The Coon Bluff IOCG Deposit

If Southern Arizona was covered by brine lakes during the mid-Tertiary Period and massive rifting was taking place at the same time, are there more IOCG deposits besides those in the Molly Marie Prospect / Goldfield area?


Ever since the Molly Marie Prospect was known to be located on an IOCG deposit, there has been a nagging suspicion that Red Mountain (Mount McDowell) is the site of another large IOCG. It’s brilliant red coloration, on some days brighter than others, seems to deserve more of an explanation than just sedimentary “redbeds”.  The rock above an iron rich intrusive can be a bright red hematite saturated rock as well, as IOCG deposits are known for.  Red Mountain can be seen driving through Mesa, Arizona on Highway 60 and from far into Gilbert looking straight down Power Road. It’s about 11 miles from the Molly Marie Prospect.
Red Mountain (Mount McDowell)

After doing some research on-line about Red Mountain, the following paper was found:


This is a link to the paper:

It was known that Red Mountain was located on the Pima Nation Reservation, so investigating it could be problematic. But, Mr. Skotnicki described “Coon Bluff” with similar geology on the south side of the Salt River four miles due east of Red Mountain; Coon Bluff is on Forest Service land and is easily accessible. Coon Bluff could be considered a “submerged” Red Mountain, analogous to an iceberg. Below is a snip of Coon Bluff from the latest 2018 USGS Granite Reef Quadrangle map.
Coon Bluff from the 7.5′ Granite Reef Quadrangle (brown area)

Coon Bluff has the dimensions of about 1 mile x 1.5 miles, it sticks out like a sore thumb on Google Earth, and I was anxious to check it out.  After work one day, I drove out to the north side of Coon Bluff via Coon Bluff road.  Almost at the end of the road, near the parking lot, outcrops of conglomerate could be seen up high on the ridge.  After a good climb, armed with a magnet, it was found that the outcrops were magnetic, and in some spots, intensely so.  Below is a photo of the outcrops of conglomerate. Up close, and even in the photo from a distance below, the conglomerate looked like dark brown, rusty iron, unlike Red Mountain.
Conglomerate on North side of Coon Bluff by Parking Lot

Below is a boulder of the conglomerate that has rolled down from above. Note the heavy manganese mineralization.
Boulder of Conglomerate with Manganese mineralization

On the following weekend, a hike was made starting from the far east end of Coon Bluff from a trailhead (can be seen in the Coon Bluff photo above). Right off the bat, an outcrop of breccia was observed near the trailhead, and the hike started off with a bang.

The hike proceeded along the ridgeline, mainly on a well-traveled trail that runs SE-NW to a point that is more than halfway across the bluff. The trek was terminated when the soles of my boots literally fell off because of the heat.  These were military desert boots, believe it or not. Below is typical hematite-saturated conglomerate and the soles. Just another one of the hazards of hiking in the desert during midday in July.
Typical Hematite Conglomerate and Boot Soles

I made it barely into Section 5 (shown in the map above), and could see down into the canyon to the south that cuts the bluff in half diagonally.  There was a dozer cut near the top of the ridge where a wide vein of Barite was exposed (about 4 to 5 feet wide). Below is a photo of the vein in the dozer cut.  Barite comes in a lot of forms, but it is very easy to identify when in large pieces. If a rock is light colored and is very dense, it is likely Barite.
Barite Vein, Exposed and Cleaned for Removal

Below is a cobble of Barite from the area. Note the crystals.
Barite Cobble

Other photos of the hike on the ridge are below. 

The next photo is believed to be from a dike and is comprised of what may underlie the entire Coon Bluff. It is a granite (granitoid?) porphyry that is highly magnetic.  Note the magnet stuck on the vertical face.
Granite Porphyry

Below is a photo of an outcrop of hematite-laden conglomerate at the top of Coon Bluff.  It appears to have a much higher percentage of hematite than the conglomerate at Red Mountain (and therefore, much softer). Note the granite cobbles scattered around the outcrop.
Conglomerate Outcrop laden with Hematite

Below are two photos of typical highly magnetic granite cobbles (and chlorite altered) on top of Coon Bluff: This alteration is the same as the magnetic granite halo that surrounds the Molly Marie Prospect.
Chlorite-altered Magnetic Granite.
More heavily Magnetic Granite

I could see into the canyon below to the south and some pits could be seen, but it would have to wait until another day.  Below is a photo from the ridge into the canyon.
The Valley with Pits from above

Mr. Skotnicki described some narrow veins of Barite in the deep canyon below.  He writes “At Coon Bluff an east-west-trending, south-dipping normal fault has been extensively mined. Deep shafts and pits, and quarries have been dug along the fault zone revealing excellent exposures of the fault plane. On the eastern section of the fault a dark manganese stain coats many of the clasts in the red beds and, locally, the fault plane itself. Calcite locally fills the fault plane as well. In the west-central part of Coon Bluff 1-15 cm-thick Barite veins are exposed in a gully and are parallel to and within this fault zone.”

