Magnetic Petrology applied to the characterization of Pegmatite Dykes in Eastern Colombia

Abstract

In the sector of San Jose, Macanal, and Tabaquen, in eastern Colombia, granitic rocks cut by pegmatite dikes and quartz veins appear with the presence of magnetite, ilmenite, and ilmenorutile. Using magnetic petrology and geochemistry concepts and methods, the main objective is to determine if these types of rocks are genetically related and how the fluid chemically evolves during its crystallization and cooling. This work was conducted in three stages. Petrography and opaque metallography for identifying the occurrence, paragenesis, and secondary processes that transform the oxides. In a second stage and utilizing an Electron Probe Microanalyzer (EPMA), 214 quantitative analyses (WDS) and four compositional maps for magnetite, ilmenite, and ilmenorutile were performed, measu- ring the oxides FeO, TiO2, V2O3, MgO, MnO, Nb2O5, Ta2O5, Al2O3, Ga2O3, NiO, CaO, Cr2O3, SnO, and WO3. Since magnetite and ilmenite are favorable geothermometers that also allow the calculation of oxygen fugacity, the ILMAT program was used to calculate these values. In closing, integrate the data with the magnetic susceptibility values. The results determine crystallization temperatures between 358-414 °C for the granitic-host rock and 402- 499 °C for pegmatites dykes, in a system where oxygen fugacity increases, the Mn2+ is enriched in the ilmenite, and magnetite preserves a low content of trace elements thorough the evolution of the fluid. Taken together with the martitization and exsolution of hematite and rutile within ilmenite found in the petrography, these results allow us to conclude that an oxide-silicate re-equilibration process controls the evolution of this magmatic-hy- drothermal fluid with a KUIlB cooling trend-type reaction. Based on the Al + Mn vs. Ti + V ratio, the signature of the magnetite is like the Lucky Friday mine’s signature studied by Nadoll. However, the analysis of the 95th percentile shows a different concentration of trace elements in the magnetite of both sectors. Therefore, a new field of discrimination is proposed for this environment of anorogenic pegmatites of the NYF family. Finally, the magnetic susceptibility is controlled only by the abundance of magnetite in each type of rock. The granitic host rocks have the highest susceptibility values, followed by pegmatites and quartz veins with the lowest.

Genesis and Evolution of Pegmatites in Eastern Colombia: Insights from Mineral Chemistry

“…In addition, according to Zen (1988), peraluminous igneous bodies can host incompatible elements such as Sn, W, and other lithophile elements such as U, Li, B, Ta, Nb, which is consistent with the presence of Nb-Ta-bearing minerals in the area (Bayona-Valderrama, 2018;Charry et al, 2023). According to Winter (2001), these magmas occur both in continental collision and postorogenic tectonic environments.…”

“…Microcline, muscovite, and biotite crystals were selected in all thin sections, except in those where such crystals were absent or altered (Table 2). Some of these minerals were in paragenesis with opaque minerals, which were analyzed by Bayona-Valderrama (2018) and Charry et al (2023) using EPMA electron microprobe and Raman spectroscopy. According to these authors, those minerals were magnetite, hematite, ilmenite, Nb-rutile, and columbite (Table 2); the last two define hereafter the Nb-Ta bearing samples, which are highly relevant to the fractionation degree discussion.…”

“…First, pegmatites had the highest concentrations of Na, indicating a high fractionation degree (Alfonso et al, 2003), except for the pegmatite P_3005, which conforms to the results from microcline. In addition, the Nb-Ta-bearing pegmatites (Bayona-Valderrama, 2018;Charry et al, 2023) showed high magmatic fractionation degree (Fig. 18).…”

“…This area has dense vegetation, high temperature and moisture levels, and a smooth relief with an average slope of 1.3% along the Guainía River. Modified from Charry et al (2023).…”

“…This motivated a research agreement between the Universidad Nacional de Colombia and the Servicio Geológico Colombiano (Colombian geological service) where granitic and pegmatitic samples were collected in the eastern border of the Guainía Department, in the surroundings of San Jose, Macanal, and Tabaquén (Guerrero et al, 2017;Morales et al, 2017;Zarate et al, 2017). However, the persistent lack of certainty regarding the magma type that formed such granites and pegmatites, their interrelations, and that with the presence of Nb-Ta-bearing minerals (Bayona-Valderrama, 2018;Charry et al, 2023) motivated the present study. With this aim, major, minor, and trace elements were analyzed in apatite, biotite, garnet, microcline, muscovite, and tourmaline.…”