I have revised my research idea to encompass the general history of yerba maté in colonial Latin America, rather than just in Argentina because I think that might have been too specific (I was struggling to find good sources). It seems to be something that was adopted by the colonizers and I think that will lead to interesting accounts and findings on the role of maté in colonial society.
I can not find the actual letter, but according to Adalberto López, in “The Economics of Yerba Mate in Seventeenth-Century South America”,
Alonso de la Madrid to Governor Hernandarias, Asuncion, 10 February 1596, cited by Aguirre, Diario, 2: pt. 2, p. 359.
had written a letter about mate (a written account of mate in colonial Latin America).
If you could give me some guidance on how to find primary sources that would be really helpful; I tried for a while with no luck.
Folch, Christine. “Stimulating Consumption: Yerba Mate Myths, Markets, and Meanings from Conquest to Present.” Comparative Studies in Society and History52, no. 1 (2009): 6–36. https://doi.org/10.1017/s0010417509990314.
Joyce, T. A. “Use and Origin of Yerba Maté.” Nature134, no. 3394 (1934): 760–62. https://doi.org/10.1038/134760a0.
López, Adalberto. “The Economics of Yerba Mate in Seventeenth-Century South America.” Agricultural History 48, no. 4 (1974): 493-509. http://www.jstor.org/stable/3741386.
Rodríguez‐Alegría, Enrique. “Eating Like an Indian.” Current Anthropology46, no. 4 (2005): 551–73. https://doi.org/10.1086/431526.