An IOCG deposit is a giant, intense area of metasomatic alteration in both breadth and height, and following will illustrate some of the alteration types on the Molly Marie Prospect.
Three main areas have been created in the photo below to aid in describing the types alteration that exist on the prospect:
This area is comprised of massive phreatic arkose breccias on the western perimeter of the collapse caldera, the basalt, which once overlaid the breccia, has been absorbed by the breccia below and/or eroded away. The breccias are what was the Whitetail formation that is approximately 500 feet thick and the Ruin granite below. The alteration here is textbook, grading from epidote on the perimeter, to sericite, chlorite and silication of the marble cobbles in the center of the area. The zones overlap, and silicification overprints all of them.
Portions of this area were mined previously as described in the “Great Mine” page. The deposit formed alongside the Rhyolite neck of the strata volcano and there are hydrothermal silica mounds covering much of the area. There are large outcrops of hematite breccia found on the hill dubbed “Cerro Negra”, which is a large hydrothermal vent.
This area has a maar diatreme on the northern end and three large hydrothermal silica mounds. The geology here is more complicated due to multiple explosive events, and there is intense magnetism of near and on the mounds in this area, and intense calcification, silication, and types of gossans not seen in the other 2 areas.
Most ore outcrops have been removed from the arkosic breccias of the claim group by hand-dug pits, but the alteration that surrounds the former outcrops remains. On a district scale, iron alteration can be seen from Google Earth, and on the ground, other forms of district-scale alteration can be found. The following are descriptions of the types of alteration can be found at the Molly Marie Prospect, starting generally from the outside of the areas working inward:
Albite – Magnetite Alteration
There is a zone of strongly magnetic granite approximately 400-500 yards wide that surrounds the caldera. Details of magnetite alteration are on the Magnetism and Iron Oxides page. Much of the rock in the caldera is highly magnetized. It is believed that this is due to iron rich magma that remained in the magma chamber after the last eruption, and the lithology above rested on this magma alllowing for extensive iron and magnetic alteration.
Clay or Argillic Alteration
The discovery that the Molly Marie Prospect was located on a collapse caldera occurred after enhancing a Google Earth photo. The iron stained “halo” over one mile in diameter can be seen below, with the west side breccias having the most clay and iron alteration. The iron gives the red coloration to the great amount of clay overlying much of Area 1 and especially the east sides of the ridges; all of the red clay is saturated with magnetite. Typically, the float and outcrops of breccia in the clay-covered areas are laced with drusy quartz, further evidence of extensive boiling that occurred below. On the right side of the photo, remnants of the slope of the strato-volcano can be seen. The volcano’s neck is on the south side of the caldera and is located just north of the First Water trailer parking lot; it is a pink color in the photo.
The Tertiary brine lake was located on the western side of the collapse caldera, and on the western side is where most of the brecciation occurred. This is believed to have been the “wet side” and and is also where the likely bulk of the ore formation took place.
Typically, the alteration zones are more exposed on the west side of Area 1 because of less red clay cover, and because the Chlorite zone is largely covered by basalt on the east side of the ridges.
There are large areas outside of the caldera where the pegmatitic Ruin granite has been altered? to Syenite, as named and drawn by Napoleon’s geologist in Syene, Egypt . One dike of this rock can be found cutting basalt in the interior of the Molly Marie caldera. The Syenite occurs in the perimeter of at least 3 of the local calderas. An outcrop on the periphery of the Molly Marie caldera is below.
This is what the Syenite looks like when sawn.
This is what the Syenite looks like where it is exposed in a wash and water-worn.
Shown in the photo below is a zone on the margin of the Molly Marie Caldera, and it is believed that this was the same Syenite as above, but heat drove the orthoclase off the Syenite in second pulse of heat and deposited the orthoclase as shown below.
The epidote alteration on the claim group on western and southern sides of ridges of Area 1 is intense and widespread, and indicates just how large this hydrothermal system is. The photos below are un-enhanced.
The west and east sides of Area 1 have a very wide zone of Sericite alteration. An example of the Sericite Phyllite that is found extensively on Area 1 ridges is shown below:
Below is a photo of an outcrop of Sericite Phyllite on the western side of the Area 1. It is suspected this it result of repeated inflation deflation of the magma chamber and fault movement.
