Contrasting structural styles of orogenic gold deposits, Reefton Goldfield, New Zealand

This paper describes the structural setting of the Globe-Progress and Blackwater orogenic gold deposits, the only two +1-million ounce (Moz) deposits discovered to date within the Reefton goldfield of central New Zealand. Most of the Globe-Progress deposit is located along the Globe-Progress shear zone, a curviplanar, brittle-ductile, structure that juxtaposes tightly folded rocks in its footwall against largely unfolded rocks in its hanging wall. At its eastern end the Globe-Progress shear zone merges with the fold-parallel Oriental shear zone, with the Globe-Progress shear zone acting as a tear fault that transferred contractional deformation on the Oriental shear zone across the structural grain of the adjacent folded rocks, during or soon after the end of greenschist facies metamorphism. An early phase of mineralization at ca. 438 Ma (K-Ar, sericite) accompanied this contractional deformation, whereas a second phase of mineralization occurred during later brittle, oblique-normal, reactivation of the two shear zones at ca. 386 Ma (K-Ar, sericite). The resulting ore shoots plunge moderately towards the southwest, along intersections between (1) the two shear zones, (2) the Globe-Progress shear zone and minor normal faults in its hanging wall, and (3) the Globe-Progress shear zone and W-dipping bedding in its hanging wall. The kilometer-long Birthday reef in the Blackwater mine is located on a narrow, mineralized, brittle-ductile fault that can be traced for 3+ km through the adjacent Greenland Group sandstones and argillites. The reef occupies that part of the fault that is close to parallel with bedding on a relatively gently ( approximately 60 degrees ) W-dipping segment of the western limb of the Waiuta anticline. The reef pinches out where the dip of the host fault steepens and its strike swings from north-northeast toward north. This change in orientation occurs where the fault refracts onto the near vertical axial plane of the Waiuta anticline above/north of the reef, and follows an open flexure in the orientation of bedding on the western limb of the Waiuta anticline below/south of the reef. Both ends of the reef, and its more intensely mineralized internal ore shoots, plunge 35 degrees toward the north-northeast, parallel to the intersection of the host fault and the axial planar cleavage associated with the adjacent Waiuta anticline.