Structural Controls on an Orogenic Gold System: The World-Class Siguiri Gold District, Siguiri Basin, Guinea, West Africa

Abstract

Orebodies in the Siguiri district, a world-class Paleoproterozoic orogenic gold camp located in the Birimian of northeastern Guinea, are typically represented by cryptic subvertical damage zones that host a high density of mineralized veins. Although no large regional fault system was recognized, observations from five representative deposits of the Siguiri district (Sanu Tinti, Bidini, Kami, Kosise, and Sintroko PB1) show that these orebodies are locally controlled by incipient structures and spread across three distinct structural and lithostratigraphic domains. Two shale-dominated peripheral domains adjoin a central domain whose lithostratigraphy is dominated by medium- to coarse-grained graywacke and this domain hosts the bulk of the gold endowment of the district. The three domains exhibit similar structural elements that can be described within a four-stage deformation scheme. The first deformation event (D 1S ) is poorly constrained and interpreted to have been an N-S compressional event. It included development of minor folds with W- to WNW-gently plunging fold axes without a clear axial planar cleavage. The main and second deformation event (D 2S ) is interpreted to have been associated with E-W to ENE-WSW- directed compression. The D 2S event was responsible for forming the dominant N-trending structural grain of the district and creating interference patterns between F 1S and F 2S folds. The third event (D 3S ) developed progressively from D 2S compression into an early-D 3S E-W- to ENEWSW-directed transpression and a late-D 3S NNW-SSE-directed-transtension responsible for most of the gold mineralization in the Siguiri district. The fourth and last event (D 4S ) was an NW-SE-oriented compressional event responsible for the localized overprinting of veining by a steep to shallowly dipping NNE-SSW ductile cleavage. Late-D 3S gold-bearing mineral occurrences formed along subvertical N-S reverse faults, NE-trending dextral shear zones, WNW-trending sinistral faults, and E-trending normal relay faults developed or reactivated early-D 3S . Mineralization is expressed as mineralized shear zones or subvertical damage zones, characterized by a 10- to 15-m-wide zone of dense quartz-carbonate-sulfide veining, or disseminated gold-bearing sulfides. The mineralized veins consistently strike ENE-WSW, are steeply dipping, and commonly have a conjugate geometry at the mesoscale. Finite strain analysis of deformation, including analysis of folds, faults, and conjugate mineralized vein sets, is consistent with a stress switch from a compressional (D 2S ) to transpressional deformation (termed early-D 3S ). Results of paleostress analysis on conjugate mineralized vein sets that formed late during D 3S indicate that the stress field ranged from extensional to strike-slip, sometimes within the same vein locality. The late-D 3S deformation is interpreted to have been a transtensional event. The first change in the orientation of the principal stress axes is related to a switch from a far field-dominated to a body force-dominated stress field reducing the deviatoric component on the stress tensor. The second change in the orientation of the principal stress axes, from early-D 3S transpression to late-D 3S transtension, suggests that σ 1 and σ 2 were similar in magnitude, which facilitated localized stress switches. In the Siguiri district, the early-D 3S and late-D 3S stress switches, which occurred at both a local and regional scale, enhanced the fracture permeability and were critical for the establishment of active fluid pathways leading to the formation of a world-class gold system.