement and deformation where possible. The results of zircon dating made it possible to place a maximum mineralization age in the area. The detrital age data is used to understand the timing of sedimentation in the area, but also set up the timing on the beginning of deformation in the area. I used the Hf isotope data to better constrain the sources and to understand the processes of crustal growth in this part of the Tanzania Craton. This chapter has been submitted for publication. Chapter 6 contains the conclusions of the thesis and provides recommendations for further work in the area

Large volcanic landslide and debris avalanche deposit at Meru, Tanzania

International audienc

Holocene explosive eruptions in the Rungwe Volcanic Province, Tanzania

The Holocene explosive eruptive record of Rungwe and Ngozi volcanoes of the Rungwe Volcanic Province in Tanzania was reconstructed based on detailed stratigraphic field evidence combined with whole-rock major and trace element analyses of tephra samples. This reconstruction is supported by 25 new radiocarbon dates on palaeosols that provide additional constraints on the Holocene tephro-chronostratigraphy. We show evidence of two catastrophic Ngozi eruptions and five Rungwe pumice fallout deposits, and also identify several more intercalated poorly preserved pumice and ash deposits. The Ngozi eruptions probably played a role in shaping the present-day caldera. The Rungwe record includes a ca. 2.2 km(3) deposit of a Plinian-style eruption dated at ca. 4 ka, a sub-Plinian one at ca. 2 ka and at least three additional smaller-scale fallout deposits. The Rungwe explosive eruptive record shows that the volcano has been frequently active in its late Holocene past. This study highlights the need for volcanic monitoring in the region and presents herewith the first basis of future volcanic hazard assessment

The world-class gold deposits in the Geita greenstone belt, Northwestern Tanzania

The Geita mine is operated by AngloGold Ashanti and currently comprises four gold deposits mined as open pits and underground operations in the Geita greenstone belt, Tanzania. The mine produces ~0.5 Moz of gold a year and has produced ~8.3 Moz since 2000, with current resources estimated at ~6.5 Moz, using a lower cut-off of 0.5 g/t.The geologic history of the Geita greenstone belt involved three tectonic stages: (I) early (2820–2700 Ma) extension (D1) and formation of the greenstone sequence in an oceanic plateau environment; (II) shortening of the greenstone sequence (2700–2660 Ma) involving ductile folding (D2–5) and brittle-ductile shearing (D6), coincident with long-lived igneous activity concentrated in five intrusive centers; and (III) renewed extension (2660–2620 Ma) involving strike-slip and normal faulting (D7–8), basin formation, and potassic magmatism. Major gold deposits in the Geita greenstone belt formed late in the history of the greenstone belt, during D8normal faulting at ~2640 Ma, and the structural framework, mineral paragenesis, and timing of gold precipita-tion is essentially the same in all major deposits. Gold is hosted in iron-rich lithologies along contacts between folded metaironstone beds and tonalite-trondhjemite-granodiorite (TTG) intrusions, particularly where the contacts were sheared and fractured during D6–7 faulting. The faults, together with damage zones created along D3 fold hinges and D2–3 hydrothermal breccia zones near intrusions, formed microfracture networks that were reactivated during D8. The fracture networks served as conduits for gold-bearing fluids; i.e., lithologies and structures that trap gold formed early, but gold was introduced late.Fluids carried gold as Au bisulfide complexes and interacted with Fe-rich wall rocks to precipitate gold. Fluid-rock interaction and mineralization were enhanced as a result of D8 extension, and localized hydro-fracturing formed high-grade breccia ores. Gold is contained in electrum and gold-bearing tellurides that occur in the matrix and as inclusions in pyrrhotite and pyrite. The gold mineralization is spatially linked to long-lived, near-stationary intrusive centers. Critical factors in forming the deposits include the (syn-D2–6) formation of damage zones in lithologies that enhance gold precipitation (Fe-rich lithologies); late tectonic reactivation of the damage zones during extensional (D8) faulting with the introduction of an S-rich, gold-bearing fluid; and efficient fluid-rock interaction in zones that were structurally well prepared