From "https://www.fs.usda.gov/recarea/whitemountain/recarea/?recid=79485", a mineral collecting permit is required for this location and "

• No fee required for permit.
• Permits can be obtained at the mineral collecting site.
• Permits are valid for 1 day at one mineral collecting site.
• Please read and comply with the conditions stated on the permit. "
  

From the reference source (which I've also attached for future reference): "Lord Hill is a granite pegmatite that was mined commercially for feldspar at one time. Over 50 different minerals have been found at Lord Hill. The more common minerals found are varieties of feldspar, quartz, topaz, phenakite, and garnet". It appears to be open to public collecting.

From "http://cashandtreasures.wikifoundry.com/page/Maine", "General description:In the past, the Lord Hill quarries have produced massive pockets containing blue apatites, gemmy quartz crystals, autunite coating feldspar, beryl, topaz, and other minerals. Massive topaz crystals, some weighing over 50 pounds have been found here. Serious digging in the mine dumps could yield the mineral collector fine specimens.

Directions from the source reference says: "From the North:Follow Deer Hill Rd. (FR9; closed seasonally) 4.7 miles from NH 113 to the Horseshoe Pond Trailhead. A small parking area is located at a curve in the road. From Deer Hill Rd., the trail descends moderately past the Styles Grave, which is on the right of the trail, then enters a gravel road and turns right onto it. The trail follows this road, keeping straight at a junction in 100 yards. At 0.3 mi, just before the gravel road ends, the trail turns right. The trail is a gradual uphill climb to the intersection with the Conant Trail.Go left onto the Conant Trail and follow the trail for 0.3 mi to the Lord Hill Summit. At the summit go left for a great view of Horseshoe Pond. Take the Mine Trail to your right to the abandoned mine."

Also "From the South: From Maine Route 5 in Lovell, Maine, turn west onto West Lovell Road and cross the narrows at Kezar Lake continuing to the right for approximately 2.75 mi. Turn left onto Foxboro Road, staying right for 2.1 mi. At the junction with Horseshoe Pond Road (right) continue straight on the Mine Road approximately 0.5 mi to the trailhead. Park your vehicle then hike up the Mine Road approximately 1.0 mi to the mine."

Also: "GPS Coordinates:•Lord Hill Mineral Collecting Site Kiosk:44.224000, -70.953800•Horseshoe Pond Trailhead: 44.23583, -70.94445 •Mine Road Trailhead: 44.211352, -70.950889"