At the end of my first Post in this series, What are Hydrogrossular Garnets?, one of the questions I raised was why hydrogrossular stones are found in New Zealand primarily in two regions more than 400 kilometres apart, in the Nelson area as well as around Orepuki, at the top and bottom of the South Island. [Writing in May 2021: As explained at the end of this Post, it has been suggested that hydrogrossular garnet can also be found on the West Coast of the South Island, and maybe also in the North Island. July 2022: I have also seen a report of hydrogrossular garnet being found on a beach in Napier.]
I have come to associate hydrogrossular stones with Gemstone Beach at Orepuki because they are not found anywhere else on the southern coast. I have heard third or fourth hand of someone who found one on a Riverton beach but the point of that story was how unusual this was. I have spent more time on Riverton beaches than Gemstone Beach but have found hydrogrossulars only on Gemstone. [Writing in May 2021: Over the past couple of years I have found a couple of small hydrogrossular stones on the Back Beach at Riverton.] Sources of information such as Te Ara The Encyclopedia of New Zealand often mention only Gemstone Beach as where you can find them in New Zealand.
When I undertook more careful research on hydrogrossular garnets it came as a surprise that the area around Nelson was treated as a key source area. In the entry on “Hydrogrossular” in “A Photographic Guide to Rocks and Minerals of New Zealand” by Nick Mortimer, Hamish Campbell and Margaret Low (2011), it is noted that the world’s first occurrence of the rock was described from the Nelson-Dun Mountain area by Colin Hutton in 1943 (more on this in a later Post). It does go on to note that hydrogrossular can be found on Gemstone Beach as well as in the streams and rivers east and south of Nelson city.
[Note: May 2022 – This next paragraph often refers to “grossular” rather than “hydrogrossular” garnet, but the two are not the same. I am at present unsure whether Thornton’s comments refer only to “grossular”.] Jocelyn Thornton, in “The Field Guide to New Zealand Geology” (2003), reports that “grossular garnet” can be found in the Dun Mountain area, “found in translucent green masses that can be polished” (page 69), as well as in the Roding, Lee and Maitai Rivers in Nelson (see first photo below for Thornton’s geological map of Nelson area). She points out that rodingite, a rock named after the Roding River, a mixture of grossular and diopside or diallage, can also be found in Cascade Creek in the Eglinton Valley near Milford Sound (see the second photo below, from Te Ara The Encyclopeida of New Zealand). “From there the grossular garnet makes its way through the lakes and down the Waiau [River], being tumbled into the rounded pebbles that can be found with a little effort on Orepuki Beach” (page 70). A 1969 research paper by J. G. Williams at the Department of Geology at the University of Otago had identified hydrogrossular in rocks in the area around Cascade Creek in the Eglinton Valley. Thornton’s book includes a photo of hydrogrossular from the Nelson and Orepuki areas (Plate 1E, opposite page 136 – see third photo below). Riverton Museum has a sample of hydrogrossular from the Eglinton Valley (see fourth photo below). In a 2018 “Southland Times” column (second item), local journalist Lloyd Ester reports: Southland’s Gemstone Beach has the best assortment of unusual pebbles in New Zealand. A combination of ocean currents, shape of the coast and the proximity of the Waiau River – the source of many of the stones – means that the rarities are concentrated along a short strip of beach. The best known of the “gemstones” are grossular garnets which are distinguished by their gloss and weight.” Nevertheless, the Waiau River is relatively young, in geological time. For instance, Thornton presents a map of what the Western Southland-Fiordland area would have looked like about 50 million years ago, the Waiau Valley being under sea (see fifth photo below). It’s possible that some of the hydrogrossular rock that became pebbles on Gemstone Beach were swept there by ocean tides millions of years ago.
To return to the question being examined in this Post: Why are hydrogrossular stones found in New Zealand primarily in two regions more than 400 kilometres apart, at different ends of the South Island? This distribution is not random but is in fact part of a much larger pattern. The first map below, found in a Google image search, shows that the sequence of terranes in Fiordland, Southland and Otago is repeated in Nelson and Marlborough. A “terrane” is a floating bit of the earth’s crust that butts up against a continent (New Zealand is part of the Zealandia continent, much of which is under water – see second map below). This disjunction in the terranes leads to the idea of there being western and eastern geological provinces making up the South Island (see third map below, from a 2013 article in “Gondwana Research”). New Zealand consists of at least 10 terranes (see pages 41-42 of Peter Ballance’s, 2017, “New Zealand Geology: An Illustrated Guide” for a list and description). But these have been partially rotated and then split by the Alpine Fault. As shown in the map by Jocelyn Thornton (fourth map below), the rocks (terranes) demonstrate that Northwest Nelson and Fiordland were once together. The two sources of hydrogrossular garnets may be 400 kilometres apart now but they are in the same terrane and 25 million years ago were in the same locality.
