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AN ABSTRACT of the THESIS of Federico Cernuschi Rodilosso For AN ABSTRACT OF THE THESIS OF Federico Cernuschi Rodilosso for the degree of Master of Science in Geology presented on February 11, 2011. Title: Geology of the Cretaceous Lascano-East Intrusive Complex: Magmatic Evolution and Mineralization Potential of the Merín Basin, Uruguay Abstract approved: John H. Dilles The 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. 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. 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. 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. 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. ©Copyright by Federico Cernuschi Rodilosso February 11, 2011 All Rights Reserved Geology of the Cretaceous Lascano-East Intrusive Complex: Magmatic Evolution and Mineralization Potential of the Merín Basin, Uruguay by Federico Cernuschi Rodilosso A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Presented February 11, 2011 Commencement June 2011 Master of Science thesis of Federico Cernuschi Rodilosso presented on February 11, 2011. APPROVED: ________________________________________________________________________ Major Professor, representing Geology ________________________________________________________________________ Head of the Department of Geosciences ________________________________________________________________________ Dean of the Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. ________________________________________________________________________ Federico Cernuschi Rodilosso, Author ACKNOWLEDGEMENTS First of all I would like to thank Orosur Mining Incorporated for their support in making this study and thesis possible. In particular, I would like to recognize George Schroer, David Fowler, Alex Raab, Devin Denboer and Ignacio Salazar. Many thanks to George for his mentoring and providing me this opportunity, to Alex for helping me become a better geologist, improve my mapping skills and his belief that I was capable of doing this research. To George, David, Alex, Devin and Bill Lindqvist for taking their time to discuss several aspects of this project. I would also like to give my full appreciation to Dr. John Dilles, my advisor at Oregon State University for the support, interest and effort that he put into this project. His visit to Lascano and help in the field and core shed are greatly appreciated. I will always be grateful to him for his effort to make me a better scientist and to push me to think critically. I am also indebted to my committee members Dr. Adam Kent and Dr. Rob Harris and my graduate representative Dr. Dave Graham for their time and help during this research. Many thanks to the geosciences professors at OSU, students and staff for being such a humane group and helping me during my research in several ways. I would also like to give my sincere thanks to all my co-workers at Orosur who provided me with abundant help in one way or another during this time: Nora Lorenzo, Néstor Vaz, Carla Lobelcho, Hugo Cicalese, Adriana de León, Daniela Spinelli, Leonardo Pintos, Victoria Flores, Diego Sarroca, María Sapelli and Marco Pérez among others. A very special thank to Martín Rodriguez for cutting, tagging and carrying kilometers of drillhole core from Lascano and shareing endless days in the core-shed in Montevideo; and to Carmen Alvez for the great help with the database, server, and in drafting the map and cross-sections in the Montevideo office, by email, phone, pager and telepathy!; and to my old friend Bruno Conti for introducing me to Lascano, taking me to the field and telling me everything he knew about it. I would also like to extend my thanks to Dr. Anita Grunder and to her and John’s children (Anita, Zoe, and Leo) for including me and Silvina in their family and for allowing us to feel at home. Also, special thanks to Chocolate (la perra) for taking care of me for the three months when I lived alone in the US! I would also like to thank my English reviewer-in-chief Morgan Salisbury for proofreading the hundreds of pages of proposals, reports, letters, thesis, etc, all the while providing sound advice. To B.J. Walker for additional help with the English and for being my open dictionary in the office answering all kinds of questions from fractional crystallization to urban English expressions, to Stephanie Grocke for the English review and suggestions, Ashley Bromley who helped me settle in when I first arrived to the US, to Dr. Adam Kent, Matt Loewen, Alison Koleszar and Luc Farmer for the help with the ICP-MS, Dr. Frank Tepley and Dale Burns for the help with the EMP, Dr. John Huard and Mark Ford for thier help in the Ar-Ar lab and to Julia Cohen for her knowledge of hydrothermal alteration. The last term of this research was possible thanks to an OSU Laurels grant for which I am extremely grateful. Thanks to the University of British Columbia professors and students that helped me using the ASD, processing the data and for receiving Silvina and I in Vancouver: Farhad Bouzari, Jaime Poblete, Santiago Vaca, Aeysha Ahmed and Shawn Hood. Many thanks to Scott Haley for his great help with the interpretation of the ASD data and for his assistance using ioGAS. Additional thanks to several geologists from Uruguay: Rossana Muzio for sharing data and discussion for the Merín basin,
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