Gold eroded from the vein systems and altered schist, such as the deposits at Oturehua and Macraes, accumulated with eroded quartz in old river channels. The most important such channels are those formed in the Miocene (about 20 million years ago). These gravels occur beneath the basins of Central Otago, and as uneroded remnants on the slopes of the mountain ranges. The gravels are common as small remnants along the Blue Lake Fault Zone. These have been extensively mined for gold historically, wherever they occur, and the most spectacular mine site is at Blue Lake. The Grey Lake mine is similar to Blue Lake. These, and the Fiddlers Flat alluvial gold mine, were developed in tilted Miocene quartz gravels resting on greywacke basement. Older (Eocene; ~40 million years old) deposits occur in the Hogburn Formation farther east, along a branch of the Waihemo Fault Zone in the Kyeburn valley, and at the northern end of the Rock & Pillar Range, especially between Hyde and Tiroiti. The Eocene gravels were deposited on top of the middle Cretaceous Kyeburn Formation, which has little or no alluvial gold accumulations.
Eocene quartz gravels
On a regional scale, the gold-bearing gravels occur close to the Waipounamu Erosion Surface. The Eocene gravels are immediately below the erosion surface, and rest on the basement unconformity. Bedding in the quartz gravels generally dips shallowly to moderately where it has been tilted by adjacent faults and/or folds. Miners pursued the gold at and near the base of the quartz gravels, where the richest gold accumulations occurred.
The Miocene gravels (Dunstan Formation) in Central Otago rest on the Miocene unconformity which is close to the Waipounamu Erosion Surface. Erosion and deposition of these gravels has slightly modified the Waipounamu Erosion Surface. River channels up to 50 m deep occur locally, although most relief is less than this. Alluvial gold occurs close to the base of the gravels, near the underlying unconformity. The gold-bearing sediments have been folded and faulted by structures along the northeast Otago margin, so the miners followed tilted or even vertical zones. Historic mining commonly ceased when excavations following the unconformity became too deep to work or drain water from.
Groundwater moving through the quartz gravels, as in the photograph above, dissolved quartz and redeposited as cement in some layers. This formed hard slabby outcrops when the rocks were uplifted and eroded, and these outcrops broke into blocks 10 cm to 2 m across. The resultant hard rounded boulders are called silcrete, or more informally, "sarsen stones". These silcrete boulders are scattered across the landscape wherever the quartz gravels have been eroded.Alluvial gold from the quartz gravels is also scattered across the landscape with the silcrete boulders, and this relationship was used extensively by early prospectors looking for new alluvial gold occurrences. The silcrete boulders are most prominent in Central Otago, where they are derived from Miocene outcrops, but the same processes affected all quartz gravel deposits back to the late Cretaceous.
Gold from both Eocene and Miocene gravels has been recycled by erosion during fault-related uplift between Pliocene (2-5 million years ago) and Recent. This recycling process can result in dilution of the gold content by eroded basement material. However, the earliest eroded material, as the basement is uplifted, is the thin veneer of gold-bearing sediments, so this gold and associated sediments occurs near the base of the new younger deposit. This type of gold concentration happened at Naseby, where Pliocene greywacke gravels are quartz-rich and gold-bearing at the base, from erosion of gold-bearing Eocene sediments. The recycling process has continued where the Pliocene gravels have been uplifted and eroded. The gold-bearing sedimentary intervals also contain abundant silcrete boulders derived from cemented Eocene quartz gravels, and these are recycled along with the gold into progressively younger gravels.
- Youngson, J.H.; Craw, D.; Landis, C.A.; Schmitt, K.R. 1998 Redefinition and interpretation of late Miocene-Pleistocene terrestrial stratigraphy, Central Otago, New Zealand. New Zealand Journal of Geology and Geophysics 41: 51-68. doi:10.1080/00288306.1998.9514790
- Barker, S L L, Kim, J P, Craw, D, Frew, R D & Hunter, K A. 2004. Processes affecting the chemical composition of Blue Lake, an alluvial gold-mine pit lake in New Zealand. Marine and Freshwater Research 55: 201-211. doi:10.1071/MF03174
- Henne A, Craw D, MacKenzie D 2011. Structure of the Blue Lake Fault Zone, Otago Schist, New Zealand. New Zealand Journal of Geology and Geophysics 54: 311-328. doi:10.1080/00288306.2011.577080
- Crustal structure and topography of the Otago northeast margin
- Initiation of the fault zones on the northeast Otago margin
- Faulting and erosion of the Macraes gold deposit
- Gold-bearing veins at Oturehua
- Blue Lake Fault Zone
- Graphite and gold on the northeast schist margin
- Gold and arsenic in pyrite
- Waipounamu Erosion Surface
- Alluvial gold along the northeast Otago margin
- Rise of modern mountains on Otago's northeast margin
- Patearoa gold: Alluvial concentrations in a dynamic environment