Geology and gold on Otago’s northeastern margin

Researchers

• Dave Craw
• Doug MacKenzie
• Phaedra Upton

Overview

The northeast margin of the Otago Schist belt is a complex geological and topographic zone, with a long history of gold mining. The geological evolution of this complex zone began with metamorphism in the Jurassic (>140 million years ago). Initial gold mineralization in the schist occurred about that time. Regional crustal extension in the middle Cretaceous (110-120 million years ago) facilitated uplift and erosion, and more gold mineralization occurred in the schist at that time. Erosion of schist basement and its gold deposits, transferred gold to the alluvial system, primarily in quartz gravels. Extensive recycling of quartz gravels and alluvial gold occurred from the late Cretaceous (about 90 million years ago) to present. The complex geology of the northeast margin of the schist belt has controlled the geology of gold deposits, both schist-hosted and alluvial. The world-class Macraes gold mine (late Jurassic-early Cretaceous) occurs in this structural zone. The following pages give an account of some of the key aspects of this complex geology, and the inter-relationships among them.

• Crustal structure and topography of the Otago northeast margin
  The underlying large-scale structural architecture and plate tectonic setting

• Initiation of the fault zones on the northeast Otago margin
  Uplift, erosion and sedimentation during regional extension in the middle Cretaceous

• Faulting and erosion of the Macraes gold deposit
  Initial exposure and subsequent erosion of the Hyde-Macraes Shear Zone

• Gold-bearing veins at Oturehua
  Middle Cretaceous gold veins in the schist

• Blue Lake Fault Zone
  Structure and rocks in a major active fault zone initiated in the Cretaceous and reactivated Miocene-Recent

• Graphite and gold on the northeast schist margin
  Metamorphic origins of the close relationship between gold and graphite at Macraes gold mine

• Gold and arsenic in pyrite
  Metals concentrated in pyrite in greywacke and argillite became available for reconcentration in gold deposits in schist

• Waipounamu Erosion Surface
  A regional flat surface formed between 100 and 25 million years ago is now one of the most prominent geological features of the Otago area

• Alluvial gold along the northeast Otago margin
  Remnants of quartz gravels host alluvial gold, and this has been recycled into younger gravels

• Rise of modern mountains on Otago's northeast margin
  The modern topography reflects the on-going active tectonics of the area

• Patearoa gold: Alluvial concentrations in a dynamic environment
  Patearoa is an historic gold mining area on the southern edge of the Maniototo Basin.