This trailhead is used to access both Argentine Pass and Horseshoe Basin. Take U.S. 6 to the west end of Keystone. If you are driving east on U.S. 6, take the exit for Montezuma Road before U.S. 6 begins its climb to Loveland Pass. If you are driving west (from Loveland Pass), turn left into the River Run area of the Keystone Ski Resort. Take another quick left and drive a bit to get to the Montezuma Road.
Drive ~4.3 miles on the Montezuma road and turn left on the 260 (dirt) road. There is a large parking area at the start of this road. From the parking area, drive 4.5 miles along Peru Creek to the Argentine Pass trailhead at 11,050', just below the Shoe Basin Mine. The road is gated beyond the trailhead.
From the trailhead (Photo #1), walk up the road a bit and pass through the gate near the Shoe Mine - Photo #2. Hike approx. 1/3 mile and there may be a sign on the right for the Argentine Pass trail - continue up the road into Horseshoe Basin - Photo #3. Continue north above 11,800' (Photo #4) and then west when the road turns left near 12,000' - Photo #5. Reach a road junction near 12,080' (Photo #6) and stay right to continue higher on the main road - Photo #7.
Reach the end of the road at 12,300'. A few feet from the end, turn sharply left to find a trail that heads west - Photo #8. Briefly hike west on the thin trail and begin a traverse up a rocky slope - Photo #9. Continue south west (Photo #10) to reach a corner below 12,400' - Photo #11. Photo #12 looks back on the climb up from the road.
Taken from Argentine Pass, to the east, Photo #13 and Photo #14 shoe the remaining route to the summit. Walk up to 12,400' to see the summit and the slope leading up to the south ridge - Photo #15. The trail turns northwest and continues towards Grays Lake, a bit higher than this point. Leave the left side of the trail and hike directly west toward the broad shoulder below the south ridge - Photo #16. The route is fairly straightforward - continue west up the shoulder to reach the southern end of the upper south ridge - Photo #17. Reach a flat area near 13,100' and then continue up steeper terrain above 13,200'. Stay just right of the shoulder's left edge to find trail segments leading up to the crux of the route - a few hundred feet of steeper, rocky terrain below the ridge - Photo #18 and Photo #19. Continue up to reach the ridge left (south) of some steep rock.
Near the left edge of the shoulder, look for a thin trail that leads up through the rocks. At 13,300', the shoulder becomes steeper. Navigate through some small ledges below the ridge and reach the ridge crest at 13,800' - Photo #20. Turn right and follow the ridge (Photo #21) up to the summit - Photo #22.
The Southeast Slope holds good snow in the spring. From the summit, hike south back down onto the south ridge and look for a starting point off to the left. If the snow is good, you can ski to the unnamed lake below 12,800', and possibly down to Grays Lake or lower. Here's a ski trip report with more photos.
The road becomes much worse after the trailhead. Most of the hiking is without a trail.