Minerals from Mt Malosa, Zomba, Malawi

5 September 2009

The alkaline Pegmatites of the Zomba-Malosa complex of Malawi are famous for their unique geology and mineralogy. The mineral specimens of Aegirines, Arfvedsonite, Feldspars, and the rare unusual REE (Rare Earth Element) minerals are some of the best in the world. The Aegirines from Mt Malosa form large free standing crystals up to 25cm and the very rare REE-mineral Parisite from Mt Malosa, are also some of the largest ever found.

Prior to the discovery of this location there were very few localities that produced Aegirine of this size and quality.  This unique setting has no comparable locality that has produced such fine and rare specimens, and found under such difficult collecting conditions.



Location of Malawi in Africa

Malawi is a land locked country in south-eastern Africa, located in the southerly part of the Rift Valley (which extends to as far north as Syria in the Middle East). Geologically the Mt Malosa region is part of the Cretaceous Chilwa Alkaline Province, which consists predominantly of alkaline intrusive and extrusive igneous rocks associated with the East African rift (Eby et al. 1995). A variety of lithologies ranging from Granite, Syenite, and Nepheline-Syenite plutons, to associated extrusive Carbonatites and agglomerates dominate the region.

The Mt Malosa-Zomba complex is the largest recognised pluton in this region of igneous emplacement, consisting of Syenite and Peralkaline Granite. The pear-shaped complex can be geologically subdivided into two zones – the southern Zomba area consisting mainly of Syenite surrounded by a periphery of Alkaline Granite and the northern Malosa are of a mixture of Syenite and Granite.

Immediately north of Mt Malosa-Zomba are a series of four west-east orientated smaller intrusions of nepheline syenite (Chinduzi and Monglolowe areas) and syenite (Chaone and Chikala areas).

Pear-shaped Mt Malosa-Zomba complex, and just to the north is the west to east orientated Chinduzi, Monglolowe, Chaone and Chikala areas (Left to right)

Of interest to the mineral collector are the two important types of specimen-bearing Pegmatites present in the area. Firstly in the Chinduzi to Chikala range of mountains, Nepheline-Syenite Pegmatites occur - these contain large well-developed Aegirine crystals "up to 5 by 2.5 cm in size" (Bloomfield 1965).

Secondly in the Mt Malosa-Zomba complex Granitic Pegmatites are found. The northwestern fault scarp in the Mt Malosa area in particular has many of these Pegmatites, and from these deposits the best specimens have been collected. The southern Zomba section of the complex has far fewer Pegmatites.

Until recent time, the exact locality of the Pegmatite deposits at Mt Malosa were never described or recorded and most  specimens have merely been labelled as coming from Zomba (the small village located south of Zomba Mountain). Several collectors have made trips to the locality to obtain specimens at the source and to determine the exact location of the Pegmatites (Cairncross, Messner, and Farquharson 1999; Cairncross 2000).

Smoky Quartz on Aegirine, Mt Malosa
Smoky Quartz on Aegirine, Mt Malosa

Mount Malosa:
 
An account of a collecting trip to the Mt Malosa plateau, undertaken in October 1997 by a group of Johannesburg mineral collectors, Karl Messner, Paul Botha and Eric Farquharson is given here:

http://wwwu.edu.uni-klu.ac.at/mmessner/sites/malawi/zomba.htm

The site is described as very remote, with a long hike down the Zomba Mountain into the valley and up the otherside to the Malosa Plateau, often involving paths so narrow one foot must be placed in front of the other. The site of the mineral excavation is located on the northwestern side of the mountain where a series of major faults cut through the mountain. These have produced scarps with vertical cliff faces up to 800 meters high, and the area is very dangerous. The Pegmatites that contain the minerals strike across the mountain and down the vertical cliffs so that some can only be reached by climbing down the cliff faces. Because the Quartz-rich Pegmatites are more resistant to weathering, they form narrow ridges that the collector can balance and walk along.

Local miners have been digging and collecting here for several years, moving many tons of overburden soil and waste rock in the process. They do this by using primitive equipment and their bare hands. There are no mines operating in the region; all the specimens are dug from outcrop. Climbing equipment, such as ropes and pitons, is unheard of, so it is astounding that under such dangerous and precarious conditions so many beautiful and often fragile specimens have been collected, let alone remain undamaged.

Aegirine, Mt Malosa
Aegirine, Albite and Zircon, Mt Malosa

Minerals found at Mt Malosa:

Aegirine - This silicate mineral occurs abundantly, and in aesthetic large and well crystallised specimens, with a huge variation in form, associations and size. The crystals are dominantly very lustrous prismatic crystals, with simple to steep tapered sharp terminations. Single crystals to 24cm have been found, and composite crystals to 40-50cm have also been collected. Clusters of elongate crystals can form aggregates resembling haystacks. The Ageririne is often associated with small white Microcline Feldspar crystals or beige doubly terminated Zircon crystals.

