YMHC #8: Aspects of the occurrence of gold deposits in Minas Gerais in the 18th century by Q. I. Lopes 

In this issue, Quelen Ingrid Lopes (Postdoc at Universidade Federal de Santa Maria, Brazil – queleningrid@gmail.com) presents a study about 18th-century gold mining in Minas Gerais, in south-eastern Brazil.

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Aspects of the occurrence of gold deposits in Minas Gerais in the 18th century
by Quelen Ingrid Lopes 

In the 18th century, the area that today is known as the state of Minas Gerais, in south-eastern Brazil, became the setting for intensive gold exploration and production during the so-called Brazilian Gold Rush. Gold production in Minas Gerais was marked primarily by alluvial gold mining, wherein gold was extracted from riverbeds and riverbanks by means of wooden gold pans, which were used to separate gold from soil and gravel. Open-pit and underground mining also took place in the surrounding rocky hills, albeit at a smaller scale. By the end of the century, when gold was largely depleted, the region had been radically transformed economically, politically, and demographically, as the Gold Rush had attracted significant numbers of Europeans and had led to the enslavement of hundreds of thousands of Africans, who were forcibly transported to South America to work in mining. In what follows, I outline some aspects of the occurrence of gold deposits in Minas Gerais and their labour implications.

The geomorphological structure of Anticlinal de Mariana and gold mining
Minas Gerais “forms part of a vast plateau, whose most prominent feature is the Serra do Espinhaço,” an area rich in “horizontal deposits and separated by deeply eroded valleys,” as pointed out by Charles Boxerⁱ. The gold deposits of Minas Gerais occur in Quadrilátero Ferrífero (the Iron Quadrangle), a complex geomorphological structure composed of several groups of rock formations. Amongst these formations is Anticlinal de Mariana (the Mariana Anticline), a fractured and heavily eroded rocky megastructureⁱⁱ, , whose quartz veins host gold mineralisation. The area saw intensive gold exploration in the 18th and 19th centuries, with gold extracted in Passagem, Mata Cavalos, Morro de Santana, and Antônio Pereira. Collectively, these localities comprised the gold-mining complex known as série de minas de ouro (gold mine series), located between Ouro Preto and Mariana.

Figure 1: Geological-structural map of the Mariana Anticline. Source: Nascimento, Stênio T., Paulo de Tarso A. Castro, and Úrsula de Azevedo Ruchkys. “Geodiversidade de compartimentos geomorfológicos do anticlinal de Mariana, Minas Gerais.” Geociências 37:3 (2018), p. 497-504.

The geological formations—i.e. mountains, hills, and valleys—and the many rivers that characterise Anticlinal de Mariana provide, in consistence with the findings of 16th-century German mineralogist Georgius Agricola, favourable metallogenic conditions for the formation of gold depositsⁱⁱⁱ. The constant flow of rivers, as well as pluvial and fluvial floods, eroded, over the millennia, rock formations and relocated sediments away from primary deposits (rock matrix). As part of this process, fragments of rock structures containing quartz veins hosting gold mineralisation—ranging in size from rocks to gravel—were detached from primary deposits and were deposited in riverbeds and along banks, as well as on hillslopes. The final place of deposition of these sediments varied due to the speed and volume of the water that brought them there, thus creating very different secondary deposits, which, given that they were deposited by running water, are classified as alluvial deposits. This in turn gave rise to the term “alluvial gold,” but also to the notion of “auriferous gravel,” for gravel conglomerates in the area were rich in gold. The first explorers who arrived in Minas Gerais in the early 18th century, thus, primarily found alluvial gold, and later used the presence of auriferous gravel to identify areas where rich deposits could potentially be located. Gold exploration did not take long, but was done in a rudimentary way and with the simple technology that was available for this type of extraction in the 18th century.

Figure 2: Johann Moritz Rugendas, Washing gold ore near the Itacolomi mountain (1835), Wikimedia Accessed on 23 June 2022. Public Domain.

Labour, tools, and technology in 18th-century mining
The use of the gold pan, locally known as bateia, has historically been an important part of gold mining in Minas Gerais. The gold pan was widely used both in prospecting the potential of a given area and in separating gold from soil and gravel.

The constant expansion of the gold mining industry in 18th-century Minas Gerais led to a growth in the numbers of enslaved people transported from West Africa to South America, who were subsequently forced to extract gold. But gold mining was only one for the many economic activities that depended on the work of enslaved Africans. Hence, together with the influx to the region of hundreds of thousands of Europeans from both Europe and northern Brazil, these processes had a significant impact on the demography, economy, and political organisation of Minas Gerais. They had also a tremendous toll on the African societies from which these individuals were taken into slavery.

Gold mining in Minas Gerais took different forms depending on whether gold ore was extracted from primary or secondary deposits. Where alluvial gold occurred, the diversion of river beds to explore deeper deposits was common. But the entire region of Mariana is characterised by an extensive fluvial system, whose rivers, streams, gullies, waterfalls, and springs conferred a significant hydraulic potential to the extraction of gold. Hence, in an attempt to tap into this potential water was channelled by means of canals to the surrounding gold-bearing hills, where gold was extracted through horizontal or vertical accessways, known as “mine holes” (here an exemple of a “mine hole” in Mariana) and “ladder mines” (ways to access gold directly in the rock matrix, i.e. lateral or vertical excavations in auriferous hills).

One of the most important technologies used in Minas Gerais mining in the 18th century so-called “water rosary” namely a complex mechanism powered by a water wheel (nora) to lift pit water from the mines. The water wheel could also be used without the rosary in order to transfer water from one location to another, even though the movement of water was necessary to set the mechanism into motion. The wheel was also used to crush waste rock to reduce material and facilitate gold mining.

Figure 3: Ore crushing mill (2007). Source: Digital collection – Museu do Ouro/Ibram, Sabará/Minas Gerais, Photographer: Daniel Mansur. Accessed on 22 November 2022.

Final considerations
Gold—as well as diamond—mining in 18th-century Minas Gerais led to an immense displacement of enslaved Africans, who were forced to mine for gold in the strongly hierarchical society of the part of South America colonised by Portugal. The enslaved were exposed to climatic diseases and physical exhaustion from extracting alluvial gold (carried out in rivers, on riverbanks, and in their surroundings) and from working in open-pit and underground mines in the hills and mountains, where they faced the additional risk of being buried alive. Historical mining also had an impact on the landscape through the removal of hills and the numerous river diversions that facilitated gold extraction.

i Boxer, Charles Ralph. The golden age of Brazil, 1695-1750: growing pains of a colonial society. Berkeley: University of California Press, 1975, p. 36.

iii Agricola, Georgius. De re Metallica. Translated by Herbert Clark Hoover. New York: Dover Publications, 1950, pp. 30-33.