The present day general appearance of the Mannum granite quarry can roughly be divided into four different sections, based mainly on visible geological features.

1. The area north of the dolerite dyke, with over 100 xenoliths scattered over the face, some of which are Porphyritic. While many of the xenoliths are dioritic, compositions vary. Size also is a variable factor. Porphyritic granite also occurs in this section.

 

2. The intrusion of the dolerite dyke, roughly in the center of the quarry. The actual dyke is in the vicinity of fifty centimetres wide, and extends right to the top of the granite in the quarry.

 

3. The southern area, which has very few xenoliths, but does have two areas of badly weathered granite as well as the extremely large xenoliths. Some of the badly weathered granite appears to be a layered structure. There is also evidence, in the form of a discard dump, to suggest that the extremely large xenoliths was once larger than it is now.

 

4. The presence of two unconformities, both on top of the granite. The first overlies the granite unconformably, and is a fossil-rich tertiary sandy limestone, which is also overlain unconformably by Pleistocene calcrete. There is also evidence at the top of the quarry, in the form of water-worn granite, to suggest that this granite outcrop was once an island in a Tertiary sea.

The main body of the Mannum Granite is about 0.8 kilometres in length and has a maximum width of 0.2 kilometres.

General View

General view of the Mannum Granite Quarry looking from south to the north.

 

Dolerite dyke

A view of the dolerite dyke cutting through the granite.

 

Xenoliths

A close-up view of a number of xenoliths consisting of dark mafic material.

 

Unconformities

A view of the unconformities, with a fossiliferous limestone overlain by calcrete, below which can be seen a number of drill holes in, what was, the highest bench of the quarry.

 

View of the large xenolith at the southern end of the quarry. Evidence of discarded material from this xenolith can be seen at the foot of the photograph.

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The best exposures of the Mannum granite are in the quarry, some 4 kilometres from the town itself, in an area known as "The Rocks". The granite is cut by a prominent dolerite dyke, and has numerous xenoliths composed of dark colored (mafic) igneous rocks, scattered mainly to the north of the dyke. In total, these xenoliths constitute around 5% of the outcrop. The most prevalent rock form present is a medium to coarse grained alkali rich granite, composed of mainly orthoclase, quartz and biotite. The crystals of the orthoclase are often mantled by plagioclase, that is, they have chemically altered outer edges. Other minerals present include mineral oxides, magnetite and titanite.

The many xenoliths show a texture ranging from fine to medium grained, and a wide range of compositions, which range from dolerite and quartz-diorite to a possible andesitic composition. Most are somewhere between 5 centimetres and 50 centimetres in diameter, although one is in excess of 5 meters in diameter. This extremely large xenolith shows prominent weathering as well as granite veins cutting through it.

In addition to the xenoliths, there are also numerous inclusions of a fine and more evenly grained micro granite. Both forms of the granite are similar in mineralogical and chemical compositions. No xenoliths can be seen in the micro granite.

One interpretation of the geology of the quarry suggests that the first intrusion of granite crystallized from high temperature magma, into a fine-grained micro granite, due to the presence of the colder Kanmantoo rocks. A later intrusion was able to physically mingle with the mafic magma represented by the xenoliths, and it is likely that these xenoliths owe their positioning due to convection currents in the molten granite. The mafic xenoliths solidified before the granite, hence the smaller grain size. This second injection of granite into the already cooling granite would cool more slowly, thus forming larger grained crystals. As this second phase took place, the granite was able to mingle with the micro granite by block stopping. Blocks of micro granite can be seen in many places in the quarry and, along with the other xenoliths, show hybridisation of the rock.

Evidence of the granite outcrop being partly covered by the sea, in Tertiary times, can be seen as water worn surfaces on the top of the quarry, as shown below.
Evidence can be seen here of the action of water on the exposed granite surface.

 

This photograph shows a small xenolith with a joint plane underneath. The joint plane shows chemical alteration. More detail of the joint plane can be seen below.

 

Detail of the chemical alteration of the joint plane.

 

The area just south of the large xenolith, which shows weathering in the granite. Closer views of this section are shown below.

 

The crescent shaped area, especially the center of the photograph, shows weathering.

 

A closer view of the center of the above photograph.

 

A view showing "stratification" of the granite.

 

Closer detail of the "stratification" of the granite.

The "stratification" shown in the photographs above, may possibly be a physical weathering feature due to the release of pressure (off loading).

An area of jointed and weathered granite.  

A closer view of the weathered area in the above photograph.

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A few kilometres upstream from Mannum there is an area known locally as "The Rocks". It was so named on account of a granite quarry, which began operating there in 1913 and from which over half a million tons of granite was quarried. The granite was quarried from the outcrop that exists there and carted, in trucks, via a small railway to the crusher. After being crushed, it was elevated in bins, and from these it was conveyed into barges. These barges were known as "Bin Barges" as they were specifically built for the work. The crushed granite was transported to wherever it was required, for use in the construction of locks and weirs, either pushed by the sternwheeler "Captain Sturt", which was finished in 1916, or pulled by the paddle steamer "Mannum", built in 1918.

Historical Photographs.

 

The crusher, about 1920.

 

Barges loaded and waiting for transport upstream, about 1920.

 

The sternwheeler "Captain Sturt, around 1920.

 

The "Captain Sturt" pushing barges loaded with crushed granite upstream, around 1920.

 

This, and a similar concrete block on the river-side of the road, are all that is left of the crushing complex.

 

Although not easily seen here, the remains of three pylons, one above water near the reeds and two below, can be seen here.

 

A closer view of what remains of two of the wharf pylons.

 

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