I grew up at Waikaka in Southland, New Zealand, and was always aware that it was an area where gold mining had taken place in the past. The Waikaka River ran though our farm, and the river flats contained lots of small ponds that were referred to as “dredge ponds”. When I was young, I remember being shown a small bottle with a few flakes of gold that the family had. And when I did a biography of a family member in my second year at high school, I chose my great grandfather James Paterson who had been involved with gold dredging. I left Waikaka to go to University in 1974. I still visit the area from time to time, even though the farm is no longer in family ownership. The following is a report on one aspect of my recent research on the geology and history of the Waikaka district. Later posts will report on other things I have found.
“THE GEOLOGY OF NEW ZEALAND” (1978)
I now live near Cambridge in the North Island of New Zealand. On the second Sunday of each month, Cambridge has a “Trash and Treasure” street market. At September’s market, I came across a secondhand bookstall that had a copy of the large two volume “Geology of New Zealand” published in 1978 by the New Zealand Geological Survey. A bargain at $40, even though a little dated. Its strength is its detailed coverage of every area of the country. During the decision to buy it, I looked up a number of places to see what what information it had on them. In the section on Southland in Volume II, I unexpectedly came across reference to “Waikaka Quartz Gravel”, the first time I became aware that there was a special designation for the gold-bearing gravel so significant to Waikaka’s history.
The authors state on page 524: “In eastern Southland the Waikaka Quartz Gravel comprises mainly quartz gravel and clay, but includes fragments of strongly leached pale blue or white schist or greywacke; these were either derived in their leached condition from the deeply weathered zone under the mid-Tertiary beds, implying local erosion and unconformity under conditions of low relief, or leached after deposition, implying a continuation in this district of peneplain conditions into the late Tertiary; probably both events occurred.”
THE NEW ZEALAND STRATIGRAPHIC LEXICON
Back home, I did an internet search for “Waikaka quartz gravel” and discovered that the New Zealand Stratigraphic Lexicon includes a brief entry on “Waikaka Quartz Gravels” (http://data.gns.cri.nz/stratlex/view.jsp?id=3492). This Lexicon, operated by GNS Science, is a list of official and recognised geological terms and units. GNS Science is a New Zealand Crown Research Institute for geology, geophysics, and nuclear science. As is stated on the home page of the New Zealand Stratigraphic Lexicon, “Precise definitions of units of rock and periods of geological time are required, especially for mapping and study of geological history…This is a list of New Zealand rock unit names, giving information on age and distribution, the hierarchy of stratigraphic units, synonyms, and references to relevant literature” (http://data.gns.cri.nz/stratlex/index.jsp).
The listing of “Waikaka Quartz Gravels” in the New Zealand Stratigraphic Lexicon provides no information on it but it means that this geological feature has official recognition, reflecting its prior appearance in geological studies. The two important prior sources noted in the Lexicon entry are the one I have already mentioned above – page 534 of Volume II of “The Geology of New Zealand” – and the research undertaken in the late 1940s associated with “The Geology of the Gore Subdivision, Gore Sheet District (S170)”, published in 1956 by the Department of Scientific and Industrial Research as New Zealand Geological Survey Bulletin 53. This is for sale as a pdf file for $20 from the GNS Science Online Shop.
“THE GEOLOGY OF THE GORE SUBDIVISION” (1956)
“The Geology of the Gore Subdivision” (1956) contains interesting information about the Waikaka quartz gravels. For example, pages 9, 14 and 15 contain the following points: the gold to be found in the area is very fine; the early 20th century saw a lot of gold dredging, with most of it in the valley of the Waikaka River; geologists referred to gold-bearing quartz around Waikaka at the end of the 19th century; the existence of lignite (“brown coal”) is also noted (this was an important fuel to operate the gold dredges); and the gold is noted as likely deriving from the mica schist further north in Otago, perhaps from the wave action of ancient seas or from the eroding action of rivers.
As an aside: The Department of Geology at the University of Otago notes that the existence of gold deposits in Otago schist is an example of “mesothermal” gold. These deposits are formed by hot water moving through rocks which are uplifted from deep (10 km) in the Earth’s crust. The gold in the deposits are extracted from the surrounding rocks by being dissolved in hot water. It is then deposited in the schist by sudden cooling or changes in pressure as the waters are forced through rocks by earthquakes. The gold deposits occurs within small quartz “reefs” or veins in cracks in the schist. I would guess that it is such deposits that have been broken down by water action in ancient oceans or rivers south of the main Otago schist area, finding its way as very fine gold into river valleys like that of the Waikaka. This is alluvial gold found in association with quartz gravel washed out of schist bedrock, the basic constituents of the Waikaka quartz gravels.
In Nick Mortimer, Hamish Campbell and Margaret Low’s (2011) “A Photographic Guide to Rocks and Minerals of New Zealand”, it is stated that gold “is concentrated by ancient hydrothermal systems into quartz veins or lodes in igneous and metamorphic rocks such as rhyolite, granite and schist” (page 46). When rivers erode these hard rock deposits, the gold is freed as individual grains. The rivers can then concentrate the gold in loose river gravels or “placers”, hence the term “placer gold”.
