ALBERT

Description: Gold production from the northern Pataz district in the eastern Andean Cordillera of Peru has been sourced mainly from mesothermal quartz-carbonate-sulfide veins hosted by the Mississippian Pataz batholith. Gold is also found in basement-hosted veins underlying the batholith, in the Vijus-Santa Filomena area of the district. Both are located within a central horst; similar vein mineralogy and proximal phengitic white mica alteration are common to both. However, comb-textured quartz, the chemical compositions of bulk ore and sulfide minerals, and the presence of barite and siderite veins suggest that the basement-hosted veins formed at a shallower crustal level. Similar expressions of hydrothermal alteration associated with anomalous gold, As, Sb, and Tl are also present in the adjacent Lavasen graben, where alteration is intimately associated with volcanic processes that deposited the Mississippian Lavasen Volcanics. K-Ar and 40 Ar- 39 Ar ages for hydrothermal illite from all three locations range between Mississippian and Late Triassic but are consistent with a single Mississippian hydrothermal event, if the data record a minimum age for original illite formation. The geologic setting, mineralization styles, and chemical data suggest a range of crustal depths, ranging from mesothermal batholith-hosted veins through shallow to intermediate depths for the Vijus-Santa Filomena area to a near-surface epithermal setting for the Misquichilca area. Telescoping of this 10- to 13-km crustal range into a 3-km topographic section of the Andes is attributed to syn- and postmineralization uplift and erosion. Sulfide-rich high-grade ore shoots and moderately saline fluid inclusions in the batholith-hosted veins are inconsistent with the original orogenic gold model and suggest a magmatic source component for the ore fluid, consistent with stable isotope (O, H, C, and S) compositions of quartz, illite, carbonates, and sulfides. The isotopic data suggest a mixed magmatic-meteoric ore fluid in the basement-hosted deposits of the Vijus-Santa Filomena area and the volcanic-hosted Misquichilca area. Both the Pataz batholith and the Esperanza Subvolcanic Complex are of the same Mississippian age as the hydrothermal alteration and mineralization. The Esperanza Subvolcanic Complex, comagmatic with the Lavasen Volcanics, contains cognate mineral clots from which a subjacent magma chamber can be inferred. It exhibits potassic, calc-silicate, and argillic alteration, and evidence for the evolution of an Fe-rich volatile phase. The Lavasen-Esperanza magma suite is ferroan and weakly alkaline, with A-type affinities. These features provide a conceptual genetic link with hydrothermal alteration associated with gold mineralization, including Fe (±As) sulfides, phengitic white mica, celadonite, Fe-rich carbonates, and less common Fe oxides. An oxidized intrusion-related model is proposed for gold and hydrothermal alteration in the northern Pataz district.

Description: 〈span〉〈div〉Abstract〈/div〉The Karouni orogenic gold deposits are located in north-central Guyana 35 km to the west of the 5 Moz Omai gold deposit. They are hosted in a 2200 to 2100 Ma greenstone belt within the Paleo- to Neoproterozoic Guiana Shield. Karouni consists of two deposits, Smarts and Hicks, located 2 km apart along the NW-striking Smarts-Hicks shear zone. Both deposits are hosted within a sequence of greenschist facies mafic volcanic rocks and felsic intrusions. The hydrothermal alteration mineral assemblages vary according to lithology and are characterized by narrow selvages (〈1–4 m in width). A chlorite-talc-calcite assemblage dominates in high MgO basalt, whereas in high TiO〈sub〉2〈/sub〉 dolerite a progression toward the vein is seen from chlorite-calcite-rutile- to albite-dominated mineralogy. Karouni is anomalous among orogenic gold deposits for its dominant sodic alteration and distinct lack of potassic alteration. Gold is located within inclusions in coarse, disseminated pyrite associated with the proximal alteration zones and as coarse native gold within the quartz-carbonate veins. Minor gold is also located within Au-bearing telluride minerals. The high TiO〈sub〉2〈/sub〉 dolerites formed a favorable chemical trap due to their high magnetite content, suggesting sulfidation via redox reaction