The Molly Marie IOCG deposit was formed when much of what is now southern Arizona was covered in deep brine lakes. The largest producing gold mine in Arizona (500,000 oz.) was the Copperstone Mine north of Quartzite, AZ, and it was an IOCG deposit also formed in the Mid-Tertiary.
The IOCG diagram below is from the the Unite States Geologic Service at this address:
The Molly Marie ore was derived in a similar fashion due to surface brines from extensive brine lakes. The Molly Marie also has extensive exhalites and a fair amount of apatite in some areas.
Because the rhyolite porphyry magma chamber beneath the Molly Marie Prospect was located beneath or bordering deep brine lakes, the IOCG deposit on the Prospect is also very similar to a high-sulfidation VMS deposit. This type of deposit typically has high gold values. As described below, it is apparent that there were two volcanic events: one before the lakes, and a resurgent event concurrent with the lakes. Due to these factors, including an excellent geologic gold trap, this resulted in a much higher than normal gold content.
Below is a diagram of the Molly Marie Prospect Ore Genesis Model created by the author (before the above diagram was seen). What is believed to make the Molly Marie Prospect much richer than even a typical gold rich IOCG deposit is the great amount of metal rich volcanic debris (from a porphyry) that was available to be leached by the brine. As plainly evidenced by the remnants of the volcano’s slope surrounding the caldera , the volcanic debris could have been as deep as 3000′.
Below is a link to an excellent paper regarding IOCG deposits in North and South America.
Below is another in greater detail from the University of Arizona regarding IOCG deposits.
The nearby Goldfield mining district, approximately 2-1/2 miles away, was the result of the same basic ore-forming processes that took place at the Molly Marie. The Goldfield district is also located on a submarine collapse caldera located above a magma chamber. The Goldfield are had many small mines, and two of them (Mammoth, Old Wasp) were very notable for not only their documented bonanza gold grades, but because of their written description and visual below they appear to have been unrecognized as the upper extensions of an IOCG deposit. As shown in the photo below, all mines were located on the periphery of the caldera in or near iron-altered breccias.
The Mammoth Mine was over 1000 feet in depth.
In the booklet, “Goldfield Mining District Geology and Ore Deposits”, John Wilburn describes the mines there and some of the ore. The Mammoth, was the highest producing mine, largely due to the “Mormon Stope” that was found in 1892 after it was struck 35 feet below “Weekes Wash”. This was supergene gold ore that ran hundreds of ounces to the ton. The Mammoth reported production of over 50,000 ounces of gold, but considering the high grade and the great amount of theft that was documented at that time, the production could have easily been more than double that.
Excerpts from Mr. Wilburn’s description of the Mormon Stope:
“This ore body was located a the intersection of a cross fault within a high-angle sheeted or shear zone in arkose dipping west 83 degrees, 30 feet wide, and 200 feet in length.” and, ” The ore body extended from near the surface to the 200 foot level carrying hundreds of ounces of gold per ton. Considerable electrum occurred abundantly free as wire, dust and flakes in white to glassy crystalline quartz stained extensively by pyrolusite, hematite, and limonite derived from oxidized pyrites. No electrum occurred in the dacite dike; brecciated arkose on the contact hosted some of the richest ore”
Wilburn’s description of the Old Wasp ore (found in 1983 with a backhoe) is as follows:
“The ore shoot was 8 feet wide and 50 feet in length. On the east footwall, free gold occurred with galena, anglesite, malachite, chrysocolla, and hematite in an extremely heavy ore 12 inches wide that assayed 244 opt gold and 56 opt silver. The sulfide ore was of limited size. At 35 feet in depth the ore values diminished. An old drift underground struck the ore shoot on the 1,022 foot level where gold values averaged 0.66 opt along 40 feet of drift. ” Author’s note: the 1022 foot level was that of the nearby Mammoth Mine. Wilburn also describes all of the ore in Goldfield as being heavy with “drusy quartz”. Drusy quartz are crystals that cannot be seen with the naked eye.
Mr. Wilburn was spoken with about the Mammoth and The Old Wasp Mine. He had held some of the Old Wasp ore in his hands, and he stated that the Mammoth ore had significant galena also.
Wilburn stated that at the lower depths of the Mammoth considerable chalcopyrite was encountered.
Here are some more excerpts for other mines from “Goldfield Mining District, Geology and Ore Deposits that indicates an IOCG deposit:
- Bull Dog Mine – “High-grade ore in the mine assayed 250 ounces of gold to the ton. The ore contained no sulfides, only pyrolusite, but at 105 feet in depth, pyrite was abundant.”
