ALBERT

Description: The Cenozoic Ambohimirahavavy alkaline complex in Madagascar consists of several syenitic to granitic intrusions (24.2 ± 0.6 Ma) the largest of which, the Ampasibitika intrusion, is characterized by the presence in its outer flanks of late peralkaline granitic dikes intruding mudstone and limestone of the Isalo Group. A network of dikelets and veinlets propagates from these dikes, intruding along bedding or obliquely to bedding. At the contact between the dikes and dikelets and a limestone, a reaction zone enriched in rare metals, dominated by calc-silicate minerals such diopside and andradite-grossular, forms a rare example of skarn resulting from peralkaline igneous activity. Much of the rare-metal mineralization (REE, Zr, Nb, Th, Sn, and Ti) occurs as secondary phases in the dikelets and skarn. In the dikelets and endoskarn, high field strength element (HFSE)-rich phases consist mainly of zircon, bastnäsite-(Ce), and Ca-REE-, and Ca-HFSE-rich phases in pseudomorphs after aegirine-augite. In the exoskarn, the main HFSE-rich phases are bastnäsite-(Ce), zircon, pyrochlore, Nb-rich titanite, and an unidentified F-rich Ca-zirconosilicate finely disseminated in a matrix composed of calcite, diopside, andradite, phlogopite, quartz, fluorite, and fluorapatite. The secondary zircon is characterized by a low Zr content and by the presence of REE, Ca, Al, and Fe. Three types of primary fluid inclusions were observed in dikelets and skarn in quartz, calcite, and diopside: liquid-rich inclusions (L-V) (20 to 40 vol % vapor) occur in all three minerals and homogenize to liquid; vapor-rich inclusions (V) (〉90 vol % vapor) occur in diopside and quartz, and homogenize to vapor; and halite-bearing L-V inclusions (L-V-H) occur in diopside and quartz, and homogenize either by disappearance of the vapor bubble or by halite dissolution. The L-V inclusions have low to intermediate salinity and homogenize at temperatures ranging from 200° to 380°C. The V inclusions have low salinity and homogenize at higher temperature (350°–395°C). The L-V-H inclusions mainly contain NaCl (35–45 wt % NaCl equiv), and homogenize by three modes, namely, bubble disappearance (mode A), halite dissolution (mode B), and simultaneous bubble and halite disappearance (mode C); the homogenization temperatures range from 260° to 380°C. We propose a model in which rare metals were transported by a Cl – , F – and HFSE-rich orthomagmatic fluid exsolved at 400° to 450°C and about 20 MPa. At these conditions, the fluid was in the two-phase region and vapor dominated. The rare metals were deposited as a result of the interaction of this fluid with limestone and mixing with an external fluid. This interaction/mixing buffered the orthomagmatic fluid to higher pH and lower temperature, resulting in the destabilization of REE-chloride complexes and deposition of fluorocarbonate minerals in the limestone; Zr and Nb, which were likely transported as hydroxyl-fluoride complexes, precipitated as zircon and pyrochlore due to deposition of fluorite and a consequent decrease in fluoride activity.

Description: The low- P /high- T Variscan metamorphic dome of Gavarnie-Gèdre-Héas, Central Pyrenees, harbors a small (1 m 3 ) but special cordierite and orthoamphibole-bearing enclave embedded within an anatectic granite. Compared to other cordierite-orthoamphibole occurrences described in the literature, the enclave shows singular characteristics: (1) its mineral composition is unique with cordierite (Crd) + gedrite (Ged) + biotite ss (Bt) + staurolite (St) + garnet (Grt) + spinel (Spl) + corundum (Crn) + ilmenite (Ilm) and sulfides (Sulf), but lacking quartz (Qtz), plagioclase (Pl), and aluminosilicate minerals; and (2) its chemical composition, with low SiO 2 , high Al 2 O 3 , K 2 O, and TiO 2 and high Fe 2 O 3T /MgO. The Crd-Oam-bearing rocks also occur as layers in stromatic migmatites that show a mineral composition (Bt + Crd + Ged + Pl + Qtz + Ilm + Grt ± St) and a chemical composition more akin to other described occurrences. The cordierite-orthoamphibole enclave records a succession of metamorphic reactions: (1) local Crd-Oam growth at the expense of Bt with release of K 2 O; (2) development of coronas of Crd ± Ilm ± Grt around St; (3) isochemical breakdown of St to Crd + Spl + Crn. In the Ged-bearing layers in interlayered migmatites, the formation of the Crd-Oam assemblage involves the consumption of Bt, Pl, and Qtz. Mass balance calculations show that in the enclave most reactions took place in an open system with addition of Si and release of K 2 O. Pseudosections indicate a clockwise P-T path, with high-temperature conditions (700–730 °C) at decreasing pressure (from 5.6 to 3 kbar). This fits the P-T path of other metamorphic domes of the Variscan Central Pyrenees.

