During the time of the Spanish Empire in the Americas, tons of silver were mined from Potosi in Bolivia starting in 1545 AD and brought to the Old World, either directly to Spain or to China by the Spanish Empire. IIRC the Spanish crown took (or tried to take) a 5th of this silver through the quinto real tax.
How important was this silver to financing the Hapsburg armies? Soon after the mind opened, the Spanish crown defaulted on its debts in 1557 and the Austrian and Spanish Hapsburgs split the Empire. Was the output of silver not enough to cover the Spanish debts 10 years after Potosi opened? And how much did this wealth finance the occupation of the Spanish Netherlands, and the wars against the Netherlands?
The short version is that while New World silver was important (I can't speak for Potosi mine specifically), it was also a double edged sword. Revenue from the sale of silver was the biggest source of variance in the Crown's finances, ranging between 40% of annual revenue in good years and almost none in bad years. Drelichman has written a few articles of 16th Century Spain, and concludes that Philip II's bankruptcies are 'excusable', since they occurred in years when revenue was unusually low (most of the variance coming from an unstable supply of silver) and expenses unusually high (usually from war, almost constantly in the Netherlands). In 1578, the Crown collected ~2 million ducats from the Indies, while in 1582, the number was more like half a million. The Crown wasn't great at keeping units of account, so these are rough conversions from escudos of 39 and florins of 20, respectively, but the Army of Flanders spent just under 3 million ducats in 1578 and 5 million in 1582. In the first year, New World silver covered 2/3rds of expenses, but in the second, just about 10%. While no one's turning down extra silver, the Spanish could not rely on the mines overseas to fund the war.
I banged out a post on Philip II's bankruptcies more generally, which you can read here.