- Black Queen Mine- “no sulfides occur in the ore, but red hematite after pyrites”, “High-grade ore carried hundreds of ounces of gold to the ton”,
- Mammoth Mine – the ore shoot on the 1,022 foot level where gold values averaged .66 opt Au along 40 feet of drift.
- Bluebird Mine – “Pyrolusite and red hematite stain the quartz and wallrock.”, “assayed 10 opt gold.”
- Mammoth 2 Mine – the crystalline quartz veins are much stained by pyrolusite and hematite.”
- Tom Thumb Mine – “quartz bearing electrum much stained by hematite and pyrolusite”
- Fair Stake Mine – “quartz contained red hematite pseudomorphs bearing electrum”, “10 inches wide assaying 3.33 opt gold”.
- Palmer Mine – “Ore occured in brecciated silicified dacite containing electrum, chrysocolla, and specular hematite”.
The Molly Marie Prospect has similar, but features as Goldfield, butThe Molly Marie caldera has a much greater volume of breccias than Goldfield, the typical host of IOCG’s.
There 3 major Areas of the IOCG on the Molly Marie Prospect and all are interconnected by hydrothermally altered basalt at the surface. The great permeability of the Whitetail formation and the high amount of limestone in the upper half (250 feet thick) gives great potential that the entire underside of the basalt is mineralized :
- Area 1 – The bulk of this area is extensive phreatic arkose breccias
- Area 2 – This area contains “Cerro Negra” descibed on other pages within that is a large “dome” in the basalt. A portion that formed beneath basalt that lies against a large intrusive in a skarn situation, there are many small diatremes and breccias in this area, some being hematite breccias, and others high in silica with visible copper staining.
- Area 3 – This area containd a 2nd hill similar to Cerro Negra in that it is a dome in the basalt also, but the basalt is altered by heat. There is significant copper mineralization near this hill. A portion of this area is large maar diatreme . Most of area 3 is also beneath basalt.
- Note: the IOCG is believed to extend beneath and between all areas.
Below is a diagram to aid in describing the sections of the IOCG deposit on the prospect by the Area that they are found.
Area 1, on the edge of the collapse caldera, is comprised wholly of massive phreatic breccias . Below is the geologic model of ore genesis in Area of the Prospect:
The below gossan is found with frequency on the ridges in Area 1. It is believed to be gossan from “vents” from a much larger IOGS system below. The actual vents are believed to have mostly been excavated and filled back in as indicated by seismic survey. This material is very friable and was not transported far. Note the heavy hematite.
The centerpiece of Area 2 is a large bulge in the basalt and a breccia pipe that is the black hill shown below and next to a large rhyolite intrusive(volcanic neck) that is behind and to the left of the hill. This hill was dubbed “Cerro Negra” by the author. There is a very large outcrop hundreds of feet wide of hematite breccias near the top of the west side of the hill that betrays what lies beneath. Rings of different alteration zones surround this hill. The photo is looking SE, and on the east side of the hill is a large subsidence zone from 19th century mining (See the Great Mine page).
Below are photos of the massive hematite breccia. This assayed over 100 ppm Lithium, and even though it has been thoroughly leached by chlorides, gossan nearby ran 158 ppm Cu and 155 ppm Zn.
This is a sawn piece of the hematite breccia, from one of the large outcrops near the top of Cerro Negra on the western side:
On the southern toe of Cerro Negra, a large outcrop of the hematite gossan breccia shown in the photo below is exposed in a wash. When sawn, copper mineral staining can be seen. Note the high amount of silica.
The photo below is looking SSW, and shows the subsidence in a hill in Area 3 that is a bulge in the basalt similar to Cerro Negra. Because of the magnetite alteration on and around this hill, especially in the wash on the north side, and the abundance of chrysocolla and apatite, it is believed the subsidence is caused by the oxidation of the deposit or collapse or by mining of the IOCG system. The subsidence zone also has a great amount of brecciation. The hill, shown below with the subsidence crater is on the southern edge of a maar diatreme. In the wash in the foreground of the subsidence zone, the basalt has been replaced by glassy jasper and milled breccias as shown on the alteration page. The glassy material is highly magnetic; the basalt is more magnetite-altered on the bottom and magnetism decreases as the basalt is tested higher in elevation.
Below is a photo of the gossan called out in the photo above. It has heavy boxwork structure indicating pyrite.
On the eastern side of the hill with the subsidence zone above, there is a large zone of limonite exposed in the wash and is shown in the photo below.
Additionally, as also described on the Peralta-Fish Map page, there is great potential for an IOCG with large vertical large extent in the maar diatreme in Area 3.
