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

Abstract

This paper provides a regional-scale background for understanding gold-mineralising processes in the Otago Schist during the Cretaceous. At this time the schist belt was in the latter stages of formation as an accretionary complex with 2000 km strike length on the Pacific margin of Gondwana. The Otago Schist is interpreted as an exhumed accretionary wedge of structurally stacked clastic metasedimentary rocks with minor metabasic rocks. Metamorphic grade reached upper greenschist facies. Gold and other related elements were mobilised from the metasedimentary rocks during metamorphism, and these elements contributed to the high levels of orogenic gold endowment (>18 million ounces in the schist belt. Mesozoic Gold deposits were emplaced in two distinct pulses, one at the beginning of the Early Cretaceous (~140-135 Ma) and the other at the end of the Early Cretaceous (~112-100 Ma). These mineralising pulses were driven by regional tectonic events that may have involved episodic underplating of subducting material, and/or subduction of spreading ridges. The earlier event was most closely associated with metamorphic processes that mobilised gold from the accreted metasediments during convergent tectonics, and appears to have been the more economically significant. The later event took place as accretionary processes ceased and the Zealandia portion of Gondwana began to undergo intracontinental rifting.

The Influence of Basement Terranes on Tectonic Deformation: Joint Earthquake Travel‐Time and Ambient Noise Tomography of the Southern South Island, New Zealand

“…The upper crust of the Otago domain shows relatively high seismic velocity compared to other domains, while the mid‐to lower crust has comparatively low velocities. This pattern is compatible with the genetic model of unroofed higher‐grade schist formed through accretionary duplexing (Mortimer et al., 2016; Reyners et al., 2017). Notably, there is an area of low velocity and low Qp in the Ranfurly area (R, Figure 7a.…”

“…Although seismotectonic studies can sometimes determine specific fault‐related features near major fault zones, many low magnitude micro‐earthquakes occur on small pre‐existing fractures that are not necessarily elements of active fault zones. The schist has many preexisting fractures from Mesozoic deformation (Mortimer et al., 2016), and additional fractures from the uplift and folding during Neogene deformation whose orientations are predictable from tectonics and rheology (Koons, 1994; Upton et al., 2009; Upton & Koons, 2007). We focus on the well‐located hypocenters from the simultaneous inversion and other COSA earthquakes, as well as relocated hypocenters of background seismicity from 2001 to 2011 Geonet manually‐assessed data (Figures 9 and 10).…”

“…The Otago domain has a surface basement geology of schist, and some semischist, of the amalgamated Rakaia and Caples terranes (Mortimer et al., 2016). The Mesozoic structure has a general northwesterly strike, but the Late Cenozoic range‐bounding faults predominantly strike northeast (Figure 1, Figure 2).…”

“…Low Q was observed in the ductile schist of the actively deforming Southern Alps crustal root, while the dry Cretaceous Otago schist showed high Q (Eberhart‐Phillips et al., 2018). As the thick schist formed through episodic underplating and corresponding uplift and extension of the shallower accretionary wedge, early extensional Cretaceous normal faults may be related to areas of crustal duplexing during subduction (Mortimer et al., 2016; Reyners et al., 2017).…”