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Título

STRUCTURAL ANALYSIS OF UNDERGROUND AURIFEROUS QUARTZ LODE VEINS AND STRUCTURAL CONTROL OF THE CATA BRANCA-CORRENTE GOLDFIELDS, QUADRILÁTERO FERRÍFERO (MG)

Texto do resumo

Gold deposits controlled by low-displacement faults clustered into several belts (goldfields) can indicate regional structural control. This phenomenon is notable in Cata Branca-Corrente, a 7 km-long historical goldfield on the eastern flank of the Moeda Syncline in the Quadrilátero Ferrífero (MG). The site comprises an approximate N–S-trending distribution of abandoned underground workings and open casts for gold, mostly from the nineteenth century. The mineralization style of the goldfields of Cata Branca’s goldfields is characterized by auriferous quartz lode veins that are hosted in the altered Archean low greenschist facies greenstone rocks of the Rio das Velhas Supergroup. As it is part of a structural-controlled goldfield, the structural analysis of the quartz veins arrays of Corrente target can be used to define the deformation history of the occurrence, possibly including the entire region. In this study, we used a cylindrical projection method to produce a detailed (1:100) geologic map of the Corrente deposit. Three main auriferous quartz lode systems that correspond to the main targets of the colonial times exploration were mapped. A large number of small lodes which may or may not be directly associated to main systems were also observed. The auriferous quartz lodes we identified strike approximately N20°W and dip 45° to 70° NE generally parallel to the main foliation (Sn). Both the quartz vein systems and the Sn displayed a slight symmetrical boudinage and were classified as foliation-parallel boudin train formed by antithetic-slip boudinage. In their map view, the distribution of the vein systems appears to represent fault arrays. To determine the principal stress orientations from two main sets of the fault arrays related to the auriferous quartz lode systems, we used the Anderson faulting theory to calculate the tensors σ1 (maximum - oriented at 30°- 45° to the fault plane), σ2 (intermediate in the fault plane), and σ3 (minor -oriented at 45°-60°). Data from the field and geologic map allowed us to divide the Corrente deposit into two segments: north and south. To the northern segment, the tensors were oriented as following: σ1=S64/55, σ2=S72/15 and σ3=N58E/59. The southern segment presented tensors distribution as following: σ1=N78E/29, σ2=S18/18, and σ3= N74W/74. These data indicate that the entire block may have undergone rotation, probably dextral sense. Our hypothesis is based on the fact that three main auriferous quartz lode veins occupy the position of the thrust faults developed at 90° to σ1, whereas mostly small veins are oriented 90° to σ3, occupying the extensional position of the system. As such, Cata Branca-Corrente goldfield may have been generated by a regional dextral strike-slip faulting event where the main auriferous quartz lode vein systems were related to reverse/thrust faults, and the small quartz vein systems filled extensional site of σ3 tensor or conjugated discrete Riedel-shear zones. However, it is possible that some quartz vein systems (possibly auriferous) were formed at a different time and under a different stress field, although this is not a predominant aspect of the Cata Branca-Corrente goldfield.

Palavras Chave

Structural analysis; quartz vein lode systems; Structural Controls; Cata Branca-Corrente goldfield; Moeda Syncline