Race and Ethnicity in Latin America

What if the mainland Spaniards and the native population were never allowed to intermarry. How would this effect the caste system and social order. More importantly, how would the West (U.S. and Europe for example) would see Mexico. How would this also affect the economy of the region?

Well, you'd still have a ton of mestizo bastards; the lack of Spanish women would see to that. And sheer numbers suggest that these mestizos would likely gain positions of power (compare with the gens de couleur among the French in Saint-Domingue).

Limpieza de Sangre is a concept that dates back to the Reconquista, so that idea is also floating around already.

Several of the more pro-Spanish tribes (most famously, but by no means limited to the Tlaxcalteca) definitely used their ancestry to solidify their status, and of course descendants of pre-conquest nobility tended to have a status in the empire anyway (consider that both the descendants of Montezuma and the descendants of Sayri Tupac were granted Spanish titles of nobility). With smaller numbers of "Spaniards," this process will likely be even more significant.