2010 Copyright mollymarieprospect.com
Setting
As depicted below, the western end and the area to the north and northwest of the Superstition Mountains is a large, volcanic complex. The diagram shown is not precise, and the Molly Marie Prospect sets inside of the Superstition Caldron
Wilburn (Goldfield Mining District Geology and Ore Deposits) names the 2 mile by 3 mile zone between Goldfield and the Molly Marie Prospect the "Goldfield Basin". The Basin is relatively un-disturbed granite, arkose, and basalt that appears from under cover south of Goldfield, is bound to the east by the resurgent? portion of the Superstition Caldron, to the West by the volcanics of the Goldfield Mountains, and to the north by the Supersition and Goldfield Caldrons.
The Superstition Mountains area is located in the southern Basin and Range Province.
As depicted below, the western end and the area to the north and northwest of the Superstition Mountains is a large, volcanic complex. The diagram shown is not precise, and the Molly Marie Prospect sets inside of the Superstition Caldron
Wilburn (Goldfield Mining District Geology and Ore Deposits) names the 2 mile by 3 mile zone between Goldfield and the Molly Marie Prospect the "Goldfield Basin". The Basin is relatively un-disturbed granite, arkose, and basalt that appears from under cover south of Goldfield, is bound to the east by the resurgent? portion of the Superstition Caldron, to the West by the volcanics of the Goldfield Mountains, and to the north by the Supersition and Goldfield Caldrons.
The Superstition Mountains area is located in the southern Basin and Range Province.
Superstition Cauldron (S), Goldfield Cauldron (G), Tortilla Caldera (T), Doggie Springs Cauldron (D), and Haunted Canyon Cauldron (H) Blackened areas are volcanic rocks. Source: Michael Sheridan
Rocks
The stratigraphy described by Wilburn regarding the Goldfield District generally also occurs at the Molly Marie Prospect, and it depicted in the below diagram.
The stratigraphy described by Wilburn regarding the Goldfield District generally also occurs at the Molly Marie Prospect, and it depicted in the below diagram.
Superstition Tuff - Occurs as a thin bed generally less than 50 feet thick, deposited over basalt and gossan breccias. Post mineralization.
Basalt - Approximately 130 feet thick. As described by Wilburn, there are 1 meter beds +/- of welded tuff in the middle of the basalt, the Saddle Rock Tuff.
The lower portion of the basalt is named the Weekes Wash basalt(Fodor, S.K. Vetter) and is higher in silica than the upper basalt, and some trends to andesite (photos 52,55).
Whitetail Assemblage - Wilburn describes the Whitetail as lying unconformably on the granite up to a thickness of 500 feet. The upper Whitetail is also approximately 250 feet thick, and contains more conglomerate than the lower Whitetail (photo 32), varying from thin conglomerate beds in the sandstone to dense cobbles at the top.
The lower portion of the Whitetail (approximately 250 feet thick) is nearly conglomerate-free. Snotnicki & Ferguson(Geologic Map of the Goldfield Quadrangle) label the lower portion of the Whitetail as Tertiary Sandstone(Ts). This arkosic sandstone is composed of fragments of eroded granite.
Pre - Cambrian Granite - This is the basement rock and varies in texture from a medium equi-granular granite to that of pegmatite granite with large phenocrysts of Orthoclase approximately 2 inches in width.
Basalt - Approximately 130 feet thick. As described by Wilburn, there are 1 meter beds +/- of welded tuff in the middle of the basalt, the Saddle Rock Tuff.
The lower portion of the basalt is named the Weekes Wash basalt(Fodor, S.K. Vetter) and is higher in silica than the upper basalt, and some trends to andesite (photos 52,55).
Whitetail Assemblage - Wilburn describes the Whitetail as lying unconformably on the granite up to a thickness of 500 feet. The upper Whitetail is also approximately 250 feet thick, and contains more conglomerate than the lower Whitetail (photo 32), varying from thin conglomerate beds in the sandstone to dense cobbles at the top.
The lower portion of the Whitetail (approximately 250 feet thick) is nearly conglomerate-free. Snotnicki & Ferguson(Geologic Map of the Goldfield Quadrangle) label the lower portion of the Whitetail as Tertiary Sandstone(Ts). This arkosic sandstone is composed of fragments of eroded granite.
