Geology of the Cretaceous Lascano-East intrusive complex : magmatic evolution and mineralization potential of the Merin Basin, Uruguay

Graduation date: 2011The Cretaceous Merín aborted-rift basin of eastern Uruguay is composed of sub-alkaline Paraná-Etendeka province basaltic flows and shallow intrusives (~133 to 131 Ma), rhyolitic ignimbrites (~130 to 128 Ma) and associated mildly alkaline to alkaline intrusions and volcanic rocks (~128 to 127 Ma). Four intrusive complexes from 20 to >30 kilometers wide are identified by circular positive gravity and cospatial magnetic anomalies and are aligned in a northeast-southwest trend. Whereas the outcropping Valle Chico complex is mainly composed by mildly alkaline syenites, drilling at Lascano East revealed mostly alkaline gabbros and trachytes. The Lascano-West and San Luis concealed complexes are inferred by the location of the aeromagnetic and gravity anomalies. \ud Ten lithogeochemical units were identified and grouped into three rock associations. A sub-alkaline group composed of Treinta y Tres A basalts, Treinta y Tres B basalts and gabbros, Lavalleja rhyolitic ignimbries and San Miguel granodiorite granophyres; a mildly alkaline group including the Santa Lucía basalts and gabbros, Aiguá rhyolites, Valle Chico syenites and India Muerta rhyolites; and an alkaline group including the Lascano alkaline gabbro to trachyte series and the Arrayán olivine basalts. The only observed sedimentary rocks are conglomerates grouped as the Quebracho Formation. Melting of a shallow mantle source (depleted mantle) combined with abundant crustal assimilation likely produced the diversity of the sub-alkaline magmatic rocks. The mildly alkaline and alkaline rocks were likely produced by mixing of this source with a deeper mantle source (ocean island basalt – like), or by progressively deeper mantle melting and lowering degrees of partial melting.\ud Hydrothermally altered and mineralized rocks were identified in the central zone of the complex where the sub-alkaline and mildly alkaline lavas are intruded by mildly alkaline to alkaline gabbros and trachyte dikes on top of inferred mafic alkaline intrusions. The mineralization and alteration can be divided into two associations. First, potassic hydrothermally altered zones and younger superimposed intermediate argillic alteration in sub-alkaline to mildly alkaline felsic rocks are cut by similarly altered Lascano alkaline series dikes. Pyrite disseminations together with pyrite, quartz-pyrite and fluorite veins in these rocks are associated with weak gold, bismuth, thallium and molybdenum anomalies. Second, local potassic alteration of mildly alkaline basalts cut by the Lascano alkaline dikes, showing sparse millimetric to one centimeter quartz-pyrite and phyllosilicate-pyrite veins associated to weak molybdenum anomalies. Local quartz-chalcopyrite-pyrite veins and copper anomalies were detected in the contact of the basalts with one Santa Lucía mildly alkaline gabbro.\ud No evidence of mineralization is found in the Valle Chico complex, the only outcropping complex of the Merín basin. The only other evidence of mineralization in the basin are fluorite veins enriched in tungsten, boron and yttrium cutting the Precambrian basement near the basin edge. The lack of mineralization in the Valle Chico complex could be explained by differences in the level of erosion throughout the basin due to the interplay of subsidence caused by mafic intrusion and different crustal thicknesses at each side of the Sierra Ballena shear zone. While Valle Chico was more deeply eroded, the possibly mineralized roof wall-rocks were preserved in the concealed complexes to the East.\ud The Merín basin was broadly contemporaneous and close in space to the magmatism in the Luderitz and Damaraland basins in Namibia and more distal complexes in Brazil. These were possibly linked to similar melt sources, evolutionary paths, and emplacement mechanisms, related to the opening of the southern Atlantic Ocean in the Paraná – Etendeka provinces. Based on typical mineralization in complexes from Brazil and Namibia the mineralization potential of the Merín basin may also expand to niobium, zirconium, phosphate, uranium, thorium and rare earths. These ores may be related to possible concealed carbonatites or other alkaline rocks not yet discovered in the Merín basin. However, the conditions for the formation of laterites, which play an important role in the economic deposits of Brazil were probably unlikely