Chicharro, Eva*, Boiron, Marie-Christine**, López-GarcÃa, José Ã.* and Villaseca,. Carlos***. *Departamento de CristalografÃa y MineralogÃa, Universidad ...
GEORAMAN 10 – Nancy – 11-13 June 2012
Combined microthermometric and Raman spectroscopic techniques to characterize the N2-CH4-CO2-H2O mixtures in the fluid inclusions of the quartz veins of the Logrosán granitic cupola (Cáceres, Spain) Chicharro, Eva*, Boiron, Marie-Christine**, López-García, José Á.* and Villaseca, Carlos*** *Departamento de Cristalografía y Mineralogía, Universidad Complutense, José Antonio Novais 2, 28040 Madrid, Spain ** G2R, Université de Lorraine, CNRS, BP 239, 54501 Vandoeuvre lès Nancy, France ***Departamento de Petrología y Geoquímica, Universidad Complutense, José Antonio Novais, 2, 28040 Madrid, Spain The Logrosán tin deposit (Cáceres, Spain) is located in the Sn-W metallogenic province of the Iberian Peninsula that belongs to the Variscan orogenic belt of Europe. The Logrosán stockwork is classified as peraluminous, perphosphorous, and volatile-rich late-tectonic Variscan leucogranite. The granitic cupola is hosted by the Schist-Greywacke Complex of Neoproterozoic age. A contact metamorphic aureole surrounds the granite affecting the adjoining Neoproterozoic rocks. Tin ore occurs as quartz-veins that cross-cut both the granite and occasionally the metamorphic rocks, and as disseminations in greisenized zones. We focus on the fluids of the intragranitic mineralised veins. The mineral association of the intragranitic veins includes cassiterite, wolframite, columbite-tantalite, rutile, arsenopyrite, sphalerite, chalcopyrite, stannite, tetrahedrite-tennantite, molybdenite, bornite, bismuth, pirrothite, quartz, muscovite and tourmaline. Two mineralizing stages are recognized: (1) An oxide-stage characterised by precipitation of cassiterite at 480-430 ºC (data from the arsenopyrite geothermometer); and (2) A sulphide-stage of lesser temperature at 460-330 ºC (data from the arsenopyrite and sphalerite-stannite geothermometers)1. Double polished wafers (250 µm) of three intragranitic veins samples were studied by microthermometric and Raman spectroscopic techniques. Two of the samples represent the oxide precipitation stage, whereas the third one represents the sulphide stage. Microthermometric measurements were performed using a Linkam THMSG 600 on quartz and cassiterite at Complutense University of Madrid. Analyses of the non-aqueous component of individual inclusions were measured using a Dior X-Y multichannel modular Raman spectrometer at G2R laboratory (Université de Lorraine, Nancy).
Type I Type II Type III Type IV
ThN2-CH4 TmCO2 ThCO2 Tmclathrate Tmice XN2 XCH4 XCO2 ºC ºC ºC ºC ºC %mole %mole %mole n.v. 8 / 13 n.v. 45 / 47 53 / 55 0 -110/-135 -57 / -85 4 / 15 -2 / -7 48 / 70 12 / 30 14 / 40 ~(-100) -57 / -79 -8 / -35 8 / 15 -2 / -4 16 / 40 4 / 34 46 / 80 0 / -4 Table 1: Microthermometric and Raman spectroscopic summarized data (n.v.: not visible)
Four types of fluid inclusions were identified, and the microthermometric and Raman data are summarized in Table 1. (1) Type I are N2-CH4-(CO2)-H2O two phases (vapour/liquid ratio of 40% approximately) fluid inclusions. They are primary in the cassiterite crystals and very scarce. Due to the optical conditions of the cassiterite crystals the microthermometric data from these inclusions are not well constrained; thus more measurements will be required to establish the partial homogenisation temperatures.
GEORAMAN 10 – Nancy – 11-13 June 2012
(2) Type II inclusions are ubiquitous in all samples, they consist on two phases (H2O(L) and N2rich+CH4+CO2(V)) or one phase (N2-rich+CH4+CO2(V)) at room temperature. The vapour/liquid ratio varies from 35 to 100%; (3) Type III inclusions are two phases (H2O(L) and CO2-rich+N2+CH4(V)) or one phase (CO2rich+N2+CH4(V)). The degree of filling of the vapour phase ranges from 15 to 100%. We noted an increasing CO2 content as the vapour bubble became bigger. Besides, the monophasic inclusions have the highest CO2 contents. Monophasic type III fluid inclusions have been so far only found in the sulfide stage veins; (4) Type IV inclusions contain low-salinity aqueous liquid and vapour with a degree of filling of about 10-50%. Raman analyses have shown different carbonic components in some of them. The microthermometric data indicate temperatures that cannot be easily explained unless we are dealing with mixtures of gaseous phases. In this regard, Raman analyses allowed recognition of a N2-CH4CO2-H2O mixture, which has not been studied in depth in the published literature; in fact, very few data of these mixtures have been published so far. Partial temperatures of homogenization (N2-CH4) have been found between -110 and -135 ºC in type II fluid inclusions. These temperatures are far away from the critical partial homogenization of N2 pure, settled at -147 ºC. Higher values than -147 ºC in partial homogenization are due to the entrance of components others than N2. At this temperature CO2 is in a solid phase, thus these variations should be explained in terms of the incorporation of methane (Kerkhof, 1985)2. We have observed a direct correlation between homogenization temperature and CH4 (Fig. 1). Type III inclusions have lower CO2 melting temperatures as the CH4 contents become higher; they also show a phase changing at about -100 ºC, that could be related to a partial N2-CH4 homogenization temperature. Melting temperature of clathrate in all fluid inclusion types are generally higher than 8 ºC. These temperatures reflect the presence of CH4 and/or N2 besides CO2 in the volatile phase.
Figure 1: CH4 %mole vs. ThCH4-N2 in type II fluid inclusions Raman spectroscopy combined with thermometric data is essential to determine the composition of fluid inclusions and their physicochemical conditions of formation, particularly in the case of gradual transitions in the proportions of the gases. Under these conditions the microthermometric data for these N2 dominant fluid inclusions are difficult to interpret (e.g., type II and type III fluid inclusions). Moreover, the lack of ternary N2-CH4-CO2 diagrams points out an additional problem for the interpretation of the thermodynamic parameters of these gases mixtures. Thus calculations on their molar volumes and bulk compositions have to be done on the basis of binary models in order to reconstruct the P-T paths in such geological context. Despite these limitations we were able to detect a trend of CO2 enrichment, CH4 impoverishment as well as a general decreasing participation of N2 over time in the Logrosán inclusions. 1
Chicharro, E., López-García, J.Á. & Villaseca C. (2011) Estudio metalogenético de las mineralizaciones de Sn-(Ta)-W del granito de Logrosán (Cáceres) Macla 15, 63-64 2 Kerkhof A.M. van den (1988) The system CO2-CH4-N2 in fluid inclusions: theoretical modeling and geological applications. Ph.D. dissertation. Free University Press, Amsterdam. 206 pp.
