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IOCG
Iron oxide associated copper--gold (IOCG) systems have become important worldwide exploration targets. Some of the largest mineral deposits in the world are classified as IOCGs. Examples include the huge Olympic Dam deposit in Australia and the Kiruna district in Sweden.
Ore deposits with IOCG affinities occur in a variety of geological settings around the world with a significant number identified in Paleoproterozoic terranes. Typically IOCG districts are associated with regional scale magnetite-hematite bearing alteration systems. Economic mineralization is generally associated with late stage epithermal events. IOCG deposits are frequently associated with intracratonic rifts. The Salobo and Igarape Bahia deposits, located in the Carajas Mineral Province, Brazil are two of the best known rift-related IOCG examples. These South American examples have geological settings which are similar to the setting of the Wilson Creek project area.
Regional Geology
Regional Geology Compiled by PK Simms 1992 |
In North America, the Midcontinent Rift-related terranes have many geological similarities to other areas elsewhere in the globe that host IOCG deposits. In the Upper Peninsula of Michigan (UP), Prime Meridian has identified a geological terrane with characteristics favorable for the formation of IOCG deposits.
The geological framework of the UP is dominated by intrusive, volcanic and sedimentary rocks of the Early to Mid-Proterozoic Marquette Range Supergroup. The Supergroup is characterized by a thick sequence of platformal clastic and chemical metasedimentary rock, with subordinate bimodal metavolcanic rocks. This terrane is distinguished by the presence of major oxide facies (Superior type) iron formation. The supracrustal assemblage was deposited on an Archean basement, intruded by ultramafic to granitic intrusions and deformed and metamorphosed during the Penokean Orogeny. A late Proterozoic (Keweenawan) extensional period of Intra-cratonic rifting, now centered on Lake Superior, split the Supergroup assemblage apart.
In this area, Prime Meridian has identified a geological terrane with the characteristics favorable for the formation of IOCG deposits.
The geological framework of the UP is dominated by intrusive, volcanic and sedimentary rocks the Early to Mid-Proterozoic Marquette Range Supergroup. The Supergroup is dominated by a thick sequence of platformal turbidite of clastic and chemical metasediments with subordinate bi-modal volcanic rocks. This terrane is distinguished by the presence of major iron oxide-facies (Superior type) iron formation and the supracrustal assemblage was deposited on an Archean basement, later deformed intruded by rare ultramafic to granitic intrusions and metamorphosed during the Penokean Orogeny.
Wilson Creek
Aeromagnetic Image of the Wilson Creek Project Area |
Geological mapping and prospecting by Prime Meridian has identified metallic mineralization and alteration of IOCG affinity in the Wilson Creek area.
The Wilson Creek Project Area is located in one of a series of four structural domes formed during the Penokean Orogeny in the central UP. The domes are characterized by a magnetically quiet center and a strongly magnetic rim (iron formation and metavolcanic rocks) that are set in a magnetically quiet background of the Michigamme Formation.
The Wilson Creek Dome was the focus of Prime Meridian's exploration because of an unusual disruption in a rimming positive magnetic anomaly at its south end. The positive magnetic signature is disrupted by a strong linear magnetic low trending roughly northeast-southwest. This low is interpreted to be the result of magnetite destruction during an epigenetic mineralizing event.
Within this trend Prime Meridian discovered copper + gold + tungsten + molybdenum mineralization in hydrothermally altered metavolcanic rocks in float. Grab samples returned assays up to 2.6% copper, 0.22 parts per million gold and 0.13% tungsten.
Altered Boulders in the Epidote Field |
These boulders are exposed in a clear cut in the Project Area, informally referred to as the Epidote Field. The float boulders are large and angular. At the western end of the clear cut, the boulders are compositionally homogenous and dominated by hydrothermally altered basalt. Geological mapping suggest that the rocks may have undergone an early epidote alteration which was overprinted by a later quartz + Kspar+ Biotite+ hematite alteration. The observed copper +/- , tungsten+/- molybdenum +/- gold mineralization is generally associated with the late alteration. However chalcopyrite mineralization also occurs disseminated and in rare veinlets associated to a lesser degree with the earlier epidote alteration assemblage.
The angular and homolithic nature of the boulders suggests that they may not have moved far from their source. Prime Meridian is mapping the Pleistocene geology of the area to determine the source of these boulders.
Wilson Creek Discovery Boulder
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IOCG Style Mineralization
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Chalcopyrite Mineralization
in Discovery Boulder
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Second Altered and
Mineralized Boulder
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Alteration Paragenisis at
the Wilson Creek Project Area
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Chalcopyrite Mineralization WC-2
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In 2007, Prime Meridian drilled 4 core holes in this area. Secondary chalcopyrite mineralization was intersected in DDH WC-2. The chalcopyrite consisted of thin intervals crosscutting 5--10 cm thick veins of massive chalcopyrite with secondary hematite, k-spar, biotite, chlorite, fluorite and tourmaline. The best assay recovered from the core was 1.53% Cu across 0.6 meters with trace amounts of gold, silver, and fluorine.
Late in the year Prime Meridian carried out an induced polarization-resistivity survey over the area. The anomalous geophysical responses are being evaluated in conjunction with drilling results to pinpoint new drill targets in this large system.
Prime Meridian also completed a ground magnetometer survey of on the Burns prospect, a discrete airborne EM and magnetic anomaly which is located 2 kilometers south of the Epidote Field and adjacent to the southward extension of the magnetite destruction zone. Ground HLEM and magnetometer surveys have confirmed the presence of the EM anomaly and coincident. Prime Meridian plans to drill test this isolated geophysical anomaly in 2008.
Ground Magentic Survey at the Burns Prospect
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