Antofagasta is a city in Chile in Region II de Antofagasta.
Take micro bus 129 out of town northward. Get out at the roundabout north of La Chimba neighborhood. The highway Ruta 1 goes underneath this roundabout and one can use the little bit of shoulder on the entrance to the highway to stop a car. Mind of a Hitchhiker stood here in early 2017 with a sign saying "Peaje" (which means "toll") to onto the highway. The toll booth is about 30 km away from here. Say yes to rides to Mejillones too because they will pass the toll.
South towards La Serena, Santiago, Valparaiso
Take micro 102 from the center (or the stadium, better) to "south/route 5" (ask to go to the forking (esp: tenedor) to "La Negra", the micro driver will understand). From this forking walk in direction La Negra, after the motorway junction (half km) you will find easily a car that goes to the mines. Ask to be brought to the carabineros station in La Negra (they will not make you problems like in Europe). In this place a lots of trucks stops, also to have a lunch in small bars along the motorway. Remember that if you want to go to Valparaiso you must go down from the truck much more before La Calera (forking), because there aren't petrol station before 50 km!!
Other useful info
Antofagasta is a very long and skinny city, stretching north to south. Truck drivers going to Mejillones from the south will drive through the city instead of via the bypass, so they are actually your best bet on being dropped off somewhere central. If you didn't discuss with your driver where exactly they are going and if it's central, you might have to walk 12 km to where you need to be. Make sure you're on the same page. Thanks to mostly Colombian immigrants and contrary to what most Chileans seem to think, there is lots of cheap-ish delicious food in Antofagasta!