Structural and lithological controls on gold mineralisation at Oturehua on the northeastern margin of the Otago Schist, New Zealand

Abstract

A set of more than 50 mineralised normal faults cuts lower greenschist facies rocks at the northeastern margin of the Otago Schist belt near Oturehua, central Otago. The faults strike northwest and dip steeply (>60°) northeast, and individual mineralised zones are traceable for up to 700 m along strike. The mineralised faults are closely associated with, and are generally parallel to, a swarm of northwest striking, steeply dipping extensional quartz ± ankerite veins (centimetre scale) that occur over an area of at least 25 km 2 in the host schist. Mineralised faults contain 10-100 cm thick quartz veins that are traceable as interlinked structures for tens of metres along strike. where faults cut massive quartz-feldspar-rich schist, the veins are planar, contain scattered sulfide minerals and visible gold, and there has been little alteration of essentially undeformed adjacent wall rock (centimetre scale). in contrast, muscovite-rich laminated schist host rock is extensively deformed and altered adjacent to mineralised faults (metre scale), with addition of quartz (silicification) and ankeritic replacement of metamorphic chlorite by kaolinite and/or sericitic muscovite. Veins adjacent to laminated (micaceous) schist contain abundant breccias with variably altered clasts. widespread addition of auriferous sulfide minerals, mainly arsenopyrite, has occurred in the laminated schist alteration zones and breccias. The mineralised faults formed in the hinge of a northwest-trending post-metamorphic antiform, and the faults may have been localised by zones of weakness associated with kink folding in the hinge of the antiform. Hydrothermal fluids responsible for mineralisation were channelled by the normal faults. These faults developed by coalescence of extensional fractures in the schist rock mass during middle Cretaceous regional extensional unroofing of the Otago Schist belt. The location of these mineralised faults near the margin of the Otago Schist belt suggests a genetic relationship between mineralisation and the regional northwest-striking normal fault systems that cut and define the schist margin.

Cretaceous tectonics and gold mineralisation in the Otago Schist, New Zealand

“…Otago orogenic gold deposits are controlled by structures that cut the metamorphic fabric of their immediate host rocks, and are clearly late metamorphic to post-metamorphic in relative timing (Williams, 1974;Craw, 2002;Craw et al, 2007;Mackie, MacKenzie and Craw, 2009). At least some of the orogenic deposits were subsequently exhumed to be eroded into Cretaceous sediments (Craw, 2010).…”

“…Both of these deposits have Au-W enrichment, with Au dominating at Macraes and W dominating at Glenorchy (Williams, 1974). The later mineralisation pulse mainly involved small quartz vein-dominated deposits hosted in normal faults, such as those at Nenthorn and Barewood (Figure 1, 4), which are scattered throughout the schist belt but occur mostly in the higher grade core (Figure 1; Mackie, MacKenzie and Craw, 2009;Mortensen et al, 2010). In addition, the compressional shear-hosted, and locally fold-related, RSSZ that has some resemblances to Macraes (Cox et al, 2006;Craw et al, 2007;MacKenzie and Craw, 2007) is grouped in this later mineralisation pulse ( Figure 4; Mortensen et al, 2010).…”

Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand

“…However, Mackie et al (2009) report Cretaceous compressional structures in the schist of North Rough Ridge (Fig. 1B), so it is not possible to link reverse-fault reactivation features to MioceneÁRecent deformation without observing an offset of Miocene features.…”