Martha Mine Educational Resources for Schools: Geology

A recipe for gold
Inside the earth
Geothermal migration

Regional geology: a recipe for gold:
a brief description of the geological history of the Waihi region.

Geysers, Mudpools and Gold
The Waihi landscape was very different seven million years ago. The land was higher, probably by several hundred metres. Geysers threw boiling, mineral rich water high into the air, mudpools bubbled and hot springs dotted the countryside, very much like Rotorua today. Hot rock far below the ground provided heat energy for those spectacular features and also provided the ingredients for the gold deposit to be formed at Martha Hill. Over time the volcanic activity dwindled as the volcanic 'hot spot' gradually migrated southeast, passing through Tauranga and Te Puke to be centred around the Central Plateau today.

Below is an impression of how the land has changed over the last 20 million years, showing how the forces of nature comine to produce a recipe for gold.

KEY

Greywacke.... Andesite.... Ignimbrite.... Lava.... Quatrz Veins

New Zealand is Born
The oldest rocks, called basement rocks, in the Coromandel region of New Zealand's North Island are greywacke. These sedimentary rocks were deposited in deep sea trenches about 150 million years ago as the land mass known as Gondwanaland eroded.

The collision of the Pacific and Australasian Plates later uplifted these compacted sediments to form the main ranges of the North Island. New Zealand was a much different shape then than it is today.

Volcanic Violence
Around 20 million years ago volcanoes burst through the landscape around what we now know as The Coromandel Peninsula; including Waihi. Tall, steep sided andesite volcanoes formed as the hot lava was thrown out, coating the sides of the volcanoes and the surrounding land. These volcanoes were similar in size and shape to Ruapehu and Ngauruhoe in the central North Island. Volcanic activity continued throughout the Coromandel for more than 15 million years.

The Mountains Wear Down
Any rock exposed to the air is subjected to a range of forces that gradually wear it away. Over time the forces of weathering and erosion can reduce mountains to hills.

20 million years of weathering and erosion has taken its toll on the local landscape. It is hard to make out the shape of those ancient volcanoes. Hundreds of metres of rock have been gradually eroded away from the original mountains.

Other Forces; Earthquakes Rupture the Land
Meanwhile, other forces were also working deep within the earth. The landmass of New Zealand was being squeezed and uplifted, helping the mountain building process. This uplift continues today, very slowly pushing up most of New Zealand.

It is thought that about seven million years ago the Waihi area was violently shaken by many earthquakes. Large deep cracks or fissures appeared in the older volcanic rocks.

Geysers, Mudpools and Gold
The Waihi landscape was very different seven million years ago. The land was higher, probably by several hundred metres. Geysers threw boiling, mineral rich water high into the air, mudpools bubbled and hot springs dotted the countryside, very much like Rotorua today.

Hot rock far below the ground provided heat energy for those spectacular features and also provided the ingredients for the gold deposit to be formed at Martha Hill.

Over time the volcanic activity dwindled as the volcanic 'hot spot' gradually migrated southeast, passing through Tauranga and Te Puke to be centred around the Central North Island today.

Weathering
Following the period of geothermal activity, weathering continued to wear down the mountains and hills. The quartz veins, produced during the geothermal activity, running up through the Waihi area were hard to erode and gradually Martha Hill began to protrude above the surrounding land.

What was once deep inside the earth was now close to the surface as a result of the forces of heat, cold, wind and rain. However Martha Hill did lose some of its original height, estimated to be about 400 metres.

Ash Blanket
About five million years ago a period of incredibly violent rhyolitic volcanism began. These volcanoes blanketed a huge areas of the North Island with thick sheets of ignimbrite and ash.

Waihi was not spared, and the gold-rich quartz veins in Martha Hill were once again covered.

Martha Exposed
The continuing erisive power of the Waihi weather kept eating away at the ignimbrite and ash over the next few million years. Eventually Martha Hill was exposed again, and in 1878 McCombie and Lee saw the quartz veins sparkling in the sun for the first time.

Inside the Earth
This diagram shows how veins of silica containing gold and silver formed in rock fissures.

Minerals like silica, gold and silver are dissolved by extremely hot water, transported by the groundwater flow and then deposited in cracks in the rock as the water cools.

These are the quartz veins which are currently being mined at Martha Mine.

Different minerals are deposited at different depths. Minerals that are soluble only at very high temperatures are deposited first far below the surface, while others are very soluble and remain in solution until they reach the surface.

1. Mudpool
2. Geyser
3. Fumerole
4. Hot spring
5. Cold dense ground water
6. Heated ground water
7. Magma body (heat source)

Geothermal Migration
This diagram shows how a geothermal hot-spot has migrated across the North Island of New Zealand over the past 20 million years.

The hot-spot is created by the collision of the Pacific and Australasian Plates.

New Zealand 'rests' at the intersection of these two plates and is part of the Pacific 'Rim of Fire'

Key Concepts & Words

20 million years ago there were andesite volcanoes in the Waihi area as big as Ruapehu.
Volcanic eruptions continued for more than 15 million years.
Weathering and erosion has significantly modified the Waihi landscape. What was deep underground is now much closer to the surface.
About 7 million years ago severe earthquakes opened up large cracks in older volcanic rocks.
Thermal activity deposited silica containing gold and silver in rock fissures. These are the quartz veins now being mined at Martha Mine.

LInks to other sections of the Martha Mine website
For a more detailed description of the geological history of the Waihi region see: A Brief Geological History of the Martha Mine

Links to other sites
To find out more about volcanoes see the New Zealand Minerals Industry Association website which has a section linking to over 50 volcano websites in New Zealand and around the world. You can go straight to the volcano page by clicking here.

The University of Otago Department of Geology website features a wide range of material about geology and gold. www.otago.ac.nz/geology/ See Gold from Mountains to Sea www.otago.ac.nz/geology/features/goldriv/goldriv.htm The website also offers a great Question and Answer section. www.otago.ac.nz/geology/askus/askus.htm

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