The McClellan Mountain and Argentine Pass trails start very near each other and run parallel to each other for the first 3.5 miles. McClellan Mountain starts after a couple of the early switchbacks on Argentine Pass, just after a brown wooden sign. If you take the switchback and stay on the main trail you'll stay on Argentine Pass, but if you continue straight ahead on a lesser trail you'll start McClellan Mountain.
This first part of the trail is narrow in places, and rocky for the entire section. It follows right next to Leavenworth Creek, and in some places the creek may actually spill over into the trail. You pass some great campsites that are right next to the creek, as well. This area is one of the more challenging areas as you climb up and over rocky hills. Some optional rocks along the trail can make things even more challenging.
The trail soon opens up as you pass under the power lines. The hill is a bit steeper and rockier as you climb to meet with the Argentine Pass trail again. This is an important intersection. The Argentine Pass trail is a flat and easy dirt road. Continue over the road to the other side where McClellan Mountain continues up a rocky hill.
This portion of the trail is even tighter, with pine trees instead of aspen trees. The branches are thicker, and could mean a greater possibility of damage or pinstriping on your vehicle. The climb has some bigger rocks in it, and is steep in places.
On this side of the trail, there is a large network of trails that continue up the mountain. Many of the spurs dead-end at mining areas. When you get to intersections, continue straight through them if you want to work your way to the Santiago Mine.
The Santiago Mine is very big and very well-preserved. It was in service as recently as the 1980s, and many of the machines inside still move smoothly. It is definitely worth exploring, but take care to leave it as you found it, taking only pictures.
After visiting the mine, continue uphill and up the switchbacks. There is one major intersection at one of the higher switchbacks, with a trail that continues north from the turn. Continue up the switchback to get to the crest of McClellan Mountain.
The trail may look like it continues over the top of the mountain, but it doesn't! Make sure you see the trail in front of you before you continue. There is a bit of parking at the top, with some amazing views of Stevens Gulch below you. You can see the often busy hiking trail across the gulch that goes to Grays Peak and Torreys Peak. You will most likely see many people on it.
Continue back down to the intersection at the switchback, and if you wish you can do more exploring down this spur. At the very next intersection you can turn right to go up a short spur to an overlook, but there isn't much to see. Continue to follow the trail around Ganley Mountain and down switchbacks. This part of the trail is not as well-used and it could be tippy in spots from debris in the trail. The recession to the east (right) of the trail has the accurate name of Wide-Awake Gulch.
At the bottom of these switchbacks is ample parking at a mine that looks like it was in use very recently. Its name is unknown to us. You can see I-70 in the very near distance. This would be a great lunch spot.
Work your way back out and down to treeline. If you take lower switchbacks that lead you further south you can go down a steep hill of loose rock and end up at the Waldorf Mine site. This is a large, open area of tailings at another intersection with the Argentine Pass trail. You can take Argentine Pass as an easy way back out to the trailhead.