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The hypozonal Renco gold deposit in Zimbabwe is hosted by a shear zone system cutting granulitefacies enderbites. The geologic history of the region testifies to a polyphase tectonometamorphic evolution. Along the exhumation path, three major events are notable during which fluid inclusions could have been formed. Detailed petrographic work and comparison of calculated isochores with the P-T evolution, allows correlation of inclusion formation with the tectonometamorphic evolution. Fluid inclusions in the mineralized lithologies of the Renco gold mine are dominated by carbonic inclusions of highly variable densities. The high density carbonic inclusions are related to the peak of metamorphism. Their lower density counterparts show a bimodal occurrence. Some are re-equilibrated equivalents of the inclusions trapped at peak metamorphic conditions whereas others clearly post-date this event and are related to gold mineralization. The latter type of carbonic inclusions occurs in distinct microstructural domains of quartz crystals.(i.e, subgrain-, grain boundaries). Therefore, these inclusions are interpreted to have formed because of selective trapping of C[O.sub.2] from a mixed [H.sub.2]O-C[O.sub.2] low salinity ore fluid. All the aqueous inclusions observed at Renco can be attributed to a late, retrograde greenschist-facies overprint.

The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded, by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ18O values of 14 to 15 per mil (V-SMOW). The total variations in δ18O values of the biotite schist and calcsilicate rock are relatively small (12-14‰), whereas the marble records steep gradients in δ18O values (17-21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ18O and δ13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid (δ18O = 12-14‰; δD = -40 to -60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°c/ km-1) in the mine area. [ABSTRACT FROM AUTHOR]

Batu Hijau is a world-class gold-rich porphyry copper deposit, situated in Sumbawa Island, Indonesia. Deep drilling indicates that several intervals of calc-silicate rock were intersected, where they are apparently interbedded with volcaniclastic rocks. The calc-silicate rocks occur at the contact with copper-gold-bearing tonalite porphyries. The rocks are fine-grained and granular with green, reddish-brown and white layers. The green layers consist mostly of fine-grained clinopyroxene (diopside and hedenbergite) and the reddish-brown layers consist mostly of garnet (andradite), whereas the white layers are commonly composed of calcite and zeolite (chabazite). The calc-silicate rocks were formed by contact metasomatism of andesitic volcaniclastic rocks, as it is calcic in composition. Paragenesis study reveals at least two stages of calc-silicate mineralization. Stage 1 (prograde) is characterized by the presence of garnet (andradite), clinopyroxene (diopside and hedenbergite), anorthite and quartz at 340–360 C (high salinity 35–45 NaCl wt percentage eqn.). Stage 2 (retrograde) is characterized by chlorite and rare epidote at 280–300 C (low salinity 1–10 NaCl wt% eqn.). Late calcite ± quartz veinlets and calcite + chabazite veins/veinlets may also be related to this stage and cross cut the oldest mineral assemblages. Mineralization (magnetite, chalcopyrite and pyrite) may occur during the retrograde stage. Clinopyroxene and garnet were modified by Fe-rich hydrothermal fluid (oxidizing condition) indicated by increase of Fe from core to rim of both the cogenetic minerals. The presence of the calc-silicate rocks associated with massive magnetite-chalcopyrite-pyrite assemblage indicates the occurrence of calcic-exoskarn surrounding the Batu Hijau porphyry copper-gold deposit. [ABSTRACT FROM AUTHOR]

The Ajjanahalli gold mine is spatially associated with a Late Archean craton-scale shear zone in the eastern Chitradurga greenstone belt of the Dharwar craton, India. Gold mineralization is hosted by an ∼100-m-wide antiform in a banded iron formation. Original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite, stilpnomelane, minnesotaite, sericite, ankerite, arsenopyrite, pyrite, pyrrhotite, and gold at ca. 300° to 350°C. Elements enriched in the banded iron formation include Ca, Mg, C, S, Au, As, Bi, Cu, Sb, Zn, Pb, Se, Ag, and Te, whereas in the wall rocks As, Cu, Zn, Bi, Ag, and Au are only slightly enriched. Strontium correlates with CaO, MgO, CO2, and As, which indicates cogenetic formation of arsenopyrite and Mg-Ca carbonates. The greater extent of alteration in the Fe-rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers. The ore fluids, as interpreted from their isotopic composition (δ18O = 6.5-8.5‰; initial 87Sr/86Sr = 0.7068-0.7078), formed by metamorphic devolatilization of deeper levels of the Chitradurga greenstone belt. Arsenopyrite, chalcopyrite, and pyrrhotite have δ34S values within a narrow range between 2.1 and 2.7 per mil, consistent with a sulfur source in Chitradurga greenstone belt lithologies. Based on spatial and temporal relationships between mineralization, local structure development, and sinistral strike-slip deformation in the shear zone at the eastern contact of the Chitradurga greenstone belt, we suggest that the Ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first-order structure. The ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics. Based on this model of fluid flow... [ABSTRACT FROM AUTHOR]