The Alpine Fault runs for about 600 kilometres along almost the entire length of the South Island. It is a segment of the boundary between the Pacific Plate and the Australian Plate (see map below, from Wikimedia). The Southern Alps have been uplifted on the fault over the last 12 million years in a series of earthquakes. However, most of the motion on the fault is sideways, with the Tasman district and West Coast moving north and Canterbury and Otago moving south. Horizontal movement of the Alpine Fault is about 30 metres every 1,000 years, which is very fast by global standards. Each time it has ruptured, it has also moved vertically, lifting the Southern Alps in the process. GNS Science estimates that in the last 12 million years the Southern Alps have been uplifted by 20 kilometres, and it is only the fast pace of erosion that has kept their highest point below 4,000 metres. Glaciers and rivers have removed the rest of the material and spread it out across the lowland plains or onto the sea floor (and beaches!). The Alpine Fault was not recognised until 1941 because the area was rugged and isolated, and earlier generations of geologists did not have the advantage of having an aerial view.
In conclusion, hydrogrossular garnets are found primarily in two regions in the South Island more than 400 kilometres apart because of the way that the Alpine Fault has split whole terranes of rocks and moved them sideways, south to north. [Writing in May 2021: A reader of this Blog has told me he has found what he thinks is a hydrogrossular stone in a West Coast river. This is the first time I have heard of a West Coast location, but it makes sense if hydrogrossular rock “travelled” north along with the Alpine Fault. On the way, rivers could certainly have moved stones away from the fault. The reader also suggested that hydrogrossular garnet can be found in the North Island.]
The next Post in this series looks at how hydrogrossular garnet is one of 13 minerals that were first described in New Zealand.
Information on the four photos at the top of this Post:
First photo: A sample of hydrogrossular garnet from the Eglinton Valley, in a drawer of rock samples in the Riverton Museum, Te Hikoi.
Second photo: A compartment of hydrogrossular stones and at least one quartz stone, from my own collection of polished stones.
Third photo: Handful of hydrogrossular garnets from Gemstone Beach, on page 155 of “The Story of Murihiku/Southland – A synopsis: An overview of Southland’s Heritage” by Russell Beck, Cathy Macfie and Lloyd Esler, April 2007 https://docplayer.net/57903929-The-story-of-murihiku-southland.html
Fourth photo: Rodingite rock sample, from Te Ara The Encyclopedia of New Zealand https://teara.govt.nz/en/photograph/9077/rodingite
Thanks for your many posts.
I found a stone in the Waiho river. The museum in Hokatika confirmed it wasn’t jade but couldn’t advise further..
Kiwi carvers in a jade forum settled on hydrogrossular, leading to months of research about global geology and specifically Westland, Franz Josef glacier , Waiho & Callery valleys geology. (including your many blogs)
Since the fault line runs north-south, (mostly close to the west coast) ..it’s also worth noting – most westland rivers also contain hydrogrossulars. (also found in the north island) ..
What a great place NZ is.
Hi Jed. Many thanks for your comment, I have found it very interesting. It helps me to open my mind more to the range of places hydrogrossular can be found. Appreciate it.
How much are these stones worth?
Hydrogrossular garnet rodingite
Hi Georgette. Hydrogrossular garnet stones are not worth much, the same as any pretty beach stone, really. I have seen polished tumbles for sale for something like $5 to $10 each at a rock shop and at a museum gift shop. I have seen pendants for sale around the $40 to $50 mark. I have seen individual polished stones for sale on stalls at shows run by rock and mineral clubs, around $30 to $40 each. I have seen craft items made partly out of them, or decorated with them, such as a human figure, a violin, and a chair – these items were priced the same as they would be if the stones used were common jaspers etc. However, hydrogrossular garnets are not often found for sale. There is no real commercial production or market for them. They tend to be found and polished by amateur rockhounds, and most of what I have seen would have come from Southland, from Gemstone Beach.