The grassroots discovery of a large VMS exploration target with excellent potential in the mainland of the USA is an unusual occurrence. However, this site describes such a discovery, in Maricopa County, Arizona near the historic district of Goldfield.
The Molly Marie Prospect has good potential for a large high sulphidation Cu-Zn-Au VMS deposit and is situated above a porphyry in coarse arkosic sandstone/conglomerate that is 500 feet thick.
The porphyry is indicated by a collapse caldera over one mile in diameter, and within and on its perimeter are found abundant phreatic arkosic breccias, gossan breccias, silica replacement, copper and gold mineralization, tufa deposits, intense propylitic alteration, and ring dikes.
Near the center of the caldera is a dacitic porphyry stock complex, the neck of a strata volcano that has been eroded to its base. Surrounding the stock are many diatremes and gossan breccias with copper and gold mineralization.
Occurring at the same time as the mid-Teritiary volcanism were vast brine lakes that bordered and eventually overlapped the Superstition volcanic complex.
The haloed Molly Marie porphyry with the stock complex and breccias is shown in the photo below:
There are many more gossan breccias, although less concentrated, that occur than are shown. (Click any photo to enlarge)
Phreatic Breccia w/ Drusy Quartz. The ridges on the western side of the collapse caldera are composed of phreatic felsic breccia. Several samples of this ran .02 opt Au.
Chrysocolla Veinlet in Basalt.
Banded quartz veinlets occur in swarms in the basalt.
Breccia from one of the many diatremes that surround the stock. Zooming reveals the silicification, veining, relic sulphides, and repeated fracturing. Vug on upper right is lined w/drusy quartz.
Massive silica left from acid destruction in the collapse caldera.
The Molly Marie Prospect has the classic zonation of a copper Porphyry as shown above. The approximate exposure level at the Molly Marie has been added.
The Molly Marie Caldera is sited on an intense aeromagnetic low, The lines indicated were flown on a one-mile spacing in 1974.
More phreatic breccia from the southern perimeter of the caldera, although the matrix is nearly black, it is not magnetic. Recharge zone?
Complementing the magnetic low is a negative gravity signature. Nearby copper porphyries are indicated by gravity lows as indicated. The Molly Marie prospect is on the flank and in the shadow of a huge gravity low.
The flaky massive silica outcrops in the collapse zone have poor core recovery, but at 15 to 20 feet some vuggy quartz can begin to be recovered.
Sawn gossan breccia from the largest gossan breccia area shown above. Note silicification.
During the Miocene (mid-Tertiary), extensive brine lakes covered most of southern Arizona. This resulted in salt deposits 10,000 feet thick at Luke AFB near Phoenix, and the largest gypsum deposit in the world which is found at nearby Pichacho, AZ. Economic potash occurs near Holbrook, AZ that was formed during this same period. The Colorado River basin had not formed yet and the water pooled in what is now Arizona in deep rift zones. The brine from these lakes leached metals from arkosic clastics 500' thick found near the Molly Marie Prospect and the water was boiled off at the porphyry located under the collapse caldera where the Molly Marie is situated.
Above is the geologic model of the Molly Marie Prospect and Caldera. Although there are other systems that overprinted the VMS deposit as the brine lake receded and advanced, indicated by bladed quartz and vuggy silica, the genesis of the VMS is shown here.
A piece of inversely-bladed quartz from the basalt in the Molly Marie Caldera.