as a possible mechanism of gold deposition. Mass balance modeling of the hydrothermal alteration indicates a wall rock-dominated system with limited addition or subtraction of major elements with the exception of C, S, and Na. Modeling of the proximal alteration has also shown strong trace element enrichment of W-Bi-Ag-Te-Mo-Pb, all of which are correlative with gold. In situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element geochemistry and secondary ion mass spectrometry (SIMS) S isotope analyses of pyrite from the gold-bearing hydrothermal system within the deposit indicate a geochemically and isotopically homogeneous system with only minor trace element variation due to differences in host rock, suggesting a single hydrothermal pulse correlative with the late stages of the Trans-Amazonian orogeny.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Karouni orogenic gold deposit is located in north-central Guyana, 35 km to the west of the 5 Moz Omai gold mine. The deposit is hosted within 2.2 to 2.1 Ga volcano-sedimentary rocks of the Barama-Mazaruni Supergroup, part of the Paleo- to Neoproterozoic Guiana Shield. Karouni consists of two zones, Smarts and Hicks, located 2 km apart along the NW-striking Smarts-Hicks shear zone, a second-order splay of the regional-scale Makapa-Kuribrong shear zone. The Karouni camp is composed of a lower sequence of mafic volcanic rocks, overlain by a lower sequence of immature sandstone and conglomerate, and an upper sequence of sandstone and laminated carbonaceous siltstone, intruded by several generations of felsic plutons and dikes. Whole-rock geochemical analysis indicates their formation in oceanic island-arc environment, and mantle-like characteristics of the high MgO basalts may indicate the presence of deep-seated structures during the early history of the camp. Regional-scale deformation during the Trans-Amazonian orogeny led to tectonic inversion of the volcano-sedimentary basins, greenschist facies metamorphism, and the development of strike-slip shear zones. Late movement on these shear zones is interpreted to be responsible for hydrothermal fluid flow, alteration, and gold mineralization within the Karouni gold camp. The Smarts and Hicks orebodies are localized within dilatational bends formed at changes in strike of the Smarts-Hicks shear zone during late dextral transcurrent movement. Rheological contrast played a dominant role in the formation of the deposits with shear-hosted, NW-striking, and steeply dipping quartz-carbonate-chlorite ± tourmaline-pyrite-gold (V〈sub〉2a〈/sub〉) veins preferentially hosted in ductilely deformed, high MgO basalts, whereas mineralized N-S, quartz-carbonate-chlorite ± tourma-line-pyrite-gold (V〈sub〉2b〈/sub〉) veins are hosted within rheologically competent high TiO〈sub〉2〈/sub〉 dolerite sills and granodiorite dikes. The interaction of these structures with favorable lithology is key for localizing high-grade orebodies.〈/span〉

Description: The Neoproterozoic Telfer deposit, one of Australia’s largest gold-copper deposits, is located in the Paterson orogen. Several highly differentiated calc-alkaline to alkali-calcic peraluminous granites intruded the metasedimentary rocks near (5–20 km) Telfer contemporaneously with structurally controlled gold-copper mineralization. Fluid inclusion assemblages with different fluid inclusion types were identified in samples from a range of different vein types. These inclusion types range from three-phase aqueous L aq + V aq + S halite , high-salinity (≤50 wt % NaCl equiv), high-temperature (≤460°C) inclusions to two-phase aqueous or two-phase aqueous carbonic, low- to moderate-salinity (2–22 wt % NaCl equiv), moderate- to high-temperature (≤480°C) fluid inclusions. Fluid inclusion trapping mechanisms and interpreted precipitation mechanisms for gold and copper include (1) adiabatic cooling between 450° and 200°C in all veins and, (2) locally, fluid phase separation at about 300°C. The trapping pressure of fluid inclusion assemblages trapped during phase immiscibility was calculated to be approximately 1.5 kbar. For fluid inclusion assemblages that lack evidence for phase immiscibility, a pressure at the temperature of final homogenization of up to 3 kbar was calculated. This high pressure value is interpreted to be related to local fluid