This photo shows the striking rings of hematite in the maar diatreme. This hematitic rock also has a high Lithium content and is magnetic.
This is a photo of breccia from the tuff ring of the maar diatreme on the northern end of the prospect. Migmatite can be seen.
On a rainy day, a good photo of the breccia of the tuff ring surrounding the maar was gained:
Doctor Michael Sheridan, former geology professor at ASU, postulated that the large quantity and richness of the gold at Goldfield was due to the re-mobilization of gold from ancient placers in the Whitetail formation. Since placers are unknown in the Whitetail formation, and a gold assay of .02 opt Au was gained from a chip sample of the contact zone of the nearby the volcanic neck(Rhyolite porphyry) of the Molly Marie caldera, a different process is proposed. It is reasoned that there were 3 major stages to the formation of the bonanza grade ore:
1) Dry Phase. A rhyolitic volcanic field was formed during the beginning of regional rifting. Note: this region is on the southern end of the Basin and Range Province. Above the magma chamber(s) a high sulfidation gold deposit formed (not IOCG). Vuggy silica is prevalent in this type of deposit, and is the bulk of massive lower-grade gold ore. The gold originates from the magma chamber. Extensive banded veinlets are found at the Molly Marie, very similarly to the banded veinlets found above the high sulfidation gold deposits of Maricunga, Peru (see below). See alteration page for more photos of the banded veinlets.
Below is a sawn piece of the oxidized Rhyolite porphry (wet) found at the volcanic neck. This is a true porphyry, enriched in metals and is a deep red in the field. It is very magnetic. There are gray varieties in the neck that are even more magnetic (unoxidized). It is believed this was the source of the gold for the high sulfidation gold deposit.
2) Wet Phase. During this phase, the volcanic field is now inundated by the brine lake(s) that have grown and now cover much of southern Arizona. The metals, including gold, are leached by brine from the regenerated volcanic debris comprised mostly of the same metal-enriched Rhyolite porphyry. The metal-enriched brine circulated to the boiling zone and the brine is boiled-off near the magma chamber. The metals and sulfur precipitate in the iron rich zones (above diagram). It is proposed that during this phase the brine also leaches gold from the vuggy silica of the dry phase, and becomes even more gold-enriched before it is precipitated at the boiling zone.
In area 2, there is ore just beneath the basalt (by proving previous mining as shown on this site). The reason for this is two-fold: The basalt acted as an aquiclude to rising fluids, and the upper portion of the Whitetail formation has a high percentage of limestone cobbles, sand, and boulders. This resulted in a hybrid IOCG-skarn-replacement style ore.
3) Supergene Phase. This phase is slow and occurred after the lakes receded. Meteoric water and residual chlorides leached the metals from the hypogene ore, and deposited them at the water table. Because of the water that is available to facilitate the process, these bonanza-grade deposits most often occur near or below washes that cut the orebodies, as they did in Goldfield.
The picture below is of one outcrops of the aquifer(arkose)-hosted jasperoid (hematite-silica) beds. Two assays of chip samples of the jasperoid beds returned values of .02 opt Au. The jasperoid beds are up-dip of the IOCG deposit and are on the west side of the caldera. These beds indicate the caldera is resurgent. The first magmatic event created the jasperoid beds shown here when they were protected from erosion in a “dry” event before the formation of brine lakes. Then, a second magmatic event sheared the outer edges of the caldera off and pushed them in the air where they reside today. Not too far away from here is granite, where the Whitetail formation and the basalt were eroded away while this portion was protected.
The google earth photo below is eastward-looking and shows the location of many of the Jasperoid outcrops in Area 1. The beds dip to the east. On the ridges above the projected origin of the beds can be found abundant hornblende, marble, and marble replace by silica. It is here, beneath these ridges, it is believed there is a very large IOCG deposit because of the porosity of the breccias (which are normally the host of IOCG deposits) and the indication of several previously mined pits. Most of the seismically tested and potential pits are rimmed with black chlorite.
There are several very shallow prospect pits in the basalt areas in Areas 2 and 3, and between them that apparently were dug to follow Chrysocolla. There is still some Chrysocolla to be found in the basalt and the arkose breccias, and its origin has been a mystery. It is now believed that sulfide debris from the underwater IOCG vents deposits once covered the basalt, and the copper was leached out and precipitated in the top 5 to 10 feet of basalt. The sulfide debris was eroded away, leaving the Chrysocolla in the basalt behind. Below is an untouched Chrysocolla vein in Area 3.
Below is a photo of another piece of Chrysocolla-saturated basalt wrested from the ground in Area 3.