The Barite vein exposed on the ridge was much wider than 1 to 15 cm wide.  And what were they mining below? Skotnicki doesn’t say.  Regardless, the description of Barite in the central portion of Coon Bluff as described by Mr. Skotnicki, and what was found in the dozer cut, was very exciting because the Olympic Dam IOCG mega-deposit has a central barren zone that is rich in Barite.

The next weekend, the hike began on the trailhead on the south side of Coon Bluff near the highway (can be seen in the photo of the USGS map above). There wasn’t any hematite exposed hiking in like on the south side of the bluff, but the conglomerate cobbles (mostly granite) were very magnetic and became more so as the hike progressed into the bluff.  The ridge on the south side of the bluff was breached and the valley below was entered. Like Mr. Skotnicki had reported, there were several Barite veins found in the valley.  Here are a few photos of them.
Barite Vein #1
Barite Vein #2
Barite Vein #3

Below is a sawn piece of Barite vein stained with hematite that came from one of the many mine dumps in the valley. Note the brecciation.

Hematite stained Barite

When I got home, much more research was done of the area.  It was learned that the entire valley with the pits (shown above) was a very large barite deposit comprised of many veins.

This paper was found:

RECONNAISSANCE GEOLOGY, Salt River – from Roosevelt Dam to Granite Reef Dam, Central Arizona by Robert B. Scarborough June 1981

The link to the paper is below:

This quote is from the paper:

“According to Brobst (1969) and Stewart and Pfister (1960), about 312,000 tons of Barite were mined from properties at Coon Bluff between 1929 and 1955, under the names of Granite Reef, Arizona Barite, and Macco mines. This production makes this the largest barite producer in Arizona to date….The Barite, according to the above authors, has a small amount of associated azurite and malachite staining.”

Repeat……… the largest barite producer in Arizona

Note: To this day, it was the largest Barite producer in Arizona.

The claims were originally staked in 1897 for silver.

This paper describes Coon Bluff as having dropped over 2000 feet on the west end, making it essentially a “sunken Red Mountain”

Since the paper above references “Stewart”, the following paper was found and downloaded.

BARITE DEPOSITS OF ARIZONA, By L. A. Stewart and A. J. Pfister, 1960

The link to the paper is here:

This paper is excellent and has a detailed description of the mines. Below is a quote from the “Barite Deposits of Arizona”:

“About 1952 the entire property was sold to the Macco Corp., which continued production until July 1955, when mining was suspended and the mill was dismantled. By the time the shaft had been sunk to 320 feet , mining was being conducted on the 300-foot level. The tenor of ore had decreased on this level, and when a heavy flow of water was encountered at the west end of the mine, operations were suspended.”

This is a map of the Granite Reef Mine from the “Barite Deposits of Arizona” below, note the many vein outcrops that are shown. It is astonishing that 2400 feet of strike length of the main vein was mined by shrink stoping via the headframe shown below.
The Granite Reef Claims, 1960

The implications of this major barite mine in the center of Coon Bluff are clear.  It is pretty conclusive that Coon Bluff is another IOCG deposit;  but this is the diatreme type. The IOCG found on Molly Marie prospect is the caldera type.  Mid-Tertiary IOCG deposits in Arizona are a reality, and the acceptance that there is the potential for many more could be the next “big one” for gold exploration.

A photo of the mill prior to the shutdown is below.
Granite Reef Mill

Below is a photo of the Granite Reef Mine headframe. A lot of tons were hoisted with this rickety structure.
Granite Reef Mine Headframe

Below is a photo of the shaft of the Granite Reef Mine reclaimed with a bat habitat protection structure over the top of it.
Granite Reef Mine Shaft

Below is an annotated Google Earth photo of Coon Bluff with the Barite deposit area outlined (this is conservative).  Considering the magnetism, alteration, hematite, copper, silver, and the Barite deposit, It is very clear that Coon Bluff is another large mid-Tertiary IOCG deposit in Arizona. Like Olympic Dam, the gold here will be outside of the barren Barite rich core and the top of the gold zone will be at a relatively shallow depth.
Coon Bluff with Barite zone in the center

Below is a photo of a sawn piece of select barite vein from the area.
Barite breccia