ON THE ROAD TO TUCSON: FOUR PEAKS AMETHYST

Four Peaks at southern end of Mazatzal Mountains.  Public Domain photo.

The Mazatzal Mountains are located immediately northeast of the Phoenix metro area trending north to their mergence with the Mongollon Rim near the small community of Pine. The Rim is the transitional boundary between the Colorado Plateau (north) and the Basin and Range (south) physiographic provinces. Much of the Mazatzal landscape is protected as the Mazatzal Wilderness Area although there is major access via AZ 87 (Beeline Highway) from Fountain Hills to Payson.  More rugged access via forest roads and mine roads may be found to the northeast of the Cave Creek area where I have briefly explored.  The south end of the Mazatzals is buttressed by Four Peaks, a mountain visible to much of the Phoenix area, especially the eastern metro area.  During my many camping trips to Lost Dutchman State Park, north of Apache Junction (far eastern metro Phoenix), I note that Four Peaks is quite visible and is the first local area to be covered by periodic winter snow.  The only named peak of Four Peaks is Brown’s Peak at 7644 feet.  I once had a hankering to hike the peak but quit midway because the upcoming more technical terrain sort of frightened me (since I did not have a hiking companion).  As I am fond of saying, “my momma didn’t raise no fools!”

 

Mazatzal Highlands as seen from northeast of Cave Creek, Arizona.

The geology of Four Peaks is an interesting blend of Precambrian rocks—Lucchitta (2001) noted that the Peaks “owe both their height and distinctive shape to roof pendants of Proterozoic-age [Precambrian] Mazatzal Peak Quartzite and Maverick Shale [Mazatzal Group] engulfed by 1500 million-year old granite.  Roof pendants are remnants of the country rock (the surrounding or preexisting rock) into which an igneous pluton is intruded. They occur at the top, or roof, of the intrusive mass.”  The Mazatzal Quartzite was cooked by the granite intrusion, silica was added, the rock “hardened” and therefore, forms a rough and erosion-resistant peak.  Although the quartzite is now a metamorphic rock, it (and other Group members) originally was deposited (~1.70 Ga) as sand and silt/clay along the trailing edge of a continent. Sometime ~1.65-1.60 Ga a plate collision of the proto-North American continent with another plate (probably a volcanic arc), accreted the trailing edge sediments to the continent.  The intrusion of the granite followed.

 

Diagram showing accretionary rocks of Mazatzal Orogeny.  Diagram courtesy of University of Wisconsin, Green Bay at: www.uwgb.edu/dutchs/CrustMvtsPPT/Subduction%20Zones.ppt

Whitmeyer and Karlstrom (2007) published a fascinating article on the growth of the Precambrian North American continent and their accompanying figures will help non-geologists understand the technical jargon (necessary words in a professional journal).  They noted that the core of the North American continent was in place by about 2.0-1.8 Ga after plate collisions of several smaller Archean (early Precambrian) continents.  Along the southern margin of this early continent, a series of volcanic arcs and oceanic terranes were accreted by a numerous plate interactions, one of which was termed the Mazatzal Orogeny.  During this interaction rocks, ~1.70-1.65 Ga in age, were added to the continent during the1.65-1.60 Ga Mazatzal Orogeny.  Whitmeyer and Karlstrom (2007) likened the Mazatzal accretionary event to the modern, and ongoing, convergence between Australia and the Indonesian island arc. Later Tertiary faulting, volcanism, uplift, and erosion created the current Mazatzal Highlands.

OK, back to Four Peaks.  It turns out that the Mountain is home to some of the most beautiful amethyst found in the U.S.  Anthony and others (1995) stated the “gem-quality amethyst crystals of a rich, red-violet color rival the best Siberian material.”  The latter seems to be the standard for quality amethyst. 

 

Display of Four Peaks Amethyst displayed at 2014 Tucson Gem and Mineral Show.

There is speculation that collection of the crystals goes back several centuries.  Blair (1992) stated that “Spanish explorers who passed through the area during the 1500’s likely paused to collect specimens.  It has been said that a portion of the royal purple amethyst found in the crown jewels of Spain came from the Four Peaks deposit.”  If the explorers did collect during their foray into the Mazatzals then they had to experience a pretty tough hike up the mountain since the deposit is in a crevice near the top.  There also is no reference for “It has been said….”  I do know that some serious mining has taken place since at least the early 1900s.  The mine has gone through numerous owners and a local rock shop employee could not tell me about current ownership.  He did say that the owners use a helicopter to access the site, dropping off miners and bring back rough.  That method of transportation means the cost of faceting or specimen rough is quite pricey and out of my budget.  It also is tough to locate good pieces in local shops (local scuttlebutt is that most of the good pieces go to China for faceting).  The mine is on a patented mining claim and collecting is strictly prohibited for us peons.  The guy at the rock shop did tell me that some more wealthy collectors may get private tours via helicopter.  However, I can’t give readers a reference on that point, or on the China rumor! 

Lowell (1976) described the formation of the amethyst: "The amethyst of this deposit occur in linings of voids in faults of the Mazatzal quartzite. The voids were intermittently filled with hot liquid solutions from intrusions below. Successive stages of quartz deposition occurred, as evidenced by alternating concentric rings of colorless quartz, hematite and amethyst around the angular fragments." The color seems the product of trace manganese but Blair (1992) noted that some of the crystals may be “greened” by heat treatment to produce prasiolite!  My question is “why”?

This entire little story is due to some fabulous pieces of Four Peaks amethyst displayed at the Tucson Gem Show.  They caught my eye and I decided to follow up as I had seen pieces of rough in a few stores.  Maybe, just maybe, I can someday stumble upon a small piece within my budget. 

ADDENDUM:  On April 3rd I located these photos of the amethyst area in The Rockhound Record (v.72, no. 10, June 2013), the newsletter of the Mineralogical Society of Arizona, and I thank them for allowing authors to use newsletter postings with attribution.  The photos were take by Alan Korwin as he "helicoptered in" for specimen collecting.

REFERENCES CITED

Anthony, J.W., S.A. Williams, R.A. Bideaux and R.W. Grant, 1995, Mineralogy of Arizona (3^rd Edition): The University of Arizona, Tucson.

Blair, G., 1992, Rockhounding Arizona: The Globe Pequot Press, Guilford, CT.

Lowell, J., 1976, Mineralization of the Four Peaks Amethyst Deposit, Maricopa County, Arizona:  Mineralogical Record, March-April. 

Lucchitta, I., 2001, Hiking Arizona’s Geology: The Mountaineers Books, Seattle.

Whitmeyer, S.J. and K.E. Karlstrom, 2007, Tectonic Model for the Proterozoic Growth of North America: Geosphere, v.3, no.4.