GENESIS AND STRUCTURAL CONTROL OF THE RINCONADA AS AN OROGENIC GOLD DEPOSIT
E.F.Bernahola, J.B.Valdivia, J.M.Llerena, F.I.Mamani Students S.E.G. chapter- U.N.S.A. Peru
The Rinconada Orogenic Deposit is located in the occidental edge of the eastern cordillera in southern Perú, it shows a main mineralization of quartz, pyrite, arsenopyrite and gold, which has an average law of 11-100g/T, placed in sedimentary rocks (Fm. Sandia y Fm.Ananea) that shown a methamorphism grade of Sub -Green Schist. The sedimentary sequence is affected by the early Hercinian tectonic that generated folding, refolding and alkaline and calc-alkaline sin-postectonic plutonism. This early Hercinian plutonism has relation with the first mineralization event that took place in weakness zones like thefoliation planes, fractures, structure intersections, etc. leading to lenticular bodies, micro lenticular bodies, vein, stockwork and stylolites showing a style of fragile mineralization (Goldfarb et al., 2005). During the late Hercinic tectonic phase (Late Permian-Early Triassic) (Dalmayrac et al., 1988), occurred a temperature rise probably associated to Permian-Triassic intrusive bodies that leadto an increase in the gold concentration and removilization of the mineralizing solutions to most favorable zones, which is evidenced by a flux foliation and an increase of the gold laws (>1000g/T) in the hinge and valley parts of the folds. The hydrothermal alterations are not extensive and are dominated by an alkali metasomatism (Chlorite – sericite) and/or silicification.
Structural ModelLocation Of The Basin
• The Orogenic deposit of the 'Rinconada' is located on the western edge of the southern Peruvian Eastern Cordillera. The mineralization is located mainly both in the Sandia formation (Ordovician) and Fm. Ananea (Silurian-Devonian), which were deposited in an extensional basin system in a context of back arc that evolved to a compressive regimeretroarc-forearc, the formations were affected by Hercynian tectonic (Eohercynian : Middle and upper Devonian ; Late-Hercynian : lower and upper Permian) developing a metamorphic level of greenschist. • The mineralization consists predominantly in Quarz, pyrite, arsenopyrite, and gold, the last one may have laws of 11-100 g/t mineralization, it is located parallel to the stratification, veins, stockwork andstylolites. the prescence of those evidence a brittle style. The Eohercynian magmatism originated fluids, (rich in CO2 and pH neutral) that carried the gold, channeling it into deep faults and being deposited in second and third order structures. The Late-Hercynian phase reheated the system, in this way Au could be remobilized, concentrating it in hinges folds and valleys, microfolds, etc. Themagmatism associated with this phase also contributed with Au to the system.
• Hercynian cycle was what caused the structural faults of the first order, which ones served as a conduit for the mineralizing fluids. But it is in the lower order faults (jog areas, areas of stress changes or bifurcations in systems of first order) where mineral deposition is more favorable. In theRinconada, is evidenced the presence of a jog serving as structural trap. This jog comes from the relative change in regional stresses to act in the Rinconada's area, own dynamics of Hercynian shortening cycle. In this deposit, second and third order faults were much more favorable for the deposition of gold.
• The predominant alteration is the chloritization (alkali metasomatism) and it is notconspicuous in the host rock, which is limited to small halos around veins or beds. It also has a calcium carbonate alteration due to the high CO2 content of the fluid. Based on this evidences, We say that fluids had a neutral pH. • Both subduction and accretion of the orogen are important processes which are related to the genesis of orogenic deposits. Subduction produce magmtism which generates...
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