The Molly Marie IOCG deposit was formed when much of what is now southern Arizona was covered in deep brine lakes (see Setttings page). The largest producing gold mine in Arizona (500,000 oz.) was the Copperstone Mine, located north of Quartzite, AZ, and it was an IOCG deposit also formed in the Mid-Tertiary.
The Molly Marie is the Magmatic-hydrothermal type of IOCG shown on the left side of the diagram below (the missing piece is the brine lake which will be shown below).
The diagram is from the paper “Magnetic Survey for Iron-Oxide-Copper-Gold (IOCG) and Alkali Calcic Alteration Signatures in Gadarwara, M.P, India: Implications on Copper Metallogeny” byand
The following link is to the paper:
The brine lake that accompanies this deposit is not shown, but is shown in the diagram below.
The Molly Marie ore was derived in a similar fashion as the left side of the diagram and was facilitated by surface brines originating from extensive brine lakes.
Below is a diagram of the Molly Marie Prospect Ore Genesis Model created by the author (years ago 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 the carapace of a porphyry) that was available to be leached by the brine (see photo of porphyry below). As plainly evidenced by the remnants of the volcano’s slope on the east side of the caldera , the volcanic debris could have been as deep as 3000′ near the vent.
An important feature that is also present where the ore fluids “leaked” though the basalt and deposited on top on the Molly Marie Prospect beneath the brine lake are hydrothermal mounds as shown in the diagram below. As shown on the Introduction page, there are 2 fully intact large mounds in Area 3 (Areas are described below), and many, many more smaller ones which altogether would constitute a FIELD of mounds. It is highly likely that there is a very large quantity of ore beneath these mounds. The mounds shown below are what is typical for a VMS deposit, and the mounds on the prospect are similar, but with much more magnetite. As stated on the introduction page, there is now evidence from deep sea mounds that there can be 5x the volume of the mound in ore below the seafloor.
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. In the Goldfield district there 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 the visual below, where 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 oxide-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 bonanza 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 at 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.
The author spoke with Mr. Wilburn about the Mammoth and The Old Wasp Mine. He said 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 features as that of Goldfield, but the Molly Marie caldera has a much greater volume of breccias than Goldfield, the typical host of IOCG’s, and a FIELD of hydrothermal silica mounds.
There are 3 major Areas of the IOCG on the Molly Marie Prospect and all are interconnected by hydrothermally altered basalt on the surface. The great permeability of the Whitetail formation and the high amount of limestone in the upper half (the upper is 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 may have formed beneath basalt that lies against a large intrusive in a skarn-like situation. There are many small diatremes and breccias and hydrothermal silica in this area, some being hematite breccias, and others high in silica with visible copper staining and significant blue apatite. There are many hydrothermal domes on the basalt between Areas 2 and 3.
- Area 3 – This area contains two large hydrothermal domes. There is significant copper mineralization near this hill. A portion of this area is a large maar diatreme . Most of area 3 is also beneath basalt. At the base of the north dome here is significant blue and yellow apatite.
- 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 similar to black smokers 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. 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. Nearby Cerro Negra, by the vent, a gold assay of .02 opt was gained.
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 across Area 3 , and shows the subsidence in a hill that is the largest hydrothermal mound. 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 hydrothermal mound. 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.
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 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 and described above. Migmatite can be seen.
On a rainy day, a good photo of the breccia of the tuff ring surrounding the maar was gained:
The basalt in Area 2 that is saturated by banded veinlets is particularly intriguing as these can be found in high-sulfidation deposits. Below is a cut piece of banded veinlet.
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 magnetic. There are gray varieties in the neck that are even more magnetic (unoxidized). It is believed this rock was ejected and formed the volcano’s slopes and the debris was the source of the gold for IOCG deposit.
Below is a photo of the striking arkose hosted jasperoid beds that outcrop on the west side of the Prospect in Area 2. Two samples from this jasperoid further to the east in returned assays of .02 ounces per ton.
Below is a diagram of the Jasperoid bed outcrops and suspected backfilled pits from Spanish and Mexican mining.
There are several very shallow modern age 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 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 rock wrested from the ground in Area 3.