We think you will agree, this is a pretty special place, the landscape is just stunning. For those interested, if you stand on tiptoes and crane your neck whilst looking towards the South, you will also be able to see The Great Australian Mine. GAM was first discovered by Ernest Henry in the 1860’s. Copper is currently mined in three open pits at a rate of approximately 700,000 tonnes per year.
We haven’t brought you here to talk about Copper though we would like to talk about the unusual calc–silicate rocks surrounding the lookout.
In short, it is thought that these particular calc-silicates were originally limy muddy to sandy sediments relating to a back reef lagoonal environment deposited in shallow marine conditions, about 180 million years ago. Over time they were buried deeply in the Earth’s crust and subjected to high pressure and high temperature so that the combination of calcium carbonate (lime) and quartz rich sands and muds were crystallised to minerals such as garnet and diopside.
Now we need to get a little more tecnichal: By definition a calc–silicate rock is a rock produced by metasomatic alteration of existing rocks in which calcium silicate minerals such as diopside and wollastonite are produced. Calc–silicate skarn or hornfels occur within impure limestone or dolomite strata adjacent to an intruding igneous rock.
Lets look a little closer at metasomatic alteration: Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different minerological and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.
Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source.
In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact metamorphic aureole adjacent to an intrusive rock mass. In the metamorphic environment, metasomatism is created by mass transfer from a volume of metamorphic rock at higher stress and temperature into a zone with lower stress and temperature, with metamorphic hydrothermal solutions acting as a solvent. This can be envisaged as the metamorphic rocks within the deep crust losing fluids and dissolved mineral components as hydrous minerals break down, with this fluid percolating up into the shallow levels of the crust to chemically change and alter these rocks.
This mechanism implies that metasomatism is open system behaviour, which is different from classical metamorphism which is the in-situ mineralogical change of a rock without appreciable change in the chemistry of the rock. Because metamorphism usually requires water in order to facilitate metamorphic reactions, metasomatism and metamorphism nearly always occur together.
Further, because metasomatism is a mass transfer process, it is not restricted to the rocks which are changed by addition of chemical elements and minerals or hydrous compounds. In all cases, to produce a metasomatic rock some other rock is also metasomatised, if only by dehydration reactions with minimal chemical change.
Metasomatic rocks can be extremely varied. Often, metasomatised rocks are pervasively but weakly altered, such that the only evidence of alteration is bleaching, change in colour or change in the crystallinity of micaceous minerals.
In such cases, characterising alteration often requires microscope investigation of the mineral assemblage of the rocks to characterise the minerals, any additional mineral growth, changes in protolith minerals, and so on.
In some cases, geochemical evidence can be found of metasomatic alteration processes. This is usually in the form of mobile, soluble elements such as barium, strontium, rubidium, calcium and some rare earth elements. However, to characterise the alteration properly, it is necessary to compare altered with unaltered samples.
When the process becomes extremely advanced, typical metasomatites can include:
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Chlorite or mica whole-rock replacement in shear zones, resulting in rocks in which the existing mineralogy has been completely recrystallised and replaced by hydrated minerals such as chlorite, muscovite, and serpentine.
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Skarn and skarnoid rock types, typically adjacent to granite intrusions and adjacent to reactive lithologies such as limestone, marl and banded iron formation.
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Greisen deposits within granite margins and cupolas.
Effects of metasomatism in mantle peridotite can be either modal or cryptic. In cryptic metasomatism, mineral compositions are changed, or introduced elements are concentrated on grain boundaries and the peridotite mineralogy appears unchanged. In modal metasomatism, new minerals are formed.
Cryptic metasomatism may be caused as rising or percolating melts interact with surrounding peridotite, and compositions of both melts and peridotite are changed. At high mantle temperatures, solid-state diffusion can also be effective in changing rock compositions over tens of centimeters adjacent to melt conduits: gradients in mineral composition adjacent to pyroxenite dikes may preserve evidence of the process.
Modal metasomatism may result in formation of amphibole and phlogopite, and the presence of these minerals in peridotite xenoliths has been considered strong evidence of metasomatic processes in the mantle. Formation of minerals less common in peridotite, such as dolomite, calcite, ilmenite, rutile, and armalcolite, is also attributed to melt or fluid metasomatism.
Investigation of altered rocks in hydrothermal ore deposits has highlghted several ubiquitous types of alteration assemblages which create distinct groups of metasomatic alteration effects, textures and mineral assemblages.
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Propylitic alteration is caused by iron and sulfur-bearing hydrothermal fluids, and typically results in epidote-chlorite-pyrite alteration, often with hematite and magnetite facies.
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Albite-epidote alteration is caused by silica-bearing fluids rich in sodium and calcium, and typically results in weak albite-silica-epidote.
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Potassic alteration, typical of porphyry copper and lode gold deposits, results in production of micaceous, potassic minerals such as biotite in iron-rich rocks, muscovite mica or sericite in felsic rocks, and orthoclase (adularia) alteration, often quite pervasive and producing distinct salmon-pink alteration vein selvages.
Rarer types of hydrothermal fluids may include highly carbonic fluids, resulting in advanced carbonation reactions of the host rock typical of calc-silicates, and silica-hematite fluids resulting in production of jasperoids, manto ore deposits and pervasive zones of silicification, typically in dolomite formations.
To log this Earth Cache we require you to consider the information given, take a look around the area, perhaps you need to do some research of your own, then message us with the following answers to the best of your ability;
1. Anywhere near the posted coordinates should find you near examples of calc–silicate rocks, describe how they feel and look?
2. Can you see any crystals within the rock, if you can where do they come from?
3. Which category do these rocks best fit: Propylitic alteration, Albite-epidote alteration or Potassic alteration?
4. There is some dark staining on the rocks, what is this from?
5. A photo of your team, GPS or the view with your log please. (Optional)
You are welcome to log your find straight away to keep your TB's and Stats in order but please message us with your answers within 24 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Source: Wikipedia, John Nethery.