New name to add to the list at the bottom...KATX
Avalon and Crosshairs hit deposits.... so what does that say about the odd...LMAO....
Their distinguishing characteristic is large concentrations of low-titanium, iron oxide minerals, mainly magnetite and hematite, as opposed to iron sulphides typical of porphyry copper-gold systems. The large amount of iron oxides can impart IOCG deposits with high magnetic and gravity signatures, making geophysical surveys an important facet of exploration.
The high levels of iron oxides have led some researchers to speculate that IOCG systems and ‘Kiruna-type’ magnetite-apatite deposits are end-members of continuum, though the former are geologically more diverse. Unfortunately, outside of the abundance of copper and gold (and uranium in some IOCG deposits), there is no set of geologic features that distinguishes one deposit from the other. Whether this reflects a fundamental difference in these systems remains to be seen.
Potassium feldspar, albite, sericite, biotite and chlorite, along with copper sulphides and pyrite, are generally present. Bulk rock analyses show varying enrichment in gold, silver, cobalt, bismuth, barium, fluorine, phosphorus, rare earth elements, uranium and thorium.
Economic mineralization consists of copper sulphides (chalcopyrite, bornite, chalcoite) scattered throughout iron oxides; hematite at shallow depths, magnetite at deeper levels. Ores may be localized along both high- and low-angle faults that may be traced back to major crustal-scale structures. Brecciated rocks are common.
Host rocks occur within hydrothermal alteration envelopes that can range from tens to hundreds of square kilometers in size. The exact alteration mineralogy depends on the host rocks and depth of formation, but there is a general trend from sodic alteration at deep levels to potassic alteration at more shallow levels. Both sericitic alteration and silicification are present at very shallow levels, though these zones are usually only a few kilometers in extent. Iron metasomatism may be locally pervasive.
Tectonic Setting
IOCG deposits are located in areas that are thought to represent intra-cratonic or continental margin environments. In many cases there is a definite spatial and temporal association with extensional tectonics, such as intra-continental orogenic collapse and extension along a subduction-related continental margin.
All of these environments were subjected igneous activity, high heat flow and contained rocks that could be the ultimate source of the metals (e.g., subaerial basalts, granitic magmas and evaporites)
Fluid source
IOCG systems require saline-rich, sulfide-poor, relatively oxidized fluids to account for the abundant iron oxides and sparse sulfides. But the data is unclear on the source(s) of these fluids and their metals, and the processes responsible for depositing the valuable minerals.
One candidate is magmatic fluid. Various IOCG deposits are found near anorogenic granites; granites produced far from orogenic, or mountain-building, zones, generally in an intra-cratonic environment