It looks to me like one of the blues or coppers, from the family Lycaenidae. my first guess would be the Lustrous Copper Lycaena cuprea, but it also could be the Lupine Blue Icaricia lupini, but it also could be one of several others. You haven't got a side view of the butterfly have you?
My first thought was also that it was one of the Skippers Hesperiidae. We have a Skipper her in Australia, that has similar markings, the Ornate Skipper Hesperilla ornata I have only seen one once, when it visited our garden and fed on the onion flowers. Here it is:
None of the American skippers seem to have underwings with black spots on a white background, so I concluded that yours was one of the Blues or Coppers and the hint of copper colouring seen inside the edge of the front wing led me to suspect that it is a copper.