Revista de la Asociación Geológica Argentina

Preface: Mineralogy, petrogenesis and metallogeny linked to metalliferous and non-metalliferous ore minerals of Argentina

Teresita Montenegro — 2024-03-30

The association of niobian rutile, columbite-group minerals and uranium-rich pyrochlore in the Rancul granitic pegmatite, San Luis, Argentina

Miguel Ángel Galliski — 2024-03-30

We describe the mineralogy of the association of accessory niobian rutile, columbite-group minerals and uranium-rich pyrochlore that occur in the Rancul property, an NYF intragranitic pegmatite of the Potrerillos group, San Luis, Argentina. Other accessory and secondary minerals include biotite, muscovite, beryl, schorl, apatite, ilmenite, pyrite, bismuthinite, clinobisvanite, hematite, and “limonite”. Niobian rutile occurs as rare cm sized prismatic crystals in the core-margin assemblage usually enclosed in quartz or albite. It has Ti# [Ti/(Ti+Nb+Ta)] variable between ~ 0.81 and 0.92 and contents of TiO₂ between 63 and 80 wt.%, Nb₂O₅ ≤ 21.66%, most of the iron as Fe₂O₃, low UO₂ and higher Sc₂O₃. The contents (wt.%) of Ta₂O₅ are variable, reaching up to 7.86, of WO₃ ≤ 1.46, of SnO₂ ≤ 1.12, of Sc₂O₃≤ 0.37 and of ZrO₂ ≤ 0.15. The structure is that of monorutile (space group P4₂/mnm) and the cell parameters are a = 4.622 (2) Å; c = 2.983 (2) Å; V = 63.73 (5) Å³. The rutile has inclusions of a columbite-group mineral, likely ixiolite or disordered columbite, and a U-rich pyrochlore-group mineral with up to 53.66 wt.% of UO₂ accompanied by niobian rutile with higher Ti# and lower content of trace elements. They are not exsolution products and were produced along subparallel cracks possibly by Ca,Mn,U-rich aqueous fluids that reworked the primary niobian rutile leaching the trace elements and crystallizing the new mineral phases.

Geological characterization of the central-west sector of the Urcuschún Pluton, La Rioja province.

María Cecilia Marchi — 2024-03-30

The Urcuschún hill is located in the morphostructural unit of the Central Precordillera, in the central-west region of La Rioja province, Argentina. In this hill, an intrusive body called the Urcuschún Pluton outcrops, which induces contact metamorphism and metasomatism in the limestones of the San Juan Formation and in the basal levels of the Paganzo Group. As a result, meta-sediments are generated from the transformation of sedimentary protoliths, along with a Fe and Mo skarn in the Ordovician limestones and, primarily, in the calcareous conglomerate at the base of the Paganzo Group.

The objective of this study is to present the petrographic and geochemical characterization of the Urcuschún Pluton. Additionally, its crystallization age was determined using the U/Pb method on zircons, resulting in a Permian age (270.39 ± 0.45 Ma).

SUPERGENE MINERALS WITH INDIUM (IN) IN THE SAN ROQUE POLYMETALLIC PROJECT, NORTH PATAGONIAN MASSIF, ARGENTINA

Veronica Bouhier — 2024-03-30

San Roque (40° 45’ 54'' S ; 68º 48' 07'' W) is a mining project located in the eastern sector of Northpatagonian Massif, Río Negro province, Patagonia, Argentina. In this project the polymetallic mineralization has epithermal characteristics and is hosted in the jurassic volcanites, volcanoclastites and sedimentites of the Marifil Volcanic Complex. It was defined as a low to intermediate sulfidation epithermal system in which the ore mineral association mainly consists of native gold, low Fe In-bearing sphalerite, chalcopyrite and galena. This contribution documents the geochemical data and the mineralogical studies carried out in representative samples of two drillholes that intercepted Del Indio structure in the supergene oxidation zone. This structure represents one of the most explored epithermal manifestations of the project scale. According to the observations with a scanning electron microscope and semi-quantitative analyses, the main supergene minerals that concentrate In, Cu, Zn, Pb, Ga, Mn, P, As and Cl are attributed to hematite, plumbojarosite, coronadite and mottramite, which are hosted by coarse-grained epiclastic subfacies from the Marifil Volcanic Complex, described in this contribution as polymictic ortobreccia. It is also documented, for the first time, a supergene mineral-association together with base and precious metals, by oxidation of the polymetallic epithermal mineralization in the San Roque project. The veinlets, stockwork and, in a leser extent, disseminated mineralization style of the hypogene ore, with In-bearing sphalerite and pyrite as the predominant sulfides, combined with the porosity and limited acid-buffering capacity of the host rock, favored the penetration of acidic, and oxidizing relatively V-rich meteoric waters, giving rise the and subsequent oxidation process.

Chemistry of hydrothermal minerals as an exploration tool in Cu-Au-Mo porphyry type deposits. Examples from the Altar Project, San Juan

Laura Maydagán — 2024-03-30

Mineral chemistry studies on hydrothermal minerals have demonstrated their potential to significantly extend the geochemical footprint dimensions of porphyry-type deposits on the weakly altered rocks surrounding these large hydrothermal systems. Studies carried out in the Altar Central (AC) and Quebrada de la Mina (QDM) deposits of the Altar project (San Juan) allowed to recognize compositional variations in phyllosilicates and clays through electron microprobe studies, X-ray diffraction and spectroscopy infrared (SWIR) analysis, as well as in sulphides and quartz by means of electron microprobe analysis and LA-ICPMS. In this contribution, a summary review of the most significant compositional differences recognized in hydrothermal minerals from Altar project is provided as well as new preliminary data on investigations that are still being developed, which have the potential to be used in mineral exploration of Cu-Au-Mo porphyry type deposits and associated Au-Ag-Cu epithermal deposits.

