Henry, D. J. and Brodtkorb, M. K. de (2006) The tourmalinites from the Pampa del Tamboreo, San Luis: Chemical aspects. VIII Congresso de Mineralogia y Metalogenia 2006, Facultad de Ciencias Exactas y Naturales (U.B.A.) Trabajo, 105-110. 105 THE TOURMALINITES FROM THE PAMPA DEL TAMBOREO, SAN LUIS: CHEMICAL ASPECTS HENRY, Darrell J.* and BRODTKORB, Milka K. de ** *Department of Geology and Geophysics. Louisiana State University, USA. E-mail: dhenry@geol.lsu.edu ** CONICET- University of Buenos Aires. E-mail: milkabro@gl.fcen.uba.ar ABSTRACT The tourmalinites of the Pampa del Tamboreo area, San Luis, Argentina, are distally associated with tungsten deposits. The tourmalinites are schistose and comprised of 70-80% tourmaline with lesser amounts of quartz, graphite, muscovite, rutile, ilmenite and pyrite and pyrrhotite. Tourmaline grains uncommonly contain small (20-30 μm) grains of detrital tourmaline with a variety of absorption characteristics implying variable compositions. Analysis of a single detrital tourmaline grain reveals that it is a Ca-rich “oxy-dravite” (Mg/ (Mg + Fe) = 0.90) likely from a high-grade, magnesian metasedimentary source. Proximal to the detrital cores there are inner domains of asymmetric tourmaline overgrowths that developed at low grades of metamorphism. The volumetrically dominant tourmaline overgrowths in the outer domain are concentrically zoned aluminous dravite and “oxy-dravite” with Al/(Al + Fe + Mg) = 0.71-0.74 and Mg/(Mg + Fe) = 0.64-0.71. Keywords Tourmalinites- chemical composition- Pampa del Tamboreo- San Luis. Palabras clave: turmalinitas- composición química- Pampa del Tamboreo- San Luis. INTRODUCTION Tourmalinites are defined as rocks with more than 20% tourmaline according to Slack (1982,1996).Tourmalinites have increasingly been recognized to occur in association with diverse types of stratabound deposits, in particular in SEDEX and VMS mineral deposits (Slack, 1996, Henry and Dutrow, 1996). They have been also recognized to occur in association with tungsten mineralization (Barnes, 1983, Appel, 1986, Raith, 1988, 1991, Leake et al. 1989) This paper deals with the chemical composition of the tourmalines from tourmalinites in the Pampa del Tamboreo, San Luis Province. The Sierra de San Luis is part of the Sierras Pampeanas Orientales and is composed of Neoproterozoic- Early Paleozoic metamorphic and igneous rocks which has been subdivided in several complexes (fig.1). The San Luis Formation is a wedge some 100 M.a. younger than the other complexes of the Sierra de San Luis (a metamorphosed dacitic and rhyodacitic tuff that are a host for some of the tungsten ores have a zircon U-Pb age of 529±12 Ma, the presumed crystallization age of the tuffs, Söllner et al. 2000, whereas a reference isochron yield an age of 660 Ma on amphibolites (Brodtkorb et al, 2006) Figure 1: Geologic map of the Pampa del Tamboreo area and turmalinite location. OC ÉAN O A T LÁN T ICO O C É A N O P A C Í F I C O 0 1 2km N Pampa del Tamboreo Tonalite Turmalinites Metaryolites and riodacites Los Cocos-El Alamo Quarcites Filites, Biotitic Squist and Quarcites Biotitic Sqchist, Filites and Quarcites La Florida Damm 1 -La Florida 2 -La Esperanza 3 -El Carlito 4 -La Donosa 5 -La Fortuna 6 -Gral. Jofré 7 -El Alamo 8 -San Pedro 9 -Los Cocos 10-La Libertad 11-El Cangrejo Deposits and mines 1 2 3 4 5 6 7 8 9 10 11 Río Río Riecito Qui n to G r a n d e Río Pampa del Tamboreo