Alteration

All gossan outcrops have been removed from the arkosic breccias of the claim group, but the localized alteration that surrounds the former outcrops remains.  On a district scale, the iron alteration can be seen from Google Earth, and on the ground other forms of district-scale alteration can be found.  The following types of alteration can be found at the Molly Marie Prospect:

Iron Alteration

The discovery that the Molly Marie Prospect was located on a collapse caldera above a magma chamber 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 iron alteration.  Of all the metals, the brine had the most affinity for iron, which was dissolved from the volcanic strata above.  Typically, the iron staining is accompanied by drusy quartz, further proof that extensive boiling occurred below.  On the right side of the photo, remnants of the slopes of the strato-volcano can be seen.  The volcano’s neck is just north of the First Water trailer parking lot.

Iron Alteration (The Halo)

Black Chlorite Alteration

Of all the alteration types found on the Prospect, this one is the most important to indicate the location of VMS deposits.  Black Chlorite alteration is exclusive to VMS deposits, and is the number one thing to look for when exploring for them.  Black Chlorite alteration replaces feldspar, occurs in the phreatic arkose breccias on the west side of the Prospect, and can be seen on the walls of the washes in exposed  breccia rock.  The seismic surveys that have been done align well with Black Chlorite outcrops, and the Black Chlorite indicates where many more should have been performed.

Black Chlorite Alteration

Four 50-foot core holes were drilled in the breccias, and extensive black chlorite alteration of the feldspar was prevalent in two of them.

Black Chlorite Alteration in Core

Because of the importance of the Black Chlorite and its part in solving the puzzle, another photo of  Black Chlorite alteration is shown below, gathered about a hundred feet from the Needle Overlook bench at the end of the sidewalk:

Select Black Chlorite alteration

Silicification and Drusy Quartz

The gossans that are present on the hill dubbed “Cerro Negra” by the author do not have Black Chlorite surrounding them because they are surrounded by basalt, but they are surrounded by a zone of silicification  as in the photo below. Some of this material is highly florescent under UV light (red).

In some areas, the basalt has been nearly completely replaced by silica through the process of metasomatism, and the augite has been replaced by epidote.  It is speculated that below the basalt in these areas there is massive vuggy silica.

Below is sawn silicified basalt.  The veinlet of quartz is about .5mm wide. What appear to be voids are augite replaced with clear quartz.

Silicified Basalt

Below is a piece of phreatic arkose breccia that is common in outcrops and quartz crystals lining the voids can be seen.  Under magnification the full extent of drusy quartz is visible.  Some arkose breccia areas have been completely replaced by silica and only faint outlines of the clasts remain.

Silicification, Drusy Quartz

Below are photos of cobbles of carbonate with fossils from the ridges of breccia that have been replaced by a very light spongy silica.  This is more evidence of boiling zones below.

Limpet and other fossils in Carbonate replaced by Silica Sponge
Fossil Snail replaced by Silica Sponge

It is reasoned that the below fossil barnacle and microbial mat (right side) were encased in granular silica in shallow water at the same time the ore was being formed .

Barnacle and Microbial mat

Epidote Alteration

The epidote alteration on claim group on the western edge of the breccia ridges  is intense and widespread, and indicates just how large this hydrothermal system really is.  The photos below are un-enhanced.

Epidote Alteration

Marble

The upper portion of the Whitetail formation is laden with limestone cobbles, sand and boulders, but below is a strange occurrence of marble that appears to be an intact bed.  A pit proven by seismic methods is nearby.  On the ridges much of the limestone cobbles have been altered to marble, some ringing like glass when struck with a  hammer.  The marble cobbles and boulders is synonomous to a “marble front” found above sulfide replacement deposits; more indication of the boiling zone below.

Marble

Metabasalt and Amphibolite

On the breccia-ridges can be found patches of amphibolite fragments that are 100% hornblende.  During ore formation, the basalt that overlaid the Whitetail arkose was fragmented by the brecciation, but was buried under at much as 2000 feet of volcanic debris.  The basalt above the boiling zones was transformed to Amphibolite by superheated steam.   Most of the Amphibolite that remains is in small pieces and takes a trained eye to recognize.  Below are some of the largest pieces found.

Amphibolite

Basalt Hornfels

Areas  of basalt that are underlain by rhyolite are “cooked” to a laminated rock that is a nondescript brown color.   In the photo below, this rock can be seen, with dikelets of intrusive rhyolite breccia that betray the rocks origin.

Basalt Hornfels

Vuggy Silica

As written in the Ore Genesis page,  it is theorized that massive Vuggy Silica was created above the magma chamber during the “dry phase”.  On the perimeter of the collapse caldera is found breccias consisting of mostly Vuggy Silica.  This piece below came from a shallow prospect pit on the southern edge of the collapse caldera.

Vuggy Silica breccia

Sulfide Replacement

The massive gossan breccia outcrops at the hill dubbed “Cerro Negra” appear to indicate that there was 100% replacement of the Whitetail formation beneath the basalt.  Whether this is due to the high amounts of limestone and marble in the Whitetail or the intensity of the boiling is not known; maybe due to both.

Gossan Outcrop

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