“MADOCITE”
Here’s a rock from Ontario having a mineral
combination you don’t see very often. This sample is from a
metamorphosed mafic dike that intrudes tremolite-talc schists hosted in
dolomitic marble. The metamorphism has converted the original mafic
igneous minerals into a mixture dominated by tourmaline
(black) and pyrite
(brassy gold). Some of the small bright white patches are talc.
Tourmaline-pyrite metamorphic rocks lack a formal name, so the term
“madocite” has been used by some geologists. Published
mineral analysis indicates that “madocite” contains tourmaline,
pyrite, tremolite, and some phlogopite mica.
Age of metamorphism - probably ~1.24 billion years, during the
Mesoproterozoic. Subsequent metamorphism occurred at ~1.1 billion years,
during assembly of the Rodinia supercontinent.
Initial metamorphic agent - probably emplacement of the nearby Deloro Granite.
Locality:
Canada Talc Limited mine at Madoc, south-central Hastings County, southeastern
Ontario, southeastern Canada.
“Madocite” (field of view ~2.7 cm
across) from the Precambrian of Ontario.
Most info. from:
Dunn (1998) - Formation of dolomite-hosted massive
talc at Madoc, Ontario. Geological Society of America Abstracts with
Programs 30(7): 229-230.