Below is another outcrop of the Sericite Phyllite.
Along most of the the eastern side of the ridges in Area 1 there is a an large intense outcropping of foliated quartz as shown below, It is known now that these were likely feeders for silica mounds located on fault zones.
Below is a closeup photo of the foliated quartz on the east side of Area 1. It is suspected that there is supergene ore beneath this zone at a shallow depth.
Black and Green Chlorite Alteration
Of all the alteration types found on the Prospect, black Chlorite (high iron) alteration is one of the most pervasive, and can be found in bottom of many of the gulches and hilltops in the brecciated areas. The Chlorite replaces feldspar, and occurs in abundance in Area 1 in the breccias. The areas where the seismic surveys have been done, and quartz outcrops, align well with Black Chlorite outcrops on their perimeters, and the Black Chlorite indicates where many more untested potential pits are located.
Four 50-foot core holes were drilled in the breccias, and extensive black chlorite alteration of the feldspar was prevalent in two of them.
Another photo of Black Chlorite alteration is shown below, gathered about a hundred feet from the Needle Overlook park bench at the end of the sidewalk:
Below is a close-up of green chlorite alteration found extensively in some brecciated areas; the boulders and cobbles of granite in the upper Whitetail formation are more susceptible to alteration. The green chlorite and greenstone are found in a wide zones in Area 1, and it has been generally been found found where the IOCG vents and quartz outcrops are located.
Greenstone, another form of chlorite alteration, is also found scattered on the hillsides:
Although not shown, Much basalt is also heavily chloritized.
There has been much recent erosion due to heavy rains after an extended drought, and more of the choritized arkose in the Area 1 breccias has been exposed as in the photo below.
Below is breccia that is chloritized and has an epidote veinlet running through it.
In Area 2, on and near Cerro Negra, the Apatite alteration is most obvious. It is the classic Apatite blue, but the brown blotches are suspected to be apatite also. What is suspected to be abundant waxy yellow apatite is found in Area 1 in breccias in the basalt and in an large exposure of iron oxide. Below is a photo of pieces of the blue variety of apatite with suspected brown also found near Cerro Negra.
Some of the basalt in all areas is a light blue and it was tested using the phosphate test that uses nitric acid ammonium molybdate. A yellow precipitate is formed if there is phosphate (Apatite). Although many rocks have some phosphate in them, when left for 24 hours, this test can be semi-quantitative.
The chlorite zone in the brecciated Whitetail formation of Area 1 has an abundance of limestone cobbles and boulders that have been metamorphosed into Marble. Below is a photo of a cobble from at first appearance is a limestone cobble, but one hit with a hammer reveals that it is red marble. Note the conchoidal flake of marble on the right.
A filled-in pit proven by seismic methods is nearby the boulder of pink marble shown below. On the breccia ridges, most of the limestone cobbles have been altered to red, gray, or banded marble, some ring like glass when struck with a hammer.
Although not normally as dense as shown below, the unaltered brecciated Whitetail formation has an abundance of limestone and marble cobbles.
The hilltop area in the photograph below has no limestone cobbles, as they have been replaced by silica, either jasperoid, saccharoidal silica, or sponge silica. There is likely a boiling zone beneath this area. The Needle Overlook parking lot is in the center of this photo.
Below is a piece of limestone with fossils that has been replaced by silica, creating a “sponge” texture.
Below is a cobble of limestone that has been nearly replaced by saccharoidal silica:
More limestone replaced by silica, creating a sponge texture:
The below fossilized barnacle and microbial mat (right side) were encased in saccharoidal silica in shallow water at the same time as the ore was being formed.
The picture below is of a typical pebble of limestone that has been replaced by silica. A buoyancy test was performed on this rock, and it nearly floats. By zooming in, it can be seen just how porous and fossiliferous this pebble is.
Although initially looking like silicification and Iddingsite, this is an outcrop of silica beds? on the north side of the north mound. This is highly magnetic material.
Below is a photo of the altered material. Cubes of iron oxide can be seen with a 20x loupe that were the original pyrite.