Redox stage in the bearing schellite quartz vein of the El Colatillo tungsten mine, in the El Realito - Río de la Quebrada shear zone, San Luis, Argentina.

Eliel Enriquez — 2024-03-30

The study area is located in the southern portion of the Nogolí Metamorphic Complex. It is composed of metamorphic rocks in which Famatinian igneous units have been intruded. Ductile shear zones have developed on these rocks, which are part of the El Realito - Río de la Quebrada system. Small tungsten manifestations are located in sheared orthoamphibolites. The mineralization occurs in the form of veins and veins of hydrothermal origin and, to a lesser extent, disseminated in the host rock by a hydrothermal metasomatic alteration. According to mineralogical, textural and chemical studies carried out on the veins, three mineral stages were characterized, controlled by oxygen fugacity, which were called: early reducing stage, late reducing stage and oxidizing stage. The first early stage is made up of a mineralogical association of Qz+Pl+Cpx, while in the late stage the association of Amp₁+Tur+Grt±Ilm was determined. In the last oxidizing stage, the replacement of the earliest minerals occurs, forming Amp₂+Sch+Cz+Ttn±F-Ap±Ccp±Py. The mineralization has characteristics of hydrothermal deposits rich in W (P, Ti, F) of the granophyll type, with a close spatial relationship with the Quebrada El Colatillo Granite, a possible source of these. The age of the mineralization would be limited to the regional shearing, which controlled the introduction of hydrothermal fluids, which has been assigned to the Ordovician-Silurian.

Paragenesis of the mineralization of the Vein 49, Calcatreu Project, Somun Curá Massif, Río Negro, Argentina

Matías Sebastián Wernert — 2024-03-30

The Calcatreu Project is a low-sulphidation epithermal deposit, located in the center-west region of the Río Negro province, approximately 60 km south of Ingeniero Jacobacci city. The mineralization encompasses an array of multiple quartz-carbonate-adularia veins, with 746 kOz of estimated resources and ore grades of 2,36 g/t Au Eq. Vein 49 stands as one of the main ore bodies of the deposit, with a well-defined 1 km long streak and a thickness of between 3 to 12 meters. It follows a NE-SW orientation and a 55° to 75° SE dip. The mineralization was grouped in 6 hydrothermal stages: 1) Tectonic-hydrothermal breccias with a pyrite-smectite-chlorite infill; 2) Monomictic carbonate breccias 3) Carbonates with banded, massive and breccia textures, associated with massive sulfide veins of pyrite, chalcopyrite, galena, sphalerite and electrum; 4) Banded silica, with occasional carbonate, adularia and ginguro layers, composed by pyrite, chalcopyrite, galena, sphalerite, tetrahedrite-tennantite and electrum; 5) Sillica with massive, breccia and banded textures, which hold no gold-silver concentrations, and is linked with occasional amethyst; 6) Late banded, massive and druzy carbonates. The vein 49 exhibits supergene enrichment, with intense oxidation weathering that reaches up to 100 m depth, represented by iron and manganese oxides, as well as anhydrite-gypsum, chalcedony, and minor secondary copper sulfides. The mineralization occurred under an extensional tectonic environment, linked to the Gastre Fault Zone mega structure, and was controlled by normal faulting and boiling processes

Epithermal activity in brecciated Permian-Triassic? arkose: an unprecedented occurrence for the region at Los Cuatro Pozos quarry, southeastern San Luis province, Argentina

Raúl Lira — 2024-03-30

Los Cuatro Pozos is an outcrop of silicified arkosic sandstones with fanglomerate levels, of continental origin and of probable Permian-Triassic age. A vertical fault zone crosscuts the quarry at least along 150 m with NE (25º) strike and a variable thickness of 5 to 10 m. In its SW edge, crackle-, mosaic-, and chaotic breccia types have formed, cemented by manganese oxides (hollandite), chalcedony, idiomorphic quartz and several generations of calcite. Anomalous contents of Ba in positive correlation with Mn, mimic the composition of Sr-rich hollandite. Anomalous geochemical values of Cu support the finding of supergenic Cu phases. Weakly anomalous Mo, V and Li are also registered. The positive correlation between of Cu and V with Mn suggests its affinity with Mn oxides; however, the occurrence of Li and Mo is aleatory. The Co+Ni vs. As+Cu+Mo+Pb+V+Zn binary ratios and the ternary Ni+Cu+Co(*10)-Mn-Fe ratios support a hydrothermal origin for the Mn oxides. The trapping temperatures of primary fluid inclusions in calcite range from ~ 192 to ~ 255 °C. The presence of Tl adsorbed onto the Mn oxides, together with Cu, might be pathfinders of the existence of sulfides at depth. The probable age of mineralization and the extensional tectonics as the dominant metallotect, could relate the Los Cuatro Pozos epithermal activity and the source of fluids to the A-type magmatism and related mineralization of the Permian-Triassic Intracratonic Magmatic Corridor of southern La Pampa province.