Similar to VMS deposits, IOCG deposits can have exhalites. For years, the co-presence of both exhalites and large hematite breccia deposits was a great puzzlement to the author, and it was thought their was a multitude of VMS deposits on the Prospect. Finally, it was reasoned that since IOCG deposits have a great amount of iron that they could have exhalites also. Through research, it has been found that this has been verified by others. Highly magnetic rock throughout the prospect verified that it was an IOCG.
The below idealized VMS diagram shows the position of the exhalites that accompany VMS deposits, and these exhalites can accompany IOCG deposits as well. This can be banded jasper (“Jaspillite”), banded chert, Jasperoid beds, and not shown, metalliferous mudstone.
There are three types of IOCG vents on the Molly Marie Prospect, and each has different exhalites. All types are similar to sub-seafloor type VMS deposits. To aid the description of deposits, below is a diagram showing the 3 main areas of the Prospect:
This area is comprised entirely of phreatic breccias up to 800 feet thick from the ridgetops to the top of the granite in the interior of the caldera. Because of the porosity and reactivity of the breccias, the great amount of alteration, and its position on the “wet side” of the caldera, this area is suspected to contain the greatest quantity of IOCG ore. Most of the exhalites are broken up here due to brecciation.
This area contains a large phreatic breccia pipe in its center (dubbed Cerro Negra by the author), and a significant portion of this pipe is comprised of hematite breccias. Area 2 is situated against the porphyry zone, and the two ridges bearing NNW/SSE are comprised of phreatic breaccias, some heavy with hematite. Between the two ridges is a “cave zone” that was heavily mined in the 19th century. See the “Great Mine” page for more information. The area between the two ridges is relatively undisturbed basalt., which was a great trap for upwards-circulating fluids next to the volcanic neck. The exhalites here are relatively weak, with the exception of hot springs sinter found near the top of Cerro Negra, and a great amount of hematite breccia that was emplaced on top of the basalt surrounding it.
This area contains 2 types of expressions of IOCG deposits; one that is beneath silicified and mineralized (chrysocolla and apatite) basalt, and one that is a large diatreme breccia pipe that has “rings” of hematite inside its maar. Although this area is not large relative to Area 1, the depth of the IOCG ore in the diatreme breccia could extend to a great depth
Area 1 has the exhalites of aquifer-hosted Jasperoid beds, Jasper, Jaspillite breccia, and metalliferous mudstone. Areas 2 and 3 have abundant gossan breccias, and Area 3 in particular has a great exposure of banded chert, although this is probably a distal exhalite of Area 1.
Aquifer-hosted Jasperoid beds
The Magma Mine exploited a great replacement-type VMS deposit, and jasperoid beds were mined up-dip from the sulfide ore for 50 years there at an average grade of .5 opt Au at the LS&A mine. Jasperoid beds were also mined extensively for gold near Wenatchee, Washington. The importance of Jasperoid indicating gold ore in Nevada is well known. The Molly Marie Prospect also has large exposures of Jasperoid beds, and they extend along nearly the entire west side of area 1.
Some of the the many large outcrops of Jasperoid beds at the Molly Marie Prospect are shown below. 2 chip samples of Jasperoid at the Molly Marie ran .02 opt Au. Often, the jasperoid beds are bordered by epidote-altered arkose.
Below is a photo of some of the Jasperoid beds(hematite-silica) at the Molly Marie prospect on the west side of Area 1. This great evidence of the hematite producing process at the boiling zone near the magma chamber.
The jasperoid was intruded as a gel, and it was micro-fractured if it cooled quickly. Considering the microfracturing, the jasperoid is sometimes more easily eroded than the host rock, the Whitetail arkose, as is shown in the photo below (west side of Area 1).
Banded Chert and “Jaspillite”
Like Black Chlorite, Jaspillite is found exclusively near VMS deposits and some IOCG deposits. Jaspillite was found in abundance at the huge VMS deposit that was mined at Jerome, AZ. Although the type shown below is not found in a great abundance, pieces can commonly be found in Area 1. Due to its brecciated texture, it was deposited near or above the IOCG deposit.
The large piece below was discovered nearly buried in clay in Area 1:
Below is a piece of sawn Jaspillite that was found in Area 1.
One of the more prominent exhalite features of the IOCG deposit at the Molly Marie Prospect is the abundance of banded chert. It outcrops as a bed from 1 to 5 feet thick for nearly a half a mile (note: this is not the Saddle Rock tuff). The chert lies on top of the basalt. Below is a piece of it in Area 3:
The outcrops can be very colorful (Area 3):
The banded chert reveals its beauty after being sawn (Area 3):
Below is a darker variety. Note the micro-bedding.
Below can be seen where “drop stones” deformed the chert when it was still soft and under water (Area 3).
Below is some of the breccia that is lying on top of the chert beds.
In the photo below it appears that there were explosions or pressure from beneath this chert bed, and gossan is intruding the chert. This contortion occurred under water when the chert was still soft.
Another important exhalite found is Metalliferous Mudstone beds. Near the outcrop below is found intense black chlorite alteration. This is in Area 1.
In the photo below is the same sample above sawn in two. Fine laminations can be seen. Although it is very erosion resistant, it can be seen that the mudstone can be accurately carved on the piece on the right. With a loupe heavy manganese, quartz, and hematite can be seen.
Below is believed to be a piece of friable IOCG gossan that was washed down to the mudstone outcrop shown above. Similar finely layered material with alternating bands of silica and hematite can be found in abundance on the ridges of Area 1, especially on the southern part.
On the ridges thoughout Area 1, pieces of banded jasper and jasper breccia can be found as below:
On the ridges of Area 1 is found abundant soft saccharoidal silica, inundated with hematite, another exhalite.