Mineralización, control estructural y modelado 3D de clavos mineralizados en la veta Julieta, distrito Casposo (Cordillera Frontal, San Juan, Argentina)
Diego Sebastián PalmaSebastián M. JovicLuciano LópezDiego M. GuidoGerardo PáezFacundo De MartinoGustavo SotarelloP. Rolando
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Abstract:
The Casposo District is located in the Cordillera Frontal, in the center-west of San Juan Province, Argentina. It is characterized by the presence of a low sulfidation epithermal system hosted in volcanic rocks of the Choiyoi Group, where the Mercado-Kamila and Julieta are the more important sectors, concentrating the largest resources of the district. Julieta consists of a quartz-calcite (Au-Ag) vein system, with colloform-banded and brecciated textures, as well as lattice-bladed replacements. In this study, a detailed analysis of the mineralogy and textures of the Julieta Vein system identifies 9 pulses grouped into 4 main events, 3 hydrothermal and 1 tectonic-hydrothermal. Pulses P1, P2 and P3 are barren and carbonated in nature. Pulses P4 and P5A are of chalcedony composition with a colloform-banded texture, where the last one shows cream-colored adularia bands and black ginguro bands, to which high Au (5,5 g/t) and Ag (52 g/t) values are associated. Pulse P5B is of silicic nature as well, although with a brecciated texture, whereas pulse P6 is siliceous carbonate. The composition of these pulses migrates to carbonates represented by P7 and P8, both of brecciated texture. Finally, the system closes with the tectonic-hydrothermal pulse P9. Julieta Vein has a dominant NW-SE strike, with minor subveins of E-W and N-S orientation, that can be divided into 3 segments: North, Central and South. The greater thicknesses, as well as the greater inclinations and high precious metals values are associated with inflections between the 3 segments. Concerning the ore shoots geometry, the structure has a main lithologically controlled subhorizontal ore shoot, while the secondary subvertical ore shoots are structurally controlled. Differences in the rheology of the host rock would control the vein morphology, forming a vertical jog. Factors such as the evolution of the mineralizing events, the vein morphology, and the precious metals distribution, allowed us to identify the importance of the lithological and structural controls in the development of mineralization in the Julieta Vein.Keywords:
Chalcedony
Silicic
Texture (cosmology)
Quartz megacrysts in Greece are systematically sampled, described and classified with respect to their morphology, solid inclusion mineralogy, and geological conditions of formation. Quartz deposition took place due to reduction in silica solubility in the hydrothermal fluids in favourable geological environments such as: alpine-type fissures close to major detachment faults, skarns and quartz veins crosscutting and generally related to granitoids and, finally, epithermally altered volcanic rocks of Tertiary age. The varieties of coloured crystals (amethyst, smoky quartz, morion, green quartz, rock crystal), the twinning of the crystals, the mineralogy of solid phases included in quartz (rutile, chlorite, sericite, feldspars) as well as the types and forms of the crystals (Tessin habit, Muzo habit, faden quartz, sceptre, window quartz, interrupted crystals, double-terminated crystals, phantom quartz, gwindel quartz, etc.) give important information on the growth mechanisms and the physico-chemical conditions during quartz formation. The quartz crystals found in several localities are gemmy and their potential for use as gemstones should be evaluated.
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ABSTRACT The extent of quartz cementation in shallow marine sandstones of the Brora Arenaceous Formation (Oxfordian) is closely related to the occurrence and abundance of Rhaxella perforata sponge spicules. Three cement morphologies are identified, chalcedonic quartz, microquartz and mesoquartz. Chalcedonic quartz forms matrix‐supported cements which preserve moulds of Rhaxella spicules. Chalcedonic quartz crystals have inequant development of crystal faces, on average 0·1 μm in diameter, and are the first formed cement and reveal homogeneous dark grey tones on the SEM‐CL/BEI. Microquartz forms 5–10 μm diameter crystals, which commonly grow on chalcedonic quartz substrates and show various grey tones under SEM‐CL/BEI. Mesoquartz crystals grow in optical continuity with their host grains, have >20 μm a ‐axial diameter crystals, and exhibit distinctly zoned luminescence. Although no opaline silica is preserved, the quartz cement is interpreted to have formed from an opaline precursor. Detrital quartz has an average δ 18 O composition of + 12·2‰ and mesoquartz (syntaxial overgrowth) has an average δ 18 O composition of +20·0‰. Estimates of the δ 18 O compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types; mixtures of the two give consistently higher δ 18 O compositions than mesoquartz, the higher estimate being +39·2‰. From oxygen isotope data the formation of quartz, microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71°C in marine derived pore waters. Organic and inorganic maturation data constrain the upper temperature limit to less than 60°C.
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Results of evaluation of natural aggregates which can be classified as slow/late expanding alkali-silica reactive rocks are presented. One common feature in these rocks was the presence of strained quartz; on the other hand, metastable silica minerals like opal or chalcedony etc, were not detected. Taking into account past service records, results of mortar-bar tests at 38 and 60 degrees C and rapid chemical tests up to 7 days, revised criteria for assessment of such aggregates are proposed. (A) For the covering abstract see IRRD 871351.
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The use of gamma radiation to induce or enhance c olor centers in g emstones is a widespread technique and applied worldwide on a industrial scale since a t l east 1970. The presence of defects and d efect structures in quartz from a border region of southern Brazil and Uruguay are the reason for the creation of a new color variety of quartz ca lled “Prasiolite” in the gem trade. This quartz has a pleasant green color produced b y gamma irradiation. The procedures of irradiation at IPEN show that the activation of these color producing defects can be monitored b y detailed chemical and spectroscopic a nalysis. For the first t ime UV-VIS-NIR spectra of this new color variety of quartz a re shown. They revealed special features of these quartz c rystals coming from basaltic terraines of the Parana Basin. Contrary to most specimen of quartz from other parts of Brazil, they have such a high water and OH content that they resemble more chalcedony or opal, but not highly crystalline quartz specimens. The cause of the color are broken bonds of Si-OH defining the so-called dangling bonds.
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