Tuesday, May 31, 2016

May Field Trip Report to Grandview Mine

May Field Trip Report

Our field trip for May was a collecting trip to the Grandview Mine up in the Mimbres/Black Range Mountains.  We headed east on 180 from our gathering point at Walmart, turned onto 152 at Bayard and to continued east the Hwy 61 junction where we turned south for several miles.  South of San Juan we turned onto Royal John Mine road crossed the Mimbres and started gaining altitude.  We remained on Royal John Mine until we entered the National Forest where we turned left onto Silver Gulch road.  Silver Gulch was a slow-go, mostly one lane, bumpy, dirt road.  It took us farther up the mountains and through some beautiful back country and into the pines.  After about a 2 hour drive we reached an old, log prospectors cabin and stopped to look around.  There was another cabin a bit farther down the road in better condition that we also visited.  Finally we reached the end of the road and the Grandview mine dumps.

The Grandview, appropriately named by the way, was actually discovered in the early 1900's, but had no recorded production until 1933.  Although it has been called a gold mine, the only production records I could find listed the ores as lead, zinc and copper.  The primary ore minerals were sphalerite, chalcopyrite, helvite and galena.  The gangue minerals included calcite, fluorite, garnet, epidote, pyrite, and quartz.

The actual mine was an underground adit with about 1000' of drift.  The ore was hosted in a late Ordovician limestone cut by the north striking Grandview fault.  The mineralizing fluids replaced the limestone along the fractured fault zone and created the ore body.  From the production 1933 to 1951 production records, the ore was 3.71% lead, 9.4% zinc, and .22% copper.

Our trip was focused on collecting some of the nice pyrite crystals located all around the tailing piles.  So everyone spread out along the road and/or down the hill with buckets and hammers in hand in quest of the shiny mineral.  Pyrite is iron disulfate, also called fool's gold, and is a very common mineral.  It is brassy yellow and commonly forms cubes often with striations on one side.  It is also brittle and breaks unevenly.  It didn't take long for the collecting to begin.  The tailing piles were glittering with pyrite sparkling in the sunshine.  The largest pieces I saw were maybe an inch on a side, but the nicest crystals were smaller and could be found loose on the tailings or as clusters in the waste rock.  Other minerals found included clear fluorite, sphalerite and some epidote.

By afternoon folks began the trek down the bumpy road down the mountain with buckets full of mineral treasures.  It was another beautiful day in the mountains and a fun rockhounding trip.