Tactites are a type of skarn.  They are multimineralic carbonate rocks resulting from contact metamorphism (alteration of country rocks by the heat of an igneous intrusion) and metasomatism (addition of elements to country rocks from magmatic or hydrothermal fluids).


The tactite shown below is from a classic locality along the York River near Bancroft, Ontario, Canada.  This is a marble having calcite (whitish-grayish - CaCO3), ferroan spinel (black - MgAl2O4 with Fe), periclase (MgO), and Mg-Ti borate minerals.


Tactite (8.9 cm across) from the York River Skarn (a.k.a. York River Tactite Zone), eastern side of York River, ~0.3 km north of Highway 28 (Highway 500), Dungannon Township, Bancroft District, Hastings County, southern Ontario, southeastern Canada (45 04 08 North, 77 43 57 West).




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