Because the rhyolitic magma chamber beneath the Molly Marie Prospect was located beneath or bordering relatively shallow brine lakes, the deposits on the Prospect are called high-sulfidation VMS deposits. This type of deposit typically has high gold values.
Below is a link to an excellent paper regarding high-sulfidation VMS gold deposits (which are gold-rich).
The diagram below is from the paper and depicts clearly how High-Sulfidation VMS deposits form in shallow water near the shoreline of an ocean or brine lake.
Ocean-floor high-sulfidation VMS deposits are rare in their own right; present-day examples being Eskay Creek and Greens Creek, but the deposits on the Prospect are another special subset of VMS deposits, called sub-seafloor high-sulfidation VMS. A famous example of the sub-seafloor type would be portions of the famed Rio Tinto VMS deposit in Spain that has been mined for millennia. The upper portions were rich in precious metals, so much so that Rio Tinto is even rumored to be the location of King Solomon’s mines.
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 on a collapse caldera located above a shallow-placed magma chamber. 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 they appear to have been unrecognized as high-sulfidation VMS deposits.
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 high amount of theft that was documented at that time, the production could have 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 consists of 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.
Considering the bonanza grade ore described, it is highly unlikely that an assay was performed for lead or zinc. The galena was reported, but it appears that zinc and the mineral “Marmatite” were not identified; zinc is the major player in high-sulfidation VMS deposits. Marmatite is the iron rich variety of Sphalerite. Marmatite is black, can be botryoidal like many manganese minerals, but has a red streak like hematite. Marmatite was the primary zinc mineral in the zinc-lead-silver-gold district of Leadville, Colorado.
It is important to mention that Mr. Wilburn is describing VMS deposits at both of these mines:
- The high sulfides at the Old Wasp
- The heat source for a VMS deposit next to the Mammoth stope (dacite dike)
- Ore contacting the Dacite dike at the Mammoth Stope
- The highest grade ore was stated as being within or replacing brecciated arkose at the Mammoth stope.
This is the same situation at the Molly Marie Prospect (replacement type, sub-seafloor, high-sulfidation VMS deposits), but it will be shown within this site that the amount of breccia and the quantity and size of ore bodies are many magnitudes larger at the Molly Marie.
Below is the geologic model of ore genesis at the Molly Marie Prospect:
Doctor Michael Sheridan, former geology professor at ASU, postulated that the large quantity of gold at Goldfield was due to the re-mobilization of gold from ancient placers in the Whitetail formation. Since a gold assay of .02 opt Au was gained from a chip sample of vuggy silica nearby the volcanic neck 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. Above the magma chamber(s) a high sulfidation gold deposit formed (not VMS). Vuggy silica is prevalent in this type of deposit, and massive lower-grade gold ore. The gold originates from the magma chamber. Yanacocha is an example of a 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 from the regenerating volcanic debris by brine, and the brine is boiled off above the magma chamber or near an intrusive originating from the magma chamber. The metals and sulphur are precipitated at the boiling zone (above diagram). It is proposed at this phase the brine also leaches some of the gold from the vuggy silica, and becomes even more gold-enriched before it is precipitated at the boiling zone. The key to this process was the discovery of high quantities of Lithium in gossans found throughout the property; the largest gossans have over 100 ppm Li.
In the geologic model above, there is a great amount of ore shown 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 VMS-limestone replacement style ore.
3) Supergene Phase. This phase is slow and occurred after the lakes receded. Meteoric water and residual chorides leach the metals from the hypogene ore, and deposit 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.
A VMS, whether sea-floor or sub-sea floor can have an “exhalite horizon as in the diagram below. The silica+hematite is called jasperoid. This will be described further on the Exhalite page, but jasperoid beds occur prominently on the Prospect, and it is believe each and every one is the edge of a VMS deposit.
The below picture/diagram below was created to show how much potential there is for VMS deposits on the property. The rhyolite intrusions were estimated from rhyolite dikes and plugs that outcrop, and the extent of basalt hornfels. The VMS deposits were determined from jasperoid beds, seismic work that indicates pits, intense black chlorite alteration, disturbed areas that were not seismically tested, amphibolite scattered above the phreatic breccias, massive epidote alteration, marbilized limestone cobbles and boulders, gossan high in Lithium, and gold assays.
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.
This is a photo of breccia from the maar diatreme. Migmatite can be seen.
This photo shows the striking rings of gossan in the maar diatreme. This gossan also had high Lithium content.
This is the massive gossan breccia that occurs near the volcanic neck at the most prominent VMS deposit on the property. There appears to have been 100% replacement of the volcanic breccias. This ran over 100 ppm Lithium, and even though it has been thoroughly leached, gossan nearby ran 158 ppm Cu and 155 ppm Zn.
Black chlorite alteraton is exclusive to VMS deposits, and the areas that tested seismically positive as pits and areas suspected to be pre-Mexican-American war pits are ringed with black chlorite alteration.
The types of alteration will be discussed further on the the Alteration page.