overpressure, as a consequence of fault zone movement, in faults and fractures that localized gold at Telfer. Phase immiscibility and gold precipitation were induced during sharp pressure decrease accompanying fault zone movement. In situ laser inductively coupled plasma-mass spectrometry (ICP-MS) analyses of fluid inclusions revealed high trace element contents in all fluid inclusion assemblages. Manganese/Fe ratios of 〈0.24 in all vein types suggest that reduced fluids dominated the system, but, locally, a switch to more oxidized conditions with Mn/Fe ratios 〉0.24 is observed. Given the high temperatures and salinities of up to 480°C and 42 wt % NaCl equiv, Au and Cu were likely transported as chloride complexes. This interpretation is supported by the observation that the highest base metal contents occur in the highest-salinity fluid inclusion. Potassium/Ca ratios of 〉1 in most assemblages, the high homogenization temperatures (≤480°C) in many fluid inclusion assemblages, and the high trace element contents (e.g., Fe, Mg, K, Na) in most of the fluid inclusion assemblages are compatible with involvement of a magmatic hydrothermal fluid during gold-copper mineralization. This fluid was probably derived from the coeval granites in the Telfer area and, thus, Telfer is interpreted to be a distal, intrusion-related, metasedimentary rock-hosted, gold-copper deposit type.

Description: This study examines the whole-rock geochemistry and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) iron oxide chemistry of itabirite and related iron ore, as well as underlying phyllitic wall rock, of the Pau Branco iron ore deposit, Quadrilátero Ferrífero, Brazil. The phyllite and iron ore of Pau Branco extend from hypogene deep-seated into supergene weathered zones. The phyllitic sequence that underlies the itabirite-hosted iron ore along a sheared contact includes (from the bottom to top) carbonaceous and carbonate-, chlorite-, and Fe-rich zones, which likely reflect distal to proximal alteration halos. The Fe-rich phyllite and iron ore comprise a complex iron oxide mineralogy: (1) magnetite-martite is hosted in itabirite and related medium-grade iron ore and present as disseminated minerals in phyllite; (2) microplaty hematite replaces chlorite and carbonate in the phyllite; (3) granoblastic hematite is a recrystallization/precipitation product and the dominant iron oxide of the hard iron ore; (4) schistose specular hematite overgrows earlier iron oxides; and (5) goethite replaces earlier hematite and amphibole within the weathering horizon. Whole-rock geochemistry and related mass balance calculations reveal that, unlike the itabirite-hosted iron ore, interpreted to be the result of a solely residual iron enrichment via depletion of most major oxides and trace elements, the iron enrichment of the phyllite is caused by addition of Fe 2 O 3 . The Fe 2 O 3 is likely sourced from the overlying itabirite/iron ore and is mineralogically reflected in the replacement of carbonate and chlorite by hematite. The rare earth elements (REE) abundances of the different ore and phyllite zones reveal seawater-like REE signatures and Y/Ho ratios of the itabirite, hypogene and supergene iron ore, shale-atypical REE patterns in the unaltered phyllite, and Eu fractionation trends. This suggests a distinct input of hydrothermal fluids during the Fe enrichment of the phyllite. Locally, restricted Ce fractionation is attributed to distinct (e.g., Mn-rich) lithostratigraphic intercalations within the itabirite. Laser ablation-ICP-MS mineral chemistry of phyllite-hosted martite and microplaty hematite suggests a strong chemical inheritance by the host rock and precursor mineral. Positive Ce anomalies are restricted to phyllite- and itabirite-hosted martite only and, therefore, may reflect the highly oxidative conditions during martitization. At Pau Branco, the fluid-rock interaction affected not only the itabirite, but also the underlying phyllite, resulting in distinct alteration halos that extend beyond the itabirite toward the footwall. The significant chemical influence and importance of country-rock lithologies as a possible elemental source, for example, must be taken into account when interpreting whole-rock geochemical data and investigating an itabirite-/banded iron formation-hosted iron ore system.