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We ran your query by both Clark County and the City of North Las Vegas, and came up empty twice over. You wouldn’t want to quarry over there, as the Las Vegas area isn’t known for precious metal and its cement-like clay defies most attempts to dig to any substantial depth (for exceptions to the rule, check out the QoD Archives for 6/25/08, regarding the massive pits, visible more from the air than the street, off Spring Mountain Road and Durango Drive).
A North Las Vegas official, however, referred us to the United Pacific Railroad, for whom the location you asked about appears to serve as a tractor yard. What seems, as far as we can tell, to be an innocuous parking lot, has become increasingly steeped in mystery, however, and the deeper we've dug, the more caliche we've hit, so to speak. A Union Pacific representative gave us several thoroughly noncommittal quotes over the phone, then retracted the lot of them via subsequent email. So, until such time as LVA staffers (or readers) get a jones to drive up to Mesquite, the mystery of the empty Union Park lot is likely to remain unsolved.