How did miners during the middle ages find metal ore deposits to mine?

Georgius Agricola wrote De Re Metallica in the mid-1500s, which is a detailed description of mining practices at the time.

The full text of De Re Metallica is available from Project Gutenberg:

http://www.gutenberg.org/ebooks/38015

From Book II:

The miner, after he has selected out of many places one particular spot adapted by Nature for mining, bestows much labour and attention on the veins. These have either been stripped bare of their covering by chance and thus lie exposed to our view, or lying deeply hidden and concealed they are found after close search; the latter is more usual, the former more rarely happens, and both of these occurrences must be explained. There is more than one force which can lay bare the veins unaided by the industry or toil of man; since either a torrent might strip off the surface, which happened in the case of the silver mines of Freiberg (concerning which I have written in Book I. of my work "De Veteribus et Novis Metallis"); or they may be exposed through the force of the wind, when it uproots and destroys the trees which have grown over the veins; or by the breaking away of the rocks; or by long-continued heavy rains tearing away the mountain; or by an earthquake; or by a lightning flash; or by a snowslide; or by the violence of the winds: "Of such a nature are the rocks hurled down from the mountains by the force of the winds aided by the ravages of time." Or the plough may uncover the veins, for Justin relates in his history that nuggets of gold had been turned up in Galicia by the plough; or this may occur through a fire in the forest, as Diodorus Siculus tells us happened in the silver mines in Spain; and that saying of Posidonius is appropriate enough: "The earth violently moved by the fires consuming the forest sends forth new products, namely, gold and silver." And indeed, Lucretius has explained the same thing more fully in the following lines: "Copper and gold and iron were discovered, and at the same time weighty silver and the substance of lead, when fire had burned up vast forests on the great hills, either by a discharge of heaven's lightning, or else because, when men were waging war with one another, forest fires had carried fire among the enemy in order to strike terror to them, or because, attracted by the goodness of the soil, they wished to clear rich fields and bring the country into pasture, or else to destroy wild beasts and enrich themselves with the game; for hunting with pitfalls and with fire came into use before the practice of enclosing the wood with toils and rousing the game with dogs. Whatever the fact is, from whatever cause the heat of flame had swallowed up the forests with a frightful crackling from their very roots, and had thoroughly baked the earth with fire, there would run from the boiling veins and collect into the hollows of the grounds a stream of silver and gold, as well as of copper and lead." But yet the poet considers that the veins are not laid bare in the first instance so much by this kind of fire, but rather that all mining had its origin in this. And lastly, some other force may by chance disclose the veins, for a horse, if this tale can be believed, disclosed the lead veins at Goslar by a blow from his hoof. By such methods as these does fortune disclose the veins to us.

But by skill we can also investigate hidden and concealed veins, by observing in the first place the bubbling waters of springs, which cannot be very far distant from the veins because the source of the water is from them; secondly, by examining the fragments of the veins which the torrents break off from the earth, for after a long time some of these fragments are again buried in the ground. Fragments of this kind lying about on the ground, if they are rubbed smooth, are a long distance from the veins, because the torrent, which broke them from the vein, polished them while it rolled them a long distance; but if they are fixed in the ground, or if they are rough, they are nearer to the veins. The soil also should be considered, for this is often the cause of veins being buried more or less deeply under the earth; in this case the fragments protrude more or less widely apart, and miners are wont to call the veins discovered in this manner "fragmenta."

Further, we search for the veins by observing the hoar-frosts, which whiten all herbage except that growing over the veins, because the veins emit a warm and dry exhalation which hinders the freezing of the moisture, for which reason such plants appear rather wet than whitened by the frost. This may be observed in all cold places before the grass has grown to its full size, as in the months of April and May; or when the late crop of hay, which is called the cordum, is cut with scythes in the month of September. Therefore in places where the grass has a dampness that is not congealed into frost, there is a vein beneath; also if the exhalation be excessively hot, the soil will produce only small and pale-coloured plants. Lastly, there are trees whose foliage in spring-time has a bluish or leaden tint, the upper branches more especially being tinged with black or with any other unnatural colour, the trunks cleft in two, and the branches black or discoloured. These phenomena are caused by the intensely hot and dry exhalations which do not spare even the roots, but scorching them, render the trees sickly; wherefore the wind will more frequently uproot trees of this kind than any others. Verily the veins do emit this exhalation. Therefore, in a place where there is a multitude of trees, if a long row of them at an unusual time lose their verdure and become black or discoloured, and frequently fall by the violence of the wind, beneath this spot there is a vein. Likewise along a course where a vein extends, there grows a certain herb or fungus which is absent from the adjacent space, or sometimes even from the neighbourhood of the veins. By these signs of Nature a vein can be discovered.

Agricola then goes on to discuss in quite a bit of detail the mystical practice of divining for veins, noting that "There are many great contentions between miners concerning the forked twig, for some say that it is of the greatest use in discovering veins, and others deny it."