The Frisco Mineral System
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Alderan´s Frisco Project forms the central part of a very large scale porphyry copper – gold – molybdenum mineral system comparable in size to other world class copper districts. Mineralization is expressed in a variety of styles within this mineral system: Outcropping and covered skarn or carbonate replacement type deposits (eg Accrington Skarn) are the result of mineralization occurring along preferential structural orientations and through contact metasomatism. Breccia pipes (eg Cactus Mine) are the direct expression of rapid energy release from the porphyry system and contain economic grade and tonnages for Cu and Au. Several phases of mineralized porphyry intrusives have been logged in historic drillcore by Alderan geologists and found in outcrop during mapping. Structural overprint post mineralization has played an influence on localized remobilization and supergene enrichment of mineralization, which is the case at the Horn Zn deposit where a pre-existing skarn deposit was brecciated through faulting and enriched through supergene enrichment.
Alderan´s Frisco Project lies in the western Cordillera of the United States, near the eastern side of the Great Basin or Basin-range geomorphic province, in southwestern Utah. It is part of the southernmost of three major, subparallel polymetallic mineral belts of mid-Tertiary age containing most of Utah´s base- and precious metal occurrences, including the Bingham Canyon porphyry deposit. Each of the three main polymetallic mineralization belts in Utah is marked by an alignment of igneous intrusive and associated extrusive, mid- to late Tertiary intermediate to felsic rocks, with which the mineralization is associated. Magmatic rocks show a distinct magnetic signature on geophysical datasets.
Rocks of the general Milford area range in age from late Proterozoic to Jurassic, and from Oligocene to Recent. Sedimentary strata accumulated in late Proterozoic through middle Jurassic time and are now removed by subsequent erosion in the Milford area containing the Frisco Project. Rocks have been affected by the mostly compressive Cretaceous Sevier Orogeny resulting in regional scale shortening including folding, and the Late Cretaceous/ Tertiary Laramide orogeny being the cause for most of the magmatic activity in the region. Very young Basin and Range type extension is the youngest tectonic event affecting Frisco.
Porphyry deposits are very large, polymetallic mineral systems that typically contain copper along with other important metals such as gold, molybdenum and variable amounts of silver, lead, zinc, tin and tungsten. Each porphyry is unique and holds different concentrations of minerals, resulting in a varying suffix given to the deposit (eg “Porphyry Copper”, “Porphyry Copper-Gold”, etc). Porphyries typically contain 100 million to 5 billion or more tons of ore with a lower grade (typically 0.2% – >1% Cu). Their size makes these deposits amenable for bulk mining with a long mine life. Infrastructure is thereby a key issue with cheap and sufficient access to both water and energy needed for mining.
Porphyry deposits are intrinsically related to large scale magmatic processes initiated by plate tectonic processes like the subduction of an oceanic plate underneath the continental crust (or in rare cases underneath oceanic crust itself). Large volumes of magma are generated by partial melting of crustal material. This magma, containing fluids and varying amounts of metals and sulphur, rises to the surface and deposits metals through cooling and pressure release.
Exploration for porphyry deposits focusses on developing an in depth understanding of the whole mineralizing system in order to vector in on the “sweet spots”. Expressions of mineralization within porphyry mineral systems are manifold and on surface depend on the depth level the mineral system is eroded to. Skarns are the direct result of metasomatic fluid and metal exchange reactions involving carbonatic hostrock into which the porphyry intruded. Breccia pipes are the result of rapid energy release from an overpressured magmatic intrusion at depth. Epithermal precious metal deposits occur in the upper levels of a porphyry system and distal to the source. Polymetallic veins also occur distal from the porphyry source, which in an ideal case is a large tonnage mineralized porphyry itself. All deposit types that form part of the very same mineralizing system can be economic deposits in their own rights.
Alderan´s Frisco project comprises all known surface expressions indicative for a large porphyry system, namely the Cactus Breccia Pipe, the Accrington Skarn system, both high and low sulphidation epithermal areas and finally mineralized felsic porphyries in outcrop and historic drillholes.