"Rock of the Month # 267, posted for September 2023" ---

MALACHITE

The copper carbonate malachite is a visually striking mineral, the bright green hues drawing the eye even in the absence of coarse crystals. Malachite is a monoclinic carbonate, formula Cu₂(OH)₂CO₃, containing some 72 wt.% CuO. Mohs hardness is 3 1/2 to 4, specific gravity 4.00 (calc). Lustre is velvet to silky in fibrous forms, earthy to dull, or adamantine to vitreous in rare crystals (Palache et al., 1951, pp.252-256). Malachite often forms by the oxidation of copper-rich precursors such as sulphide ore minerals (e.g., chalcopyrite, bornite), in the same fashion that goethite or limonite ("rust") form from iron-rich minerals. In the oxidized zone, malachite may be associated with copper oxides (e.g., cuprite and tenorite) or with native copper. Unlike the associated blue carbonate azurite, malachite seldom forms distinctive crystals, and is more commonly found as secondary crusts and masses. These are often curved, sometimes colour-banded, and, since the mineral is soft, readily shaped into attractive lapidary forms or larger decorative items.

ANCIENT HISTORICAL NOTE

Malachite has been known, and indeed worked for its copper content, for a very long time. Caley and Richards (1956) presented an annotated version of "On Stones", the dissertation of the writer Theophrastus, student of Aristotle, born circa 372 B.C. on the island of Lesbos. At that time, numerous minerals were recognized, if imperfectly. Thus chrysokolla referred to green Cu salts such as malachite and the silicate chrysocolla. Malachite was apparently a minor green pigment known to the Greeks (ibid., p.174). While in modern times some huge masses of decorative value have been found in the Urals of Russia, malachite was a Cu ore in the Sinai peninsula, exploited by the Egyptians as ore and as a decorative stone, with the larger masses known as false smaragdos (ibid., pp.101-102, 104-105). In a resurrected, updated historical manuscript, Boyle (2024, vol. 1, chapter 3) refers to many metals and minerals, including gold, silver and copper, as they were known in ancient times, including minerals such as malachite and native copper.

THE CENTRAL AFRICAN COPPERBELT OF ZAMBIA AND THE CONGO

A genetic model for some of the sediment-hosted, stratiform Cu deposits in the region notes the coarse to fine-grained, terrigenous and carbonate sediments of the late Proterozoic Shaba Supergroup. A cyclic, fault-controlled, fining-upward and shallowing-upward intracratonic setting is inferred, with the Cu and Co probably derived from basement rocks via hydrothermal fluids introduced along rift fractures in the Shaba basin (Lefebvre, 1989). As summarized by Master (1998), the Central African Copperbelt of Zambia and the Congo is hosted by the Neoproterozoic Katangan Sequence, age dated at circa 879-650 Ma. The older basement rocks are the Lufubu schists (early Proterozoic arc terrane with metavolcanics, intruded by 2000 Ma granodioritic plutons and, after the Ubendian collision, by 1800 Ma post-tectonic granites). In latest Precambrian time the strata were affected by first the 850 Ma Lusakan orogeny and then the 650-500 Ma Lufilian orogeny. The major mineralization probably occurred in early diagenesis, "with sulphide precipitation occurring via bacterial reduction of seawater sulphate" (Sweeney et al., 1991). Subsequent metamorphism and deformation in the Lufilian orogeny generated cross-cutting, high-temperature mineralized veins, but does not seem to have caused significant remobilization of sulphides. Selley et al. (2005) affirm that the Zambian section of the Central African Copperbelt accounts for 46% of the total production and reserves, and that these are the richest and largest sediment-hosted stratiform Cu deposits known.

L'ETOILE DU CONGO MINE

The specimen is from the L'Etoile du Congo (Star of the Congo, Kalukuluku) mine on the edge of Lubumbashi, the second city of the country. The mine lies in Katanga province, in the southeast corner of the Democratic Republic of Congo. During the long rule of the dictator Mobutu Sese Seko, during which the country was called Zaire, Katanga was known as Shaba province (1971-1997). Today Katanga itself is split into 4 administrative districts, with the mine located in Haut-Katanga. Modern exploration at L'Etoile du Congo commenced in 1907, leading to discoveries of subsequently-mined orebodies in 1911 and 1919. Mining has been intermittent in the past century, by state, corporate and artisanal actors. According to a Wikipedia page accessed in August 2023, "A 2008 assessment confirmed that there was 13.5 million tonnes of ore in the pit, graded 3.3% copper and 0.55% cobalt" (see the Wiki article here). These are remarkably high grades, far above values typical of porphyry deposits of Cu or magmatic sulphide deposits of Ni and Co. An oxide cap 50 m thick, developed upon sulphide ore, has been mined. At depth the ore horizon is complicated by folding and thrust faulting. The region of southeastern DRC hosts other rich deposits, such as Kamoto (Cu- Co), Dikulushi (Cu- Ag) and Musonoi (Cu- Co- Mn- U).

The mine has yielded a range of copper and cobalt minerals. As reported in the specific literature of fine mineral specimens, these include: malachite and chrysocolla (Moore, 2020); malachite (Sielecki, 2020); fine chrysocolla coatings on malachite (Moore, 2018); cornetite (Moore and Stefano, 2020); and native copper (Moore, 2022). The material that has come from the mine to mineral dealers in the past decade all seems to be from the oxide cap, sitting atop the primary sulphide ore at depth.

