Charles Miller - fought with US Army at battle at Wounded Knee - this photographer took images during and after the Battle. Anyone recognize the insignia? He later became a Mason? Could it be from that organization?
It's not Masonic - it's the U.S. Army Marksman Button from the 1880's.
Most Masonic Jewelry will feature one or more of: a carpenter's square; compasses/dividers; and the letter 'G'. However jewelry for associated organization such as Shriners, York Rite and Scottish Rite will employ additional symbolism.