Upper-amphibolite facies mylonitization of mafic-ultramafic rocks and gneissic-migmatitic country rocks, Sierra de San Luis, Argentina: implications in the remobilization of ore sulfides

DELPINO, S; BJERG, E A; FERRACUTTI, G; DIMIERI, L; MOGESSIE, A; KOSTADINOFF, J

Buenos Aires

Congreso; Mineralogía y Metalogenia 2002; 2002

Asociación Mineralógica Argentina

&amp;lt;!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:21.0cm 842.0pt; margin:70.9pt 3.0cm 70.9pt 3.0cm; mso-header-margin:35.45pt; mso-footer-margin:35.45pt; mso-paper-source:0;} div.Section1 {page:Section1;} --&amp;gt; Around mafic-ultramafic bodies of the Sierras de San Luis Cc:otral Block, a shear zone several kilometers wide was developed. Several authors have partially studied the mylonitic rocks as part of petrologic and/or structural works, but a great controversy exists among them with respect to the timing and metamorphic grade at which this event occurred. On the other hand, a post-magmatic remobilization of sulfides±PGM of mafic-ultramafic rocks has been proposed in previous works. In the present contribution the attention was focused on the mylonitic deformation affecting mafic-ultramafic intrusives and their gneissic-migmatitic adjacent country rocks, both previously metamorphosed within the granulite facies. Physical conditions of deformation are evaluated through deformation mechanisms in minerals, the construction of a suitable petrogenetic grid and internally consistent geothermoberometry. Considered together, deformation mechanisms in mafic-ultramafic (ol, opx, hbl and pl) and gneissic-migmatitic (pl, kfs; and qtz) country rocks, indicate a ductile behavior characteristic of very high-temperature deformation. The stability field for the country-rock mineral association in equilibrium during the mylonitization (Pl+Qtz+Kfs+Ort+Bl+Sil), indicate thermal conditions of deformation within the upper-amphibolite facies. Internally consistent geothermobarometry applied to this association in equilibrium with an H2O+C02 fluid phase, constraints the conditions of deformation within the range 668-762 °C and 6.3-6.9 Kb at O.29<XCO2<O.72. Textural relationships in mylonites (distribution along foliation planes and crystallization within porphyroclast extension sites) clearly indicate that primary magmatic sulfide of mafic-ultramafic rocks, were remobilized during the mylonitic event. On the base of the obtained results and previous works in the area, a general discussion about the mylonitic event and related processes is raised.