The Proterozoic Vazante hypogene zinc silicate district, Minas Gerais, Brazil: a review of the ore system applied to mineral exploration

The Proterozoic Vazante zinc silicate district in Minas Gerais, Brazil, hosts world-class hypogene willemite deposits in dolomitic rocks interbedded with siliciclastic rocks deposited in subtidal to supratidal environments. Willemite ore bodies are structurally controlled along regional NE-trending structures which are interpreted as being active during the Neoproterozoic Brasiliano orogeny. The hydrothermal alteration is characterized by an early stage of Fe-dolomite, which replaced the host dolomitic rocks, followed by precipitation of minor sphalerite and then hematite and willemite. Elements commonly enriched in the zinc ore include As, Ba, Be, Bi, Cd, Co, Fe, Ge, In, Mo, Ni, Sb, Se, U, V and W. Mineralogical, fluid inclusion and isotopic data indicates that mixing of S-poor metalliferous saline fluids with meteoric water favored the formation of willemite ore. Carbonaceous phyllites from the underlying thick siliciclastic sequence show evidence of early enrichment in zinc (and ore-related metals) and remobilization, respectively, prior to and during the Brasiliano orogenic event. This unit is interpreted as a possible source of ore-related elements. It is proposed that during the Brasiliano orogeny, hot (T > 170 degrees C) saline fluids (>15 wt % eq. NaCl) leached metals from siliciclastic source rocks and precipitated willemite ore in the overlying dolomitic sequence along structures that favored mixing with oxidizing meteoric water

Contrasting Patterns of Alteration at the Wheeler River Area, Athabasca Basin, Saskatchewan, Canada: Insights into the Apparently Uranium-Barren Zone K Alteration System

Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors con- trolling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of min- erals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz over- growths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal al- teration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement flu- ids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high by ~250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls For- mation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid- and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric wa- ters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicat- ing low probability of this area to host uranium mineralization. However, areas of possible unconformity-re- lated uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed

Geochemical, isotopic, and geochronlologic constraints on the formation of the Eagle Point basement-hosted uranium deposit, Athabasca Basin, Saskatchewan, Canada and recent remobilization of primary uraninite in secondary structures

The Athabasca Basin hosts many world-class unconformity-related uranium deposits. Recently, uranium reserves for the Eagle Point basement-hosted deposit have increased with the discovery of new mineralized zones within secondary structures. A paragenetic study of Eagle Point reveals the presence of three temporally distinct alteration stages: a pre-Athabasca alteration, a main alteration and mineralization comprised of three substages, and a post-main alteration and mineralization stage that culminated in remobilization of uraninite from primary to secondary structures. The pre-Athabasca alteration stage consists of minor amounts of clinochlore, followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization of plagioclase and K- feldspar caused by surface weathering. The main alteration and uranium mineralization stage is related to three temporally distinct substages, all of which were produced by isotopically similar fluids. A major early alteration substage characterized by muscovite alteration and by precipitation Ca–Sr–LREE-rich aluminum phosphate- sulfate minerals, both from basinal fluids at temperatures around 240°C prior to 1,600 Ma. The mineralization substage involved uraninite and hematite precipitated in primary structures. The late alteration substage consists of dravite, uranophane-beta veins, calcite veins, and sudoite alteration from Mg–Ca-rich chemically modified basinal fluids with temperatures around 180°C. The post-main alteration and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at a minimum age of ca. 535 Ma. U– Pb resetting events recorded on primary and remobilized uraninites are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb to a distance of at least 225 m above the mineralized zones

Sulfide evolution during prograde metamorphism of the Otago and Alpine schists, New Zealand

Sulfide minerals in the Otago and Alpine schists, New Zealand, a metasedimentary belt exposed from unmetamorphosed greywackes up to amphibolite facies, underwent systematic changes in abundance, composition and texture during prograde metamorphism. In unmetamorphosed rocks, the most common sulfide mineral is framboidal pyrite, which contains abundant As (up to 14000 ppm), Co (up to 4000 ppm), Cu (up to 14000 ppm), Ni (up to 1100 ppm) and, locally, Ag (up to 270 ppm), Au (up to 90 ppm), and Sb (up to 240 ppm). Chalcopyrite, sphalerite, and galena also occur as isolated grains. Chalcopyrite and sphalerite contain few trace elements, whereas galena contains significant Se (up to 1600 ppm) and locally abundant Hg (up to 600 ppm). The distribution of these trace and minor elements is extremely heterogeneous. In subgreenschist-facies rocks, pyrrhotite replaces pyrite, and there is a clear textural change from framboidal pyrite to composite grains of pyrrhotite, sphalerite, chalcopyrite, galena and cobaltite. Pyrrhotite contains Co (average values 1100 ± 490 ppm), Cu (up to 17000 ppm), and Ni (up to 11000 ppm). Antimony and Hg are above detection in rare individual grains, but none of the other trace and minor elements sought are detectable. Sphalerite, galena and cobaltite increase in proportion in subgreenschist-facies rocks, and also contain higher concentrations of Ag (up to 1480 ppm in galena), Au (up to 230 ppm in galena and 110 ppm in cobaltite), As (38 ± 6 wt. % in cobaltite), Co (26 ± 4 wt. % in cobaltite), Hg (up to 4500 ppm in galena and 1100 ppm in sphalerite), and Sb (up to 1280 ppm in cobaltite and 770 ppm in galena). Pyrite, sphalerite, galena, and cobaltite become less abundant from subgreenschist- to amphibolite-facies rocks. In amphibolite-facies rocks, only pyrrhotite, chalcopyrite and trace amounts of galena and molybdenite occur, and none of these minerals contain detectable levels of Ag, Au, As, or Hg. Mass-balance calculations between sulfide minerals and whole rocks show that sulfides are important host minerals for S, Cu, and As, but host a minor proportion of Fe, Zn, and Pb. As pyrite, sphalerite, galena, and cobaltite become less abundant at higher metamorphic grade, Pb, Zn and Co are retained in the rock incorporated in other minerals, whereas As, Sb, Hg, Au, and Ag are removed from the rock, most likely by metamorphic devolatilization