Aegirine, Mt Malosa
Aegirine, Mt Malosa

Albite - This Feldspar forms prismatic lustrous cream coloured crystals, often with intergrown elongate Aegirine crystals

Arfvedsonite - The Amphibole occurs as sharp prismatic to euhedral crystals. The crystals are black and typically display striations on the prism faces, parallel to the c-axis. The crystals are commonly stubby measuring 4-5 cm long but can be found over 10 cm in length. Associated minerals include Aegirine, Microcline, Zircon, and Quartz, and Arfvedsonite may be found as fine hairlike crystals included in Quartz, giving it a dark green colour.
 
Bastnasite-(Ce)

Beryl

Biotite

Calcite

Caysichite-(Y)

Epididymite - one of the largest Epididymite crystals yet reported from Mt Malosa is a terminated hexagonal crystal 4.5 cm long and 4 cm in diameter, on a matrix of Aegirine and Smoky Quartz. The crystal is colourless and displays a highly micaceous texture. Other specimens of Epididymite are more typically elongate, thin prismatic crystals, very commonly included within Quartz crystals.

Epidote

Eudialyte

Eudidymite

Fergusonite-(Y) - Rare - These display the typical tapering habit and are pale yellow.

Galena - Unusual considering the geology but rarely crystals have been with attached Aegirine crystals confirming its source as indeed originating from Zomba.

Goethite - Goethite is prevalent on most of the specimens that come from the area. It forms a brown rust-coloured coating to most specimens. Pseudomorphs after what is believed to be Parisite have been found - the crystal that is being replaced is flat, tabular and hexagonal in outline, but is now composed entirely of iron hydroxides.

Hematite

Hingganite-(Y)

Ilmenite

Mangan-Neptunite

Monazite

Neptunite

Orthoclase-Microcline - All the Feldspar found at Mount Malosa is potassium Feldspar. Although no definitive analyses were carried out, a few random Feldspars were examined and proved to be Microcline. The largest euhedral crystals typically do not exceed 25cm in length. Groups of Microcline are common and often associated with Quartz, Smoky Quartz, Aegirine and Arfvedsonite. The Feldspar is white to very pale-cream in colour and twinning is very common. Some K-feldspar crystals have pitted or corroded crystal faces.

Parisite-(Ce) - Parisite-(Ce) has been reported from the Zomba complex. One of the largest crystals to date is a terminated, 3.2 cm hexagonal crystal (2.1 cm diameter) associated with Aegirine and a second, smaller naturally etched Parisite. The Parisite displays a common feature of having a tapering profile towards the flat, pinacoidal termination. As with other smaller crystals that were found on other specimens, this large specimen is has typical corroded faces.

Parisite, Mt Malosa
Parisite with Aegirine and Orthoclase

Polycrase-(Y)

Polylithionite

Pyrochlore

Quartz - An abundance of Quartz is common within any batch of Mt Malosa material. The crystal sizes range from thumbnail to over 25 cm. The Quartz is most commonly clear and colourless. However, inclusions of acicular Aegirine produce green to almost black crystals. Smoky Quartz is also common. The habits of some of the Quartz specimens are somewhat unusual. Most are the common hexagonal shape but some crystals display an extremely flattened form. Many Quartz crystals have overgrown Aegirine. A few Quartz specimens have included yellow to dull orange impurities that are identified as oxidised included fibrous Aegirine. Quartz is associated most commonly with Aegirine and Potassium Feldspar, as well as Zircon.

Smoky Quartz on Aegirine, Mt Malosa
Smoky Quartz on Aegirine

Riebeckite

Scheelite

Siderite - A few specimens of rhombohedral, tan-coloured Siderite crystals form the matrix to Quartz and Feldspar

Thorite

Titanite

Xenotime

Zircon - Zircon crystals up to 4 cm have been collected. Some are opaque, light brown tetragonal crystals, but rarely gemmy, transparent orange crystals.

Zircon, Mt Malosa
Zircon from Mt Malosa

References:

Bloomfield, K. 1965. The geology of the Zomba area. Geological Survey of Malawi bulletin 16.
Cairncross, B. 2000. Mineralogie des pegmatites du mont Malosa, district de Zomba, , Le Regne Mineral 35:27-37.
Cairncross, B., K. Messner, and E. Farquharson. 1999. Die pegmatite des Mount Malosa bei Zomba, . Lapis 24 (4): 22-32.
Eby, G. N., M. Roden-Tice, H. L. Krueger, W. Ewing, E. H. Faxon, and A. R. Woolley. 1995. Geochronology and cooling history of the northern part of the Chilwa alkaline province, . Journal of African Earth Sciences 20:275-88.

Author: Robin, Ed and Luke
Categories: Collection Acquisitions , News & Information