In the mid-seventeenth century Portugal reasserted its independence from Spain and reclaimed its empire, notably in Brazil. The discovery of rich gold mines there, at Minas Gerais ('General Mines') in 1692-4 boosted Portuguese wealth. Huge gold coins with the names of Portuguese kings were circulated across Europe. This inspired the first gold rush of the Americas. Over 2,000 slaves a year were forced from Africa to work the mines. The output of gold peaked around 1720, with enough to last 'for as long as the world shall endure', a contemporary noted. In a couple of decades the world's supplies may have been doubled. Great Britain in particular absorbed large quantities, following the Methuen Treaty of alliance with Portugal in 1703. The English fleet protected Brazilian trade, and England received complete freedom to trade with Brazil. As the trade balance was always in England's favour, it was paid in Brazilian gold.
The gold of Minas Gerais was minted into two series. One series (including the 12,000 and 24,000 reis) was intended to circulate in Brazil itself. They had the Portuguese arms on the front and the Cross of Christ on the back, with the initials of the mint, M for Minas Gerais, between the arms of the cross. Oddly, the value in reis (here 20,000) written on them is wrong, as the coin had been revalued to 24,000 several years earlier.