Description: The Ianapera emerald deposit is hosted in the Neoproterozoic Vohibory domain of southern Madagascar, within a tightly folded metamorphic sequence of mafic and ultramafic rocks (M-UMR), gneiss, and marble, a few kilometers north of the major tectonic break known as the Ianapera-Ampanihy Suture. Late-collisional metaluminous to peraluminous felsic intrusions outcrop in the area. Emerald occurs mostly within metasomatic phlogopitite and quartz-tourmaline veins, developed within lenses of M-UMR. Recent work recognized granitic pegmatites as the source of the emerald-forming fluids, leading to the classification of proximal and distal mineralization styles, based on whether or not pegmatites are associated with the phlogopitite, respectively ( Andrianjakavah et al . 2009 ). Considerations of the chemistry of tourmaline and scapolite associated with emerald and beryl, data from fluid and solid inclusions in these minerals, thermodynamic calculations, and mineral thermobarometric data confirm that Be originated from the pegmatites, transported in a moderately saline aqueous fluid that exsolved at about 600 to 680 °C and 4–6 kbars. This fluid was enriched in alkali elements, incompatible elements and dissolved sulfur, and produced the proximal mineralization. Migration along fractures caused loss of at least part of the pegmatitic elements and local mixing with a metamorphic CO 2 -rich fluid, leading to the more distal mineralization at somewhat lower temperatures (520 to 650 °C). Oxygen and hydrogen stable isotopes were measured from beryl and emerald; the data corroborate a magmatic-hydrothermal origin for the mineralization and a common source for proximal and distal fluids. The calculated 18 O H2O of 12.5 at 520 °C for the distal emerald and the D H2O range measured from the channel of the emerald samples fit within the oxygen and hydrogen isotopic range of values defined for peraluminous granitic magmatism. The occurrence of a distal style of emerald mineralization such as typified by Ianapera suggests that metamorphic origin models for similar occurrences worldwide should be taken with care and possibly reconsidered in terms of source and origin of fluids and metals.

Description: The El Callao mining district is the most important gold-producing region in Venezuela. It is hosted in the Paleoproterozoic Guasipati-El Callao greenstone belt, which forms part of the Guayana craton, the Venezuelan extension of the Guiana Shield of South America. It consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. The Colombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 Moz) and grades of up to 60 g/t. Gold mineralization is concentrated within a vein network in the Colombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. Gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures. Based on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. Both types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating As-rich and As-poor bands. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. The highest concentrations (5–23 ppm Au) are found in oscillatory-zoned pyrite rims, which correlate with the highest As concentrations (16,000–23,000 ppm). In As-poor bands, Au (up to 1.5 ppm) and As (300–6,000 ppm) concentrations decrease by about an order of magnitude. Copper, Bi, Te, Sb, Pb, and Ag always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. Visible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930. At an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of ankerite, which, in turn, resulted from alteration of Fe-Ti oxides. Gold, together with other chalcophile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. Subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid-rich and vapor-rich aqueous-carbonic fluids. This process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the As- and Au-rich overgrowths in pyrite: (1) removal of H 2 O into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in pH due to partition of H 2 S and CO 2 into the vapor, thus decreasing the solubility of sulfide minerals; and (3) an adiabatic decrease in temperature, lowering the solubility of As and Au in the liquid. Waning of this process restored precipitation of As-poor pyrite, until the onset of a new cycle. Because pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. Previously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. This newly created porosity facilitated fluid circulation and remobilization of structurally bound gold, as well as of other chalcophile elements (Ag, Cu, Bi, Te, Pb, and Sb), which reprecipitated together with pyrite in the form of native gold, sulfides, and tellurides, either as small inclusions or as larger grains within fractures. This remobilization process facilitates the exceptionally high gold tenor found in the deposit, where the Colombia corridor is intersected by the Santa Maria fault.

