Field Trip to Ghana - May 10-23, 1998
The New Mexico Institute of Mining and Technology (aka New Mexico Tech) Student Chapter participated in a field trip to Ghana in May of 1998. Drs. David Norman and Henry Appiah coordinated the trip, which was sponsored in part by SEG. The 11 participants included 7 students and 4 professionals. The trip included visits to paleoplacer and lode gold deposits, manganese, bauxite, and diamond mines. The trip was the final course activity in a class on the ore deposits of Ghana. The first week of the trip was spent in the Tarkwa region, and the students were guests at the U.S.T. School of Mines. The second week was spent in the area of Kumasi, and the final leg brought us to Korfuridua, near the Akwatia diamond mine. The lack of rains caused by El Nino and consequent hydroelectric power rationing (12 hours on, 24 off) made lodging and dining an adventure akin to impromptu camping. However, the tropical misadventures were taken in good humor.
Ore deposits are located predominantly in the Paleoproterozoic aged Birimian and Tarkwaian Group formations exposed in western and far northern Ghana. The Tarkwaian conglomerates lie unconformably over the Birimian metavolcanic and metavolcaniclastic schists. Comagmatic and syndeformational "Cape Coast" granitoids intruded the Birimain units. Metamorphism of both rock groups reached greenschist grade at about 2.1 Ga in a single event. Regionally, the mineralization is controlled by a NE-SW oriented megashear extending through both the Tarkwa-Prestea and Obuasi areas. The region is host to many gold paleoplacers and shear-zone lode-gold deposits. Diamonds are found dominantly in kimberlite-derived recent and paleoplacers.
The Tarkwaian rocks form prominent hills, and in the Tarkwa area the paleoplacers at the base of the unit occupy one or two principal strata. At each of the paleoplacer mines (the Teberebie Mine, Ghana Australian Goldfields-Iduapriem Mine and the Ghana Goldfields-Tarkwa Mine) the gold grades were highest in the coarsest, matrix supported, quartz pebble conglomerates. Underground operations were producing 7-8 g/t ore at the headframe and open pit operation grades were generally 1.5 g/t gold. The Tarkwa underground mine had just restarted production after a $120 million development and major maintenance project.
Gold mineralization in the Birimian schist is typically in quartz-sulfide veins with locally intense graphitization. At some of the lode gold mines (the Aboso Goldmines-Damang Mine, the Barnex-Prestea Mine, and the Bogosu Gold-Bogosu Mine), there are some ongoing efforts to understand the complex structures controlling mineralization. On a regional scale, the mines occur near the Birimian-Tarkwaian contact. In the Ashanti Goldfields-Obuasi Mine current operations are focused on the disseminated oxide and sulfide deposits which accompany the lode gold. A visit to the new Biox Plant was particularly interesting, with total recovery exceeding 80%.
Other highlights of the trip were the Ghana Manganese Company-Nsuta Mine, the Akwatia diamond mine, the Ghana Bauxite Company-Awaso Mine and the Bonte Goldmines-Bonte Mine. Dredging at the Bonte Mine turns up archeological finds as well as placer gold: small pieces of jewelry and weight standards from the Ashanti gold trade! The conveyor belt at the Awaso Mine brings ore down from so high up on the hill that it actually generates electricity for the mine.
We would like to thank the many mine employees who made this trip a success. We are indebted to the Ghana Bauxite Company for the kindness extended to us upon the breakdown of our vehicle. Thank you also to the folks at the Ghana Manganese Company and Ghana Goldfields for the use of their swimming pools and after hours hospitality. We especially appreciate the cooperation of the U.S.T. School of Mines and the efforts of Dr. Henry Appiah. This trip would not have been possible without the financial support of SEG, to whom we extend our gratitude.
The open pits at Iduapriem are more than a mile in total length. Gold is extracted from the Banket Conglomerate in the Lower Proterozoic Tarkwaian sequence. The better grades of ore occur in the matrices of the coarsest grained conglomerates. The source of the gold may be the Birimian greenstones which extend unconformably below the Tarkwaian sequence.
The leach heap at Ghana-Australian Goldfields in Tarkwa has some interesting features. The crushed material must be transported under cover in the rainy season. To prevent the agglomerate from compacting in the leach heap it is mixed with concrete. The concrete also prevents the build up of hydrogen cyanide gas by adjusting the pH in the leach heap.
Travel in Ghana can be difficult at times, but strangers are always ready to help out. Here the van is being towed by a logging truck on the lonely jungle road to Awaso. The passengers waited for a rescue vehicle, sitting on a giant log as dusk and the mosquitos approached. Passers-by stopped to ask for pen pals, and got them!
The bauxite mine in Awaso is located on a very high hill where weathering of the Lower Proterozoic Birimian schists is deep. The highest grade aluminum ores are located along deep fractures in the face of the open pit. The ore is crushed on the hill top, and conveyed directly to railroad cars one mile down the hill. The conveyor belt generates enough electricity to run the mine, quite an asset for the remote village of Awaso.
The diamond mine in Akwatia is a river bed dredging operation, however, a deeply weathered kimberlite seems to be the source of the diamonds. Here the kimberlite dike is exposed along a hill top. One of our graduate students used REE analyses to confirm the origin of this outcrop. Deep weathering (and commonly dense vegetation) makes it difficult to map in Ghana.