Northern Michigan University is a relatively sizable academic institute. Plenty of research and industrial equipment would be hosted there. Training and teaching technicians and electricians involves functional equipment to troubleshoot and maintain. Nothing on the scale of a leftover factory or storage depot, but definitely something worthwhile to small town like Marquette. Then you mentioned mining. 

[quote=wikipedia]Marquette continues to be a shipping port for hematite ores and, today, enriched iron ore pellets, from nearby mines and pelletizing plants. About 7.9 million gross tons of pelletized iron ore passed through Marquette's Presque Isle Harbor in 2005.[/quote]

It's certainly nothing like Detroit or Chicago, but it holds a lot of commodities and equipment generally outside the reach of nearby counties. I've been there myself. A whole lot of nothing separating Marquette from the villages surrounding it. Only place that could compete up there would be Sault St. Marie and Houghton/Hancock. Repair garages only get you so far before looting is necessary. Having localized mining infrastructure and equipment, plus the shipping equipment at the docks, gives Marquette an advantage. Put them up against an industrial region and they pale, but locally it dominates.

Titanic........I'll rek u bruv