Structural control of Au-Pd mineralization (Jacutinga): An example from the Caue Mine, Quadrilatero Ferrifero, Brazil

摘要

Precambrian banded iron formations (BIFs) in the Quadrilatero Ferrifero host a special kind of Au-Pd mineralization known as Jacutinga. The main orebodies are hosted within the Caue Syncline, a SW-verging fold that involves Paleoproterozoic metasedimentary rocks in the Itabira District, a regional synclinorium with BIFs in the core of synclinal folds in the northeastern part of Quadrilatero Ferrifero, Minas Gerais. Structural analysis reveals two important features of the district: the polydeformed character of the rocks and the importance of brittle structures in the control of the orebodies. Two deformational events are recognized in this area. The first event developed the main foliation, S1, that is the enveloping surface of the Caue Syncline. The second event is better defined in the northern boundary of the structure where it is represented by a right-lateral wrench fault zone that has developed a foliation, S2, that truncates S1. This wrench fault was also responsible for the development of a system of fractures (Frm) that host the Au-Pd mineralization. The auriferous bodies of Caue Syncline (Y, X, Area Central, Aba Norte, Noroeste and Aba Leste/Aba Leste Inferior) were generated during this second event. Shear fractures (R, R' and P) and tension fractures (T) developed in response to the wrench fault system under brittle conditions. The best-developed, and most commonly mineralized fractures are R and T in all auriferous bodies. Elsewhere, the best mineralization occurs in the contacts of hematite bodies (soft/hard) and intrusive rocks with fractured itabirites. Other mineralization (Aba Norte, Area Central and X) is hosted on the contacts of other units. A system of fractures, as well as their intersections, thus represents the structural control on Jacutinga bodies and is responsible for the geometry of the orebodies. Of importance, there is no control by mineral/stretching lineations, fold axes and other ductile structure on the geometry and plunge of the orebodies.