Please Note: Mariquita Peak is on private land on both the east and west sides. The peak is not included in the holdings for the Cielo Vista Ranch. As best as we can tell, the west side of the peak likely belongs to the Trinchera Ranch, another vast holding, owned by Louis Bacon now and includes a conservation easement managed by the US Fish & Wildlife. This easement does not open the land to public access. The property owners still maintain all rights to who accesses their property. At the time of this writing, we have no information on the property owners on the east side of the peak. The two closest vehicle accesses to Mariquita are the Whiskey pass Road on the west and FR437 on the east. If you use the Whiskey Pass access, you will need permission from the Cielo Vista Ranch to drive back in there and will likely be charged a fee. While the road to the pass and the pass itself along with De Anza Peak are on their property, Mariquita is not, so they cannot technically grant permission to hike Mariquita.
The other access from the east is presented here as the "trailhead" of choice and utilizes the road to the Purgatoire Campground and FR437. This is only a "proposed" route and is only offered because it provides the closest vehicle access to the peak. While you will be on San Isabel National Forest land up to the summit of Mt. Maxwell, anything south of there is on private property. Climb/hike at your own risk if you choose not to contact the property owners and obtain permission.
From a vehicle park on FR437 at 11,700 feet approximately, hike west up the broad, prominent east ridge of Mt. Maxwell. The terrain will be a tundra/grass mix with embedded rock and intermittent areas of rock talus. As you hike higher, the ridge narrows and becomes almost all rock as you approach the summit of Maxwell. There is a national forest boundary marker at the summit. If you choose to head south along the ridge crest to Mariquita, you'll lose almost 300 feet elevation and will then regain nearly 400 ft. The ridge walk is almost all broken, rocky talus and hence slow. Class 2. From Maxwell to Mariquita and beyond, the west slopes below the ridge crest are impressive broken cliffs. The main ridge south from Mariquita continues rocky and slow to Pt.12,925, then finally relents to more tundra coverage the remainder of the distance to De Anza Peak. For a return route, it is advisable to return as you came, but it's possible to contour below the summit of Maxwell on the east side and avoid regaining some elevation, however, there are some short cliffs and plenty of rock to deal with.
If approaching from the Whiskey Pass Road on the Cielo Vista, from the vehicle park on the Whiskey Pass Road at 11,520 ft., you can continue walking on the old roadbed up past two switchbacks and through open trees to where the road comes against the slopes directly below the pass. Then ascend for a steep 1,000 feet on the best route you can find, avoiding minor rock bands and across talus & scree to the pass. Class 2+. From the pass, head north along the ridge crest. As you gain the soft rank 12er summit of 12,955, there will be some minor low cliffs to avoid and the ascent will be mostly rocky until you begin to level out on the south end of UN12,955. Walk on across the easy, dual summit, descend north to the next saddle and then continue to the summit of De Anza on mostly "thin" tundra with small embedded rock. Class 2. From the summit of De Anza, it's a little over another mile to Mariquita. It's easy tundra walking to Pt. 12,925, then becomes a rockier traverse from there to the summit of Mariquita, still Class 2. As with all the peaks in this area, wildlife is abundant.
An alternate way to climb De Anza from the vehicle park on the Whiskey Pass Road would be to head NNE up the broad, west shoulder/ridge of UN12,955. This route would avoid the talus and scree of the direct ascent to Whiskey Pass and offer more of an easier tundra ascent. Once on the main north-south ridge, continue as described above.