In pages 93-100 of “The Field Guide to New Zealand Geology” (2003), Jocelyn Thornton describes the “Haast schist region” (see below). The southern edge of this region lies just to the north of Waikaka.
Schist is a metamorphic rock that develops under the weight and pressure of overlying sediments. Mica, a shiny flake, grows in pressurised rock and can commonly be found in Otago schist. Mica and other flat mineral grains align to give schist cleavage, foliation and splittability. A good example of schist and its use is Mitchell’s cottage at Fruitlands in Central Otago. This building made of schist was erected between 1880 and 1904 by Andrew Mitchell, a gold miner and stonemason from Shetland. The photo below is of the cottage in its setting in the schist landscape.
Quartz veins bearing gold can be found throughout the Haast schist region. “There must have been a long-lasting fluid flow through the buried schists. As pressure and temperature increase, minerals that contain water in their structure will lose the water and this water will move up through the pile from the deepest hottest parts, leaching even trace elements [like gold] from the rocks and concentrating them in veins” (page 99 in Joyce Thornton’s, 2003, “Field Guide to New Zealand Geology”). Schist, when weathered or eroded, tends to eventually break down into fine clay.
WAIKAKA QUARTZ GRAVELS DEFINED
Back to “The Geology of the Gore Subdivision” (1956)… On pages 16 and 17, Table 5 sets out the main stratigraphic units of the area, arranged in terms of geological age. As shown on page 16 (see below), the Waikaka quartz gravels are grouped with the very similar Waimumu quartz gravels (Waimumu lies about 35 kms south of Waikaka, not far from the Mataura River south of Gore) and are described as follows: “Dominantly subangular to rounded white quartzite pebbles, rare quartzite red jaspilite and decomposed schist, in a micaceous clay matrix. A fluviatile deposit on a land of low relief but not a peneplain.”
The next six paragraphs break down and interpret this description of Waikaka quartz gravels:
Dominantly subangular to rounded white quartzite pebbles = This refers to roundness, size and type of the most common stone. Geologists often work with six categories of roundness for stones, going from sharp and jagged to smooth and round – 1) Very angular (where corners sharp and jagged), 2) Angular, 3) Sub-angular, 4) Sub-rounded, 5) Rounded, and 6) Well-rounded (where corners are completely rounded). Geologists usually also categorise stones in terms of size. One set of categories is as follows: A “grain” (e.g., of sand) has a diameter of 2 to 4 millimetres, a “pebble” is between 2 and 64 millimetres, a “cobble” is 64 to 256 millimetres in diameter, and a “boulder” is larger than that. [Note that “gravel” is a loose collection or aggregation of stones which can be of various sizes, though it is often of pebbles mixed with granular material and may contain larger stones as well.]
Quartzite is formed when a quartz-rich sandstone has been exposed to high temperatures and pressures which fuse the quartz grains together forming a dense, hard rock. Quartzite has a high degree of hardness as well as a high quartz content. It generally comprises greater than 90% percent quartz. Quartzite also tends to have a sugary appearance and glassy lustre. The purist quartzite is white in colour (and we often simply refer to it as quartz), though quartzite can be of a variety of colours dependent on minor amounts of impurities being incorporated with the quartz during metamorphism.
Quartzite red jaspilite = Jasper is an aggregate of microgranular quartz and/or chalcedony and other minerals. It is an opaque, impure variety of silica, usually red, yellow, brown or green in color. The common red variety of jasper is due to iron inclusions. Jaspilite is a banded rock made up of layers of jasper with other material, such as hematite and quartz.
A micaceous clay matrix = This is a fine-grained mass of material, predominantly clay, with fine scales of mica present, this clay and mica coming from the break-down of schist, as noted previously.
A fluviatile deposit = This means “of, found in, or produced by a river”.
A land of low relief but not a peneplain = A peneplain is a gently undulating, almost featureless, plain that is produced by river erosion that would, given enough time, reduce the land almost to sea level, leaving so little gradient that essentially no more erosion could occur. An area of low relief is one that tends to be flat but is not completely flat, having some slope to it (and therefore not being a peneplain).
WAIKAKA QUARTZ GRAVELS ON THE MAP
“The Geology of the Gore Subdivision” (1956) contains a geological map. Right at the top margin is the southern part of the Waikaka district, with the Waikaka township two miles (just over 3 kms) north of the map border. A section of Waikaka Quartz Gravel is shown on the eastern side of the Waikaka River, starting just south of Fleming and extending northwards as a narrow band, coinciding with the river terrace. There are also three outcrops noted on the western side of the Little Waikaka River, not far from what is locally known as the Queen Hill on Turnbull Road.
The nearby river flats are classified as “river alluvium” and the ridge in between consists of “Gore Piedmont Gravels”, defined in the Key as “deeply weathered rusty brown auriferous quartz-greywacke gravels”. This suggests to me that the Waikaka quartz gravels are a series of beds, overlain by more weathered gravels, which they may be the source for, and they are probably also one of the main sources of the river flats alluvium which will no longer have the clay matrix of the beds. It also means that gold is to be found not only on the river flats but also above the river terraces – which is why sluicing was used by miners in this area.