In some locations on the Silica- vent mounds, narrow dikes of what is assumed to be rhyolite cut the layers of silica.
Silicification and Drusy Quartz
Most of the breccia zones in Area 1 are heavily inundated with drusy quartz.
The hematite breccias that are the vent of the IOCG beneath the hill dubbed “Cerro Negra” in Area 2 are surrounded by basalt, and they are surrounded by a zone of silicification as shown in the photo below. Some of this material is highly florescent under UV light (red). This basalt is highly magnetic, a result of metasomatism, which was caused by mineralized fluids boiling beneath the basalt. Interestingly, this basalt assayed .2 ppm Tellerium.
The outcrop in the photo below was thought to be silicified basalt, but it is thought now to be an intrusion of highly magnetic material that has a “knobby texture” :
This sawn rock below was thought to be hematite altered basalt, because of the large clasts in the rock, many shaped like augite.
Below is a photo of a sawn piece of another type of silicification of basalt. The veinlet of quartz is about 1/2 mm wide. What appear to be voids lined with hematite is clear quartz. This material is highly magnetic.
Silicification and drusy quartz is found throughout all alteration zones in the brecciated arkose.
Below is a piece of arkose breccia that is common in outcrops in Area 1, and quartz crystals lining the voids can be seen. Under magnification, the full extent of drusy quartz is visible. Some brecciated arkose areas have been completely replaced by silica and only faint outlines of the clasts remain.
Some silicified basalt has created mounds between Areas 1 and 3 because of its resistance to erosion and is saturated with banded quartz veinlets.
Below is a piece of veinlet material that was cut and polished:
Metabasalt and Amphibolite
On the ridges in Area 1 can be found patches of amphibolite fragments that are nearly 100% hornblende. During ore formation, the basalt that overlaid the Whitetail arkose was fragmented by brecciation, but could have been buried under as much as 2000 feet of volcanic debris. It is assumed that the basalt above the boiling zones was transformed to Amphibolite. Or was it intruded? Below are some of the larger pieces found.
On the southern perimeter of the collapse caldera in Area are found breccias consisting of mostly Vuggy Silica. This piece below came from a shallow prospect pit on the southern edge of Area 1.
Rather than alteration, it is known now that the hematite is typically found internally in the silica mounds. There are at least 2 separate areas in Areas 2 and 3 that have large outcrops of milled hematite breccia, some of it containing noticeable apatite when a fresh face is magnified.
Near the Cerro Negra in Area 2 and other places on the Molly Marie Prospect, many calcite veins are found that exhibit red fluorescence under short wave UV light. Some of it is shown below. Some basalt on the property glows like charcoal briquettes at night under UV. In the famous zinc mines of Franklin, New Jersey, core was “lamped” with UV to look for red calcite. If red calcite was “lamped” it was known that a zinc orebody was nearby. It is believed the same phenomena is present at the Molly Marie Prospect. It is known now that the calcite veins are located at prior vents.
Below is a photo of some pieces of calcite-quartz veins found in abundance in some areas:
Below is the same calcite under UV light:
Near the maar diatreme in Area 3, there is a large outcrop of breccia that is comprised of rounded fragments of mafic magma and is exposed for approximately 200 feet in a wash . The matrix is milled arkosic material that is inundated with Apatite? when magnified. A photo of the breccia is below.
One of the rounded fragments is shown below after it is sawn. The dark portion in the center is where dilute HCl was applied. The pencil lead is .7mm. The sawn face of this cobble literally glistens in the sunlight due to calcification. This mafic material appears to have cooled very quickly.
Advanced Argillic Alteration
Large portions of basalt in Area 2 have been altered to a red clay by acid alteration? or by hematite replacement. Some of these areas are inundated with quartz veinlets. Some of these altered basalt areas can be seen with Google Earth. This material is highly magnetic.
After recent erosion, some quartzite cobbles of the Whitetail formation have been exposed that have been exposed to acid or hematite replacement? as in the photo below.
Arkose breccia in Area 1 that is incised by washes has intense advanced argillic alteration including resultant kaolinite and alunite .