Pre - Cambrian Granite - This is the basement rock and varies in texture from a medium equi-granular granite to that of pegmatite granite with large phenocrysts of Orthoclase approximately 2 inches in width.
Mineralization and the Model
The alteration in and around the Molly Marie caldera is a classic progression and varies according to distance from the volcanic neck and rock type. The basalt is silicified near the gossan breccias, and it is heaviliy chloritized in areas intermingling with advanced argillic alteration (photo 68). The granite to the south and west of the caldera has albite and muscovite alteration (photo 35). The arkose is heavily silicified and pyritized, and is often intensely epidote altered (photos 2,3,33), especially the lower portions of the arkose. Arkose hosted Jasperoid beds (photos 28,29,30) and epidote extend to the west past Highway 88 a mile and more in distance.
Alteration
Click to enlarge
Wide silicified zone that surrounds the gossan breccias. (click to enlarge)
Supergene Gold
The bonanza gold found at nearby Goldfield was largely supergene enriched as per the local miners, and the same is expected at the Molly Marie Prospect after further exploration.
120 fire-assays were conducted on all rock types at the Molly Marie Prospect including basalt, Rhyolite,arkose, quartz veins, breccias, float and intrusive dikes. All assays were performed by Jacobs Assay Office in Tucson, Arizona. Several samples returned Au in the .02 opt range, and there were two high silver samples of .5 and 1 opt. The assays from .003 oz/ton Au and above were placed on this map click here. For a list of assays and description click here.
Recent total rock analysis of the gossan breccias by Skyline Labs of Tucson returned assays of over 100 ppm Li, and over 150 ppm Cu, and Zn.
Recent total rock analysis of the gossan breccias by Skyline Labs of Tucson returned assays of over 100 ppm Li, and over 150 ppm Cu, and Zn.
Assays
Discussion
Click to enlarge
Tertiary Lakes
As also shown on the cover page, the southern portion of Arizona was covered by vast brine lakes during the Miocene epoch. The Superstition volcanism occurred simultaneously. The document "Arizona has Salt" descibes the salt and gypsum deposits that were formed during this period. Several are found nearby the Superstition Mountains including those at Higley, which is now Gilbert, Arizona.
Diagram compliments of an.ucc.nau.edu/rcb7/terpaleo.html
Are the Superstition Mountains really a resurgent caldera? It is proposed that they are not, but rather the remnants of strata-volcanoes that have been water-worn similar to Kauai, HI. It is suggested that deep brine lakes washed the bulk of the volcanic debris from the caldron, including those from the Molly Marie Prospect into the rift that is between and under Higley-Gilbert and Florence, AZ.
By enhancing a Google Earth photo, 4 collapse calderas can be identified in the Goldfield District. The Goldfield, Golden Hillside, and Goverment Well calderas have had extensive historic shallow gold mines on their perimeter. The Molly Marie caldera is virgin, but contains the most extensive gossan breccias found. None of the calderas have been drilled in their interior for VMS deposits. (click to enlarge)
Because of their position on the edge of the brine lakes, the calderas shown above are in the ideal position to contain high-sulphidation VMS deposits. An excellent paper about the subject is GOLD-RICH VOLCANOGENIC MASSIVE SULPHIDE DEPOSITS . Click title to view.
Similarly, the zinc mines of Ireland were formed near and under brine lakes.
The different varieties of VMS deposits. The high sulphidation VMS is the lower left. From the paper VOLCANOGENIC MASSIVE SULPHIDE DEPOSITS by GALLEY, HANNINGTON , JONASSON (click to enlarge)
Arizona has been known for Laramide age copper porphyries, and most copper exploration in the state has been centered around trying to find them. The possibility of Tertiary VMS deposits, and specifically, high sulphidation VMS deposits, appears to have been overlooked.
The re-discovery of the Goldfield District may be the "next big one", in regards to mineral exploration, with small-footprint, high-grade underground mines being the norm instead of large footprint low-grade open pits.
The re-discovery of the Goldfield District may be the "next big one", in regards to mineral exploration, with small-footprint, high-grade underground mines being the norm instead of large footprint low-grade open pits.