Description: The Gaoua mining district, underlain by Paleoproterozoic volcanic and intrusive rocks of the Boromo-Goren greenstone belt in southern Burkina Faso, is a rare case where economic copper and gold resources occur together in the Birimian terrane of West Africa. The main country rock lithologies consist of flows and volcaniclastic sediments of basaltic to andesitic compositions. These are intruded by subvolcanic diorite and quartz diorite stocks and dikes, granophyre dikes and, less commonly, gabbroic dikes. The stocks are porphyritic in texture, locally brecciated, and characterized by development of a carbonate, anhydrite, and quartz stockwork system. Structural observations, detailed petrography, and sulfide geochemistry reveal that copper and gold in the stockworks and surrounding altered rocks cannot be associated with the same mineralizing event. Copper mineralization, mainly in the form of chalcopyrite, is typically observed in hydrothermal breccia and is associated with early arsenic-poor and gold-free pyrite. The geochemistry of the plutonic rocks and associated basalts and andesites implies formation in an arc setting. Textural evidence indicates that gold mineralization is closely tied to a second generation of sulfides, with the later pyrite hosting visible gold and minor chalcopyrite that clearly postdates the copper ore. Analyses of these pyrites by LA-ICP-MS reveal compositions rich in Ag, As, Au, Bi, Cu, and Pb. Pyrite with this signature was only observed in local shear and fracture zones that crosscut the host rocks and copper-rich breccia. Therefore, our data indicate the strongly deformed and metamorphosed porphyry copper mineralization in the Gaoua district is overprinted by orogenic gold mineralization. Formation of the early porphyry copper deposit took place at 2165 ± 24 Ma based upon Re-Os dating of the early pyrite; the age of the younger gold event, which is similar in style to that described throughout the West African craton, is not known but may be related to one of the lode gold episodes that are widespread through the West African craton at ~2150 and ~2100 to 2040 Ma. The Gaoua copper deposit, one of very few Precambrian porphyry copper deposits known in West Africa, formed in a volcanic-arc setting that predated the Eburnean orogeny in the Boromo-Goren greenstone belt.

Description: The Alamoutala gold deposit is located in the Kédougou-Kénieba inlier, a window of Paleoproterozoic rocks that crop out in eastern Senegal and western Mali. The deposit is part of the ~3-Moz Au Yatela district and produced 308,400 oz Au between 2002 and 2012. Country rocks in the Alamoutala open pit consist of carbonate rocks, arenites, wackes, and siltstones. The sedimentary rocks have been subjected to polycyclic deformation and regional greenschist-facies metamorphism. Synkinematic, calc-alkaline, intermediate quartz-feldspar porphyry stocks intruded the metasedimentary rocks at 2083 ± 7 Ma (U-Pb zircon). Amphibolite-facies contact metamorphism of carbonate rocks resulted in the formation of a high-temperature-low-pressure magnetite-bearing skarn. Field relationships and microtextural data, however, indicate that economic gold mineralization is shear hosted and occurred during retrograde contact metamorphism. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral-reverse displacement at the time of mineralization. Ore minerals deposited synchronous with a potassic hydrothermal alteration assemblage defined as biotite + calcite + pyrite ± K-feldspar, actinolite-tremolite, quartz, muscovite, and tourmaline. Petrographic studies reveal that ore-related sulfides largely consist of pyrite with lesser pyrrhotite and chalcopyrite. The ore also contains accessory Ag-Sb-As-Hg-Bi-Te-Mo-W-Zn-Pb-Ni-Co-U-bearing mineral species. The Alamoutala deposit represents two distinct hydrothermal systems in which a locally developed magnetite-bearing skarn was overprinted by a region-wide orogenic gold event in the late Eburnean.

Description: The ~8-Moz Sadiola Hill gold deposit is located in the Kédougou-Kénieba inlier, a window of deformed ca. 2200 to 2050 Ma rocks that crop out in eastern Senegal and western Mali. The geology of the inlier differs from other Paleoproterozoic granite-greenstone belts and sedimentary basins by the abundance of carbonate rocks. The Sadiola Hill gold deposit occurs within 3 km of the Senegal-Mali shear zone, and country rocks in the region have undergone polycyclic deformation. The lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks, which include arenite, wacke, and siltstone. The sedimentary rocks have been subjected to regional greenschist-facies metamorphism and have been intruded by multiple generations of synkinematic, calc-alkaline stocks and dikes. The bulk of the ore is hosted within a N-S- to NNW-trending, 10- to 50-m-wide, brittle-ductile dilational shear zone defined as the Sadiola shear zone. Gold lodes also occur along an array of steep NNE-trending shear zones. Geometric and kinematic analyses indicate that the ore-hosting structures were undergoing sinistral displacement at the time of mineralization, locally defined as the D 3s NNW-SSE-shortening event. Hydrothermal alteration is polyphase and includes an early high-temperature calc-silicate phase (i.e., porphyroblastic growth of actinolite-tremolite) followed by a potassic phase (i.e., biotite-calcite-quartz ± K-feldspar-tourmaline-actinolite) that was synchronous with ore mineral deposition. Paragenetic studies reveal a multistage ore development that includes an As-rich sulfide stage, followed by an Au-Sb stage. The ore is associated with a metal enrichment suite of Au-As-Sb ± Cu-Fe-W-Mo-Ag-Bi-Zn-Pb-Te. The Sadiola Hill deposit shares a similar relative timing and structural setting to that of other world-class orogenic gold systems in the West African craton (e.g., Ashanti, Loulo), with gold deposition occurring during a period of transcurrent tectonics, after the cessation of region-wide compressional deformation. The high-temperature paragenesis at Sadiola Hill is atypical of gold mineralization in the craton and indicates that the late Eburnean tectono-magmatic activity between ca. 2080 and 2060 Ma played an important role in the dynamics of hydrothermal fluid circulation along the Senegal-Mali shear zone.