The town Aggeneys is located some 65 kilometres west of Pofadder, on the N14 highway to Springbok, on the farm Aggeneys. Located immediately west of the town lies zinc/lead/copper/silver mine. Initiated in around 1976, Aggeneys town was established to facilitate the Black Mountain Mine, which is an underground base-metal operation that currently employs over 600 permanent staff.
The area is immediately surrounded by various mountain peaks and boasts some of the most diverse and complex geology in Southern Africa, offering some magnificent walking and hiking opportunities. Some of the richest copper, lead and zinc concentrations occur within the immediate area, which is obviously the main reason for Aggeneys’s existence in the first place, as Aggeneys, was established primarily for the purpose of mining.
Deposit Description
The zinc and lead deposits at Aggeneys are exploited in three main ore deposits (Black Mountain, Broken Hill and Gamsberg) within the Aggeneys District of the Northern Cape Province.
All three are hosted by the Bushmanland Group of the Namaqualand Metamorphic Complex, which in turn lies within the broader Namaqua Mobile Belt on the south-western and southern margin of the Kaapvaal/Kalahari Craton. This complex comprises an older metamorphic suite (locally a quartzo-feldspathic augen gneiss) dated at around 2000 to 1900 Ma, overlain by a 1700-1600 Ma supra-crustal succession. This latter succession comprises a lower intrusive to extrusive l euco-gneiss, the Hoogoor Suite, and an upper schist/quartzite succession, the Bushmanland Group. The latter is extensive, and is largely preserved as infolded enclaves or thrust slivers, and is relatively thin, generally less than 1000 m thick.
At Aggeneys the Bushmanland Group comprises a lower 80 m thick aluminous schist, the Namies Schist composed of quartz-muscovite-K feldspar, locally with up to 25% sillimanite and biotite. This is overlain by a 5 to 900 m thick white crystalline white to grey quartzite, followed by the up to 200 m thick Aggeneys Ore Formation and amphibolite, leucocratic grey gneiss and conglomerates.
The Aggeneys Ore Formation is strongly deformed and in general comprises a footwall schist of sillimanite-quartz-biotite with minor garnet. This is followed by a ferruginous garnet-quartzite composed of bands of garnet and magnetite in a matrix of quartz. This zone is in turn followed by an interval containing magnetite-quartzite (medium grained magnetite & quartz) and amphibole-magnetite (quartz, spessartine, magnetite, ortho-pyroxene, grunerite, cummingtonite & fayalite) with some bands of ferruginous garnet-quartzite. The lower massive sulphide falls within this interval. Above this amphibole-magnetite is the main massive sulphide body and sulphide-quartzite. This is in turn overlain by magnetite-quartzite and amphibole-quartzite, another zone of ferruginous garnet-quartzite and the hangingwall sillimanite-quartz schist.
There are rapid thickness changes within the orebody sequence in the mine, with for instance over a strike length of 30m within a stope, a magnetite-quartzite changing from 20m to 5m in thickness. Over the same interval the adjacent massive sulphide remains fairly constant at 3 to 4m in thickness, although it may vary by up to 1m.
The massive sulphides (>25% sulphide) generally contain <10% magnetite and range from banded to brecciated textures to heavy disseminations. Pyrrhotite and galena dominate, followed by sphalerite, chalcopyrite and pyrite. The gangue is predominantly quartz, with variable garnet and some barite.
References:
http://www.portergeo.com.au/database/mineinfo.asp?mineid=mn016
http://www.rockstone-research.com/index.php/en/geology-exploration/182-Porphyry-Copper-Deposits-in-the-Course-of-Time
https://en.wikipedia.org/wiki/Aggeneys
Your Tasks – the C.S.I. work!
In order to substantiate your visit and be able to claim this EarthCache a number of questions need to be answered and submitted to the cache developer, via e-mail, to “cincolcc [at] gmail.com”
1 – Have a look around you and comment, in your own words, what you think the surrounding hills are composed of. Are they igneous rocks or sedimentary? Why do you think they are what you say they are?
2 – If you traveled to GZ from Pofadder do you think the surrounding area is different to what you observed en route? If you traveled here from Springbok, comment on the differences that you observed.
3 – From GZ looking NORTH, how many mine headgears can you see?
4 – Describe any other interesting things you saw or experiences you had on site (you can add these to your log for others to enjoy too). Photos are always welcomed in the logs.
NOTE: You may log your visit prior to approval, but e-mail submissions that do not meet the above criteria will be deleted.
