On 8 September 1546 Juan de Tolosa, leading a small force of Spaniards and Indian auxiliaries, made camp under a hill crowned by a peculiar semi-circular crest of bare rock. The place lay 150 miles north-north-east of Guadalajara. From the summit of the Cerro de la Bufa, as the Spaniards later called the hill, a group of Zacatecos Indians watched the strangers' activities. Tolosa in due course made friendly approaches to them, and the Indians, in appreciation of his good intentions, showed him stones which, on subsequent examination, were found to be rich in silver. And in this way, according to the traditional account, was the wealth of Zacatecas uncovered to the civilised world. How did Tolosa come to be there?

His arrival on the future site of Zacatecas proved to be the culmination of a movement of exploration and expansion in search of wealth that had started immediately after the conquest of Tenochtitlan. The search for a route to the East, and then the quest for the fabled Seven Cities of Cíbola, had led Spaniards westward and northward from central Mexico in the very early years of the settlement of New Spain. By 1528, Cortés' lieutenants and followers had explored large areas of land to the south of the Lerma–Santiago river system, in what is today the state of Michoacán. And in 1529 began the conquests to the north of the Santiago of Beltrán Nuño de Guzmán, traditionally the blackest of figures among Spanish conquerors of Mexico.