Mexican Deco 980 silver "surrealist" mask Brooch

Possibly the most "surrealist" early Mexican Deco "mask" brooch I have ever had, this piece is the source of so many questions for me. What is it exactly that it depicts with its "portrait" surrounded by an elaborate headdress, the two arms emanating right from its top with no actual body present? And what is the "mask" itself made of? It registers as a stone in my gem-tester but I am not sure if it is onyx or a different type of opaque, black stone. I find myself in one of those instances in which I wish I had specialized in Mesoamerican art and history yet regardless of my shortcomings, this one is a fabulous find!

Size / Weight: 2 1/8" tall by 2 3/8" wide; 35.8 grams

Hallmarks / Date: "TAXCO 980"; 1930s-40s

Condition: excellent vintage with elaborate, layered repoussage, great patina and no damage or repairs; please note that the clasp is missing its roll-over part but the brooch stays firmly "pinned" in place when you have it on

Inv# 15896