Introduction
The Hillgrove Suite is part of a series of magma intrusions, or plutons, that have pushed up from the Earth's mantle into the layers of older sedimentary rock above, over different periods of time. Hillgrove Plutonic Suite is located at Hillgrove, 30km East of Armidale, NSW. The word "Pluton" comes from the Roman god Pluto of the underworld, a reference to the formation of these rocks under the earth before being exposed by erosion.
Formation of Hillgrove Plutonic Suite
The Hillgrove Suite is part of the Granite Belt, an extensive complex of granitic rocks of different ages. This is known to geologists as the New England Batholith, an aggregation of plutons extending from the Darling Downs region near Warwick, Queensland, to the New England Region, as far as Armidale, New South Wales; over 250 kilometres.
Fig 1: Formation of a pluton. From Rymich.com
The New England Batholith forms part of the Tablelands Complex of the New England Orogen. An Orogen is a part of the Earth's Crust that has resulted in mountain formation due to the interaction of tectonic plates (Orogeny). The New England Orogen was formed by one tectonic plate sliding underneath another. This subduction caused the uplift and buckling of the upper plate, allowing magma to intrude into the upper layers, resulting in the volcanic rocks and plutons of the Tablelands area we see today.
Fig 2: Location of the New England Orogen in New South Wales. From Earthscrust.org.au
Fig 3: Structural units of the southern New England Fold Belt showing elements of the New England Batholith in blue, scattered throughout the Sandon Association (Sandon Beds) in greenish yellow. Armidale is marked. From Macquarie University
Associated Mineral Bearing Strata
At GZ you are standing on the Granite of the Hillgrove Suite on the Eastern side of the Gorge. But you can look across the gorge, and a bit to the south, to see Metz Lookout on the Western side, where you find the metamorphosed Greywacke. This had its beginning as the nutrient-poor soil of the Sandon Beds, common to the Armidale area, laid down during the earlier Devonian and Carboniferous periods. It is composed of angular grains of quartz, feldspar and small fragments of other rock, and was formed into this dark, cherty metamorphic rock by the extreme heat from the neighbouring intrusion of magma during the early Permian period. So when you look from GZ across to the Metz lookout you are also looking back in time. The point of contact between the Greywacke of the Sandon Beds and the Granite pluton of the Hillgrove Suite is a few hundred metres West of GZ, on the Eastern side of the Gorge.
The cherty Greywacke contains the minerals of interest to commercial mining, including veins of gold (aurum) bearing quartz and stibnite, giving rise to gold and antimony mining in this area. Alluvial gold was discovered in 1859, but antimony (stibnite) was discovered in the late 1870's, stimulating commercial mining that continues to the present day. The historic mines included Eleanora, Baker's Creek, Sunlight (East and West), Syndicate, Cosmopolitan and Damned If I Know mines, among others. Many of the mines operated steeply incline-tramways down the treacherous sides of the gorge to transport ore and workers. You can see the path of some of these old tramways from the Metz lookout across the gorge. More than 20km of known veining occurs in the steeply inclined overlapping strata, the deepest mine shafts extend vertically for more than 500m. More than 720,000 oz gold had been extracted in the past by the various mines of the day. There is still upward of 1.3 million gold oz equivalents estimated remaining. Recent exploration in 2016 by Hillgrove Mines (who now hold all the mining leases) has found significant new lodes of gold associated with the historic Sunlight mine sites.
Fig 4: Cherty Greywacke with quartz veining from Metz lookout. (Photographer: L Kelly).
Questions:
Please message/email the cache owner separately with your answers to the following questions when you log your find. Do not make references to the answers in your online log.
- In what geological period were the Sandon Beds of the New England area laid down? ______
- In what geological period did the Hillgrove Suite plutons intrude the Sandon Beds? ______
- What is the rock type at GZ? How does it differ visually from the rock type from Metz Lookout as pictured above? ______
- Stage 2 Metz Lookout (from the information board): What is the word used to describe the mineral-bearing stone? Identify the root word given in the lesson above. What is the likely meaning of the word from the information board? ______.
References:
- Ryan, V., and Ryan, C. (2009) Girraween National Park - Granite Foundations
- Finlayson, D. (2016) The New England-Lachlan Orogen boundary across the Peel Fault and Tamworth Belt
- GEMOC ARC National Key Centre (2006) Macquarie University. From Research Highlights 2006 - Heavy Rock(s) in Tamworth
- Rod (2017) Stunning in Red and White
- Cartoscope Pty Ltd (2017) Geological Sites of NSW - Oxley Wild Rivers National Park
- Hooper, B., Ashley, P., and Shields, P., (c. 2010) The Hillgrove Gold-Antimony-Tungsten District, NSW, Australia