A hydrothermal clay mineral assemblage at the Late-Proterozoic unconformity (Buenos Aires Complex - La Tinta Formation) of the Barker area, Tandilia ranges, Argentina.

MARTÍNEZ JUAN C.; DRISTAS A. JORGE; MASSONNE, HANS -J; THEYE THOMAS

Castellaneta Marina, Italia

Conferencia; 14th International Clay Conference.; 2009

Istituto di Metodologie per l'Analisi Ambientale, Consiglio Nazionale delle Ricerche (CNR),

Alteration profiles at the unconformity zone between the Paleoproterozoic igneous-metamorphic Buenos Aires Complex and the Late Precambrian La Tinta Formation in the Tandilia Ranges has been largely on debate since weathering, hydrothermal metasomatism and even metamorphism were assigned to explain their mineral paragenesis. The significance of this clay assemblage at the Tandilia Late-Proterozoic unconformity (TLPU) from Barker area has been evaluated through petrography, X-ray diffractometry (XRD), geochemistry of major, trace and REE, and by electron microprobe analyses (EPMA) studies. Natural outcrops of the above mentioned contact are hardly recognized at the northern lower part edges of the plateau-like ranges mainly composed by mature quartz-arenites. Mineral alteration patterns have been established according to pervasiveness and type of alteration, directly related to unconformity distance. Eight alteration profiles widespread into the study area show, in a general way, asymmetric alteration patterns confined to the contact, basement - sedimentary succession, with local variations. Pyrophyllite + sericite (2M1-1M) + Ti-rich hematite ± aluminum phosphate-sulphate (APS) minerals ± tourmaline ± rutile occurred where elements leaching and temperatures were higher (~ 260 ºC), at the unconformity. Sericite (1M) + chlorite + calcite ± anatase-rutile ± kaolinite (1T) ± secondary quartz developed where lower temperature was reached, usually farther from the unconformity (Martínez and Dristas, 2007) or in marginal zones respect to the different alterations focus. Petrographic observation reveals typical alteration and replacement textures present in both altered basement and sedimentary rocks into the unconformity zone. Identical alteration pattern were mentioned for the uranium deposits unconformity related from the East Alligator Rivers Uranium Field, northern Australia, and the Athabasca Basin, Canada (Komninou and Sverjensky, 1995; Lorilleux et al, 2003; Beaufort et al, 2005; Gaboreau et al, 2005). Geochemical behaviour of REE from non-altered to altered samples was established by means of REE pattern plotted against chondrite. Non-altered basement is characterised by negative Eu anomaly (Eu/Eu* =0.24). A strongly positive Eu (Eu/Eu* =11.74) anomaly is seen where incipient chloritisation and sericitisation are developed. The former anomaly gradually disappears whereas a positive Ce anomaly is characteristic of the most altered basement and sedimentary rocks at the unconformity. Light REE (Ce, La, Nd) are enriched at this alteration zone and correlated mainly with P, Sr and S. Electron microprobe analyses and backscattered images reveal textural setting of replacement minerals. Some of them exhibit either interlayer cation variations or mechanical mixing undefined by EPMA resolution. Source for Al, Ca, K, Ba, Sr, P, S and REE were identified in minerals from the meta-igneous basement. Secondary phyllosilicates (phengite, pyrophyllite, paragonite-component), whose structures were defined by X-ray diffractometry, crystallised either into the altered rocks from the basement and the base of the sedimentary succession. Chemical variations among different structural possition are evaluated for phyllosilicates at both sides of the unconformity. This paper deals with the role that the lithostratigraphic discontinuity plays over a regional hydrothermal fluid flow, as is reflected on the clay assemblages.