ROAN ANTELOPE: A STUDY IN CONTRASTS

The Roan Antelope deposits were discovered in Luanshya district in Northern Rhodesia in 1902 (the deposit area is now within the territory of Zambia). The deposits later formed the subject of a thesis submitted at the University of the Witwatersrand in 1951, by Johannes (Joe) Brummer, who became a well-known and respected exploration geologist. The Roan Antelope ore deposits occur in a tightly folded basin of Katanga sediments, completely surrounded by granites and metasediments of the basement complex. Mineralization is confined to an argillite horizon circa 6-27 metres (20-90 feet) thick, of which approximately the middle third contains economic values. These deposts are dominated by sulphides (pyrite, chalcopyrite, bornite and chalcocite). The host argillite is a recrystallized siltstone, in which the sulphides show marked preference for the coarser, sandy dolomitic partings. The ore components are thought to have been derived by erosion of the basement complex. Gangue minerals include micas and quartz with lesser carbonates and sulphides, with tourmaline as the main accessory, plus trace apatite, zircon and microcline K-feldspar. Supergene enrichment is limited in the Roan Antelope Cu deposits (Brummer, 1955).

A SOBERING HISTORICAL PRÉCIS

Given the concerns in Africa in the past two decades over child labour, conflict minerals (e.g., "blood diamonds" and "coltan") and the flow of resource wealth to unaccountable armed groups, it is impossible to avoid mention of Congo's tortuous past. The post-colonial history has been disappointing, to say the least, yet the bandit origins of Congo / Zaire are simply appalling. Hochschild (1998) provides a detailed, often harrowing review of the remote despotism of one man who fashioned Belgian Congo, without ever visiting, in search of plunder beyond belief. King Leopold II of Belgium (1835-1909), who ascended the Belgian throne in 1865, essentially held the country and its people as a personal fiefdom (not a true Belgian colony). Millions died under his sway, in a brutal period that was at its worst in 1890-1910. Towards the end, at Leopold's command, Congolese archives in Belgium and Congo were burned in August 1908, before the handover of Congo to Belgium: the tyrant did not want his archives to be seen. 50 years after the original rubber tyranny came to an end, in 1960, Congo was granted independence. The short elected term (and life) of Patrice Lumumba was soon followed by the rise of a native despot, Mobutu Sese Seko, whose legendary corruption milked the country for another 32 years, 1965-1997. Much more could be said, but the Congo is perhaps the classic case of the "resource curse", wherein resources (timber, rubber, ivory and minerals) benefitted a few, while local people and the environment paid a terrible price. Given the recent history of the northeastern DRC since the Rwandan genocide, it is clear that sustained vigilance is required to prevent the exploitation of local people and the illicit export of ores of cobalt and other metals. L'Etoile mine is a hopeful sign, in the peaceful southeastern DRC. The mine has been operated since 2003 by Chemaf, a professional and legally accountable mining and metallurgical company producing copper and cobalt in the DRC (the DRC government has a 5% stake).

REFERENCES

Boyle, R.W. (2024) A History of Geochemistry and Cosmochemistry. Prehistory to the end of the Classical Period. Cambridge Scholars Publishing, Newcastle upon Tyne, England (Wilson, G.C., Butt, C.R.M., Garrett, R.G. and Robinson, H.A., editors), circa 600pp., chapters 2 and 3B, in press.

Brummer,JJ (1955) The geology of the Roan Antelope orebody. TIMM 64 part 6, 257-318.

Caley,ER and Richards,JFC (1956) Theophrastus on Stones. Ohio State University, Columbus, OH, 238pp.

Hochschild,A (1998) King Leopold's Ghost: A Story of Greed, Terror and Heroism in Colonial Africa. Mariner Books / Houghton Mifflin Company, Boston, 367pp.

Lefebvre,J-J (1989) Depositional environment of copper-cobalt mineralization in the Katangan sediments of southeast Shaba, Zaire. In `Sediment-Hosted Stratiform Copper Deposits' (Boyle,RW, Brown,AC, Jefferson,CW, Jowett,EC and Kirkham,RV editors), GAC Spec.Pap. 36, 710pp., 401-426.

Master,S (1998) A review of the world-class Katangan metallogenic province and the Central African copperbelt: tectonic setting, fluid evolution, metal sources, and timing of mineralization. GAC/MAC Abs. 23, 120, Quebec.

Moore,TP (2018) Munich show 2017. Mineralogical Record 49 no.2, 333-350, March.

Moore,TP (2020) Tucson show 2020. Mineralogical Record 51 no.3, 487-521, May.

Moore,TP (2022) Munich show 2021. Mineralogical Record 53 no.1, 165-177, January.

Moore,TP and Stefano,CJ (editors) (2020) Mineral Collections in Texas, III. Mineralogical Record 51 no.6, supplement, 286pp., November.

Palache,C, Berman,H and Frondel,C (1951) The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana. Volume 2, Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, Etc. John Wiley and Sons, Inc., New York, 7th edition, revised and enlarged, 1124pp.

Selley,D, Broughton,D, Scott,R, Hitzman,M, Bull,S, Large,R, McGoldrick,P, Croaker,M, Pollington,N and Barra,F (2005) A new look at the geology of the Zambian copperbelt. In "Economic Geology One Hundredth Anniversary Volume" (Hedenquist,JW, Thompson,JFH, Goldfarb,RJ and Richards,JP editors), SEG, Littleton, CO, 965-1000.

Sielecki,R (organizer) (2020) Mineral Collections in Australia. Mineralogical Record 51 no.5, supplement, 128pp., September.

Sweeney,MA, Binda,PL and Vaughan,DJ (1991) Genesis of the ores of the Zambian Copperbelt. Ore Geology Reviews 6, 51-76