Below is the geologic model of the Olympic Dam IOCG deposit. Although not as large as Olympic Dam deposit, the Molly Marie formed similarly. One feature of the Olympic Dam deposit that the Molly Marie does not have is high amounts of uranium; the porphyry at the volcanic neck assays only about 1 ppm. Olympic Dam was formed in a maar; the Molly Marie was formed in a collapse caldera.
Because the rhyolite porphyry magma chamber beneath the Molly Marie Prospect was located beneath or bordering relatively shallow 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.
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 Molly Marie Prospect is the surface or basin-derived type).
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, but the magma had less iron and the magma chamber may likely be deeper indicated by less alteration. Goldfield 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.
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.
Wilburn stated that at the lower depths considerable chalcopyrite was encountered.
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.
The Molly Marie Prospect has similar, but different features than Goldfield:
- The Molly Marie has a much greater volume of breccias than Goldfield.
- The Goldfield volcanic neck is comprised of dacite.
There 3 major portions of the IOCG on the Molly Marie Prospect:
- The largest portion is in the abundant arkose breccias (Area 1)
- A portion that formed beneath basalt that acted and against a large intrusive in a skarn situation. (Area 2)
- A portion that formed in a large maar diatreme (Area 3). Much of area 3 is also beneath basalt.
- Note: the IOCG extends 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 1 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 heavy hematite and magnetite.
The centerpiece of Area 2 is a large 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. Because of the 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 a sub-seafloor VMS-like deposit or collapse (and/or mining) of the IOCG system. This hill shown below with the subsidence crater is on the southern edge of a maar diatreme. Note the subsidence cracks on the left side of the crater and the adjoining ridge. In the wash, the basalt has been replaced by glassy jasper and milled breccias as shown on the alteration page. The glassy material is highly magnetic and indicates that the basalt is magnetite-altered on the bottom and magnetism decreases as the basalt is tested higher in elevation.
Below is a photo of the gossan depicted 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 hematite 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. Yanacocha is an example of a high sulfidation gold deposit. 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 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 darker varieties in the neck that are even more magnetic. 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 circulates to the boiling zone and the brine is boiled-off near the magma chamber. The metals and sulfur precipitate (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. The reason for this is two-fold: The basalt acted as an aquaclude 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 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-hosted jasperoid 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.
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.