I think I solved my mystery. I found a similar brooch on eBay with the exact markings. It is saying it's 1930s and crediting it to Artemio Navarrete. Here's what the description says:
Artemio Navarrete was a talented silversmith from Iguala who moved to Taxco to assist William Spratling in opening his Taller de Las Delicias. He taught and supervised many of the artists in the workshop, many who went on to their own successful careers.
Spratling's influence can be seen in many of Navarrete's designs.
A .940 silver bar pin with his AN hallmark is available.
Here's the a picture of the eBay brooch.