The Los Pijiguaos bauxite mine in the western part of the State of Bolivar, Venezuela, represents the only currently exploited bauxite deposit in the country. Since mining commenced in 1987, total crude bauxite production has amounted to more than 43 million tonnes (Mt), with a present annual production close to 5 Mt. Proven and probable reserves are around 570 Mt, at an economic cut-off grade of about 44 wt.% alumina (Al2O3) and a total silica (SiO2) content of less than 20 wt.%.The orebody has an irregular, dismembered geometry as a result of strong and prolonged fluvial erosion during and after uplift of the region. The isolated minable blocks are located on plateau remnants separated by deep valleys. As a result, only half of the entire lease area is accessible for mining.Los Pijiguaos represents a typical laterite bauxite deposit that formed on a flat topped plateau from weathering of the underlying mid-Proterozoic Rapakivi-type Parguaza granite. The complete laterite profile is characterized by a vertical zonation comprising a concretionary zone (the bauxite horizon), and a mottled zone (the saprolite horizon) on granitic bed rock. The economic bauxite zone attains an average thickness of 7.6 m. On a mine scale, best ore grades and greatest thickness of the ore horizon are at topographic highs. Laterally, grade distribution is also not isotropic but follows distinct WNW–SSE trends. The spatial control on the mineralization is a result of the westerly inclination of the planation surface and the formation of major NW-striking fault systems in the region.Gibbsite is the most abundant mineral in the economic-grade bauxite horizon. The occurrence of minor nordstrandite is also restricted to that zone. Kaolinite is not present in the bauxite zone, but in the lower saprolite zone kaolinite can be more than 5 wt.%. An inverse correlation exists between gibbsite and quartz. Abundance ratios for the two minerals decrease from around 45 in the bauxite horizon to around 2 in the lower saprolite zone. Hematite is commonly more abundant than goethite. In the bauxite horizon, however, the goethite/hematite ratio is greater than 1. Anatase occurs as an accessory phase mainly in the bauxite horizon.Mass balance considerations demonstrate that bauxitization resulted in progressive desilication, hydration, and Al2O3 and Fe2O3 concentration in the residual weathering blanket. Silica as well as alkali and alkaline earth metals were almost completely leached. Mass and volume losses in the order of 61% and 77%, respectively, attest to severe mineralogical, textural, and chemical changes.From an economic point of view, the Los Pijiguaos bauxite represents a high-quality almost pure gibbsitic ore that allows processing to alumina by the low-temperature Bayer process around 140 °C. In terms of material consumption, 2.3 tonnes of dry bauxite are used for every tonne of alumina produced. The accompanying mass of red mud amounts to 550 kg per tonne alumina. The relatively high abundance of hematite in the bauxite has a positive effect on the red mud settling rate and, thus, favors a high extraction yield of the alumina plant.Los Pijiguaos is based on a huge reserve that will provide bauxite for more than 100 years at the current rate of mining. The entire region may contain as much as 6 billion tonnes of probable bauxite resources. [Copyright &y& Elsevier]

The gold mineralization of the Hutti Mine is hosted by nine parallel, N–S trending, steeply dipping, 2–10 m wide shear zones, that transect Archaean amphibolites. The shear zones were formed after peak metamorphism during retrograde ductile D2 shearing in the lower amphibolite facies. They were reactivated in the lower to mid greenschist facies by brittle–ductile D3 shearing and intense quartz veining. The development of a S2–S3 crenulation cleavage facilitates the discrimination between the two deformation events and contemporaneous alteration and gold mineralization. Ductile D2 shearing is associated with a pervasively developed distal chlorite–sericite alteration assemblage in the outer parts of the shear zones and the proximal biotite–plagioclase alteration in the center of the shear zones. D3 is characterized by development of the inner chlorite-K-feldspar alteration, which forms a centimeter-scale alteration halo surrounding the laminated quartz veins and replaces earlier biotite along S3. The average size of the laminated vein systems is 30–50 m along strike as well as down-dip and 2–6 m in width.Mass balance calculations suggest strong metasomatic changes for the proximal biotite–plagioclase alteration yielding mass and volume increase of ca. 16% and 12%, respectively. The calculated mass and volume changes of the distal chlorite–sericite alteration (ca. 11%, ca. 8%) are lower. The decrease in δ18O values of the whole rock from around 7.5‰ for the host rocks to 6–7‰ for the distal chlorite–sericite and the proximal biotite–plagioclase alteration and around 5‰ for the inner chlorite-K-feldspar alteration suggests hydrothermal alteration during two-stage deformation and fluid flow.The ductile D2 deformation in the lower amphibolite facies has provided grain scale porosities by microfracturing. The pervasive, steady-state fluid flow resulted in a disseminated style of gold–sulfide mineralization and a penetrative alteration of the host rocks. Alternating ductile and brittle D3 deformation during lower to mid greenschist facies conditions followed the fault-valve process. Ductile creep in the shear zones resulted in a low permeability environment leading to fluid pressure build-up. Strongly episodic fluid advection and mass transfer was controlled by repeated seismic fracturing during the formation of laminated quartz(-gold) veins. The limitation of quartz veins to the extent of earlier shear zones indicate the importance of pre-existing anisotropies for fault-valve action and economic gold mineralization. [Copyright &y& Elsevier]

Gold mineralization at Renco mine in the Northern Marginal Zone of the Limpopo Belt, Zimbabwe, is hosted by a set of intersecting steep and shallow dipping shear zones, locally termed ‘reefs’. Both steep and shallow reefs are closely associated with quartz-mylonites. The crystallographic preferred orientation of quartz from samples of these quartz-mylonites indicate that deformation in steep reefs occurred under amphibolite-facies conditions. Pole figures and microstructures of samples from shallow reefs, in contrast, show evidence for an origin of the auriferous mylonites at amphibolite-facies conditions that were subsequently overprinted by deformation under lower-greenschist-facies conditions. Since mineralization is hosted by both reef geometries, gold mineralization is inferred to have occurred during deformation and associated fluid infiltration along mylonites at amphibolite-facies grades, coeval with Late-Archaean thrusting of rocks of the Northern Marginal Zone onto rocks of the Zimbabwe Craton.The crystallographic preferred orientation patterns of quartz point to a normal sense of movement during reactivation of the shallow reefs, probably related to the Meso-Proterozoic tectonometamorphic event that has affected the central parts of the Limpopo Belt at ca. 2.0 Ga. Normal faulting along the boundary between the Northern Marginal Zone and Zimbabwe Craton has, to date, not yet been documented, and may provide some important constraints for the understanding of the kinematics of the Meso-Proterozoic tectonics in the Limpopo Belt. [Copyright &y& Elsevier]

Abstract: Gold mineralization in the Mesoarchaean New Consort gold mine in the Barberton greenstone belt, South Africa, is situated in the immediate hanging wall of an exhumed, composite granitoid gneiss dome known as the Stentor Pluton. The granitoid-greenstone contact is characterized by a contact-parallel high-strain zone that separates the mid-crustal gneisses from the greenschist facies greenstone belt. In contrast to the lower greenschist facies deposits in the central parts of the greenstone belt, the mineralized shear zones at New Consort are hosted by distinctly higher grade metamorphic rocks, ranging from upper greenschist to upper amphibolite facies conditions. A distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the Stentor gneiss dome against the overlying greenstone belt. Examination of the structural and petrologic evolution of the mineralized high-strain zone, combined with new U–Pb and Ar–Ar data and previously published ages, indicates a multistage tectono-metamorphic evolution. Early high-T, medium-P metamorphism (ca. 600–700°C and 5±1kbar) in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen-parallel (NE–SW) extension and associated detachment shearing during the main (D2) accretionary event in the centre of the greenstone belt, between ca. 3260 and 3225Ma. Recent work has shown that these early high-T fabrics were reactivated at ca. 3105Ma during the subsequent D3 tectonism and associated emplacement of large potassic granite batholiths. The gold mineralization was coeval with NE–SW extensional exhumation and solid-state doming of the Stentor gneiss dome during the subsequent D4 deformation. This late-stage deformation and the gold mineralization postdate the emplacement of the D3-related potassic granites by more than 50 million years, as indicated by (1) a U–Pb age of 3046±27Ma for a retrograde titanite generation in a D2-related amphibolite (sample 113), and (2) a 3027±7.5Ma age for hydrothermal titanite from the so-called footwall lens mineralization, which temperature has been estimated at 590±40°C. This high-T event is also recorded by a “corrected” 3040±4Ma hornblende Ar–Ar age from the same sample, suggesting that cooling of the Stentor gneiss dome shortly followed the exhumation. A considerably younger Ar–Ar age (2966±4Ma) has been obtained for an amphibolite from the contact-parallel high-strain zone, pointing to non-uniform cooling following the mineralization. The gold mineralization was largely contemporaneous with the emplacement of NNW trending, syn-kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca. 3030–3040Ma. The mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the Barberton greenstone belt, which has previously been dated at ca. 3080Ma. In conjunction with previously published ages, the data indicate the Au mineralization in the Barberton greenstone belt to have occurred during, at least, two mineralization episodes at ca. 3080 and 3040Ma. The main phase of Au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid-greenstone terrain that may have lasted some 200Ma. [Copyright &y& Elsevier]