The shear zone-related gold mineralization at the Turmalina deposit, Quadrilatero Ferrifero, Brazil: structural evolution and the two stages of mineralization

摘要

Turmalina is an important orogenic gold deposit located in the NW region of the Quadrilatero Ferrifero. The deposit is hosted in an Archean greenstone belt composed of ortho-amphibolites and pelites with interleaved tuffs metamorphosed under amphibolite facies conditions and intruded by a granite stock. The orebodies are controlled by WNW-ESE-trending shear zones, associated with hydrothermal alteration. Three deformation events are recognized in the Turmalina gold deposit: D-1 and D-2 are the result of a progressive Archean deformation under ductile conditions between 2749 +/- 7 and 2664 +/- 35Ma; D-3 is characterized by a transpressional event under ductile-brittle conditions with the age still unclear. The three generations of garnet observed show that Grt(1) blastesis is pre- to syn-D-1 and Grt(2) growth during the late to post-deformation stages of the D-2 event. The initial temperature (Grt(1) core) is around 548-600 degrees C, whereas during late D-2, the temperatures reached 633 degrees C (metamorphic peak-Grt(2) rim), likely as a result of granite intrusion. The Grt(3) resulted from re-equilibration under retrograde conditions. Two gold-bearing sulfide stages were identified: pyrrhotite-arsenopyrite +/- lollingite +/- chalcopyrite +/- gold stage I precipitated below a metamorphic peak temperature of 598 +/- 19 degrees C associated with S-1 foliation (D-1), and pyrrhotite-pyrite-arsenopyrite +/- chalcopyrite +/- gold stage II is located commonly along V-3 quartz-carbonate veinlets with a temperature range between 442 +/- 9 and 510 +/- 30 degrees C. We suggest that the granite intrusion imposed an additional thermal effect that promoted further dehydration of country rocks. The Au derived mainly from a metamorphic fluid source but potentially mixed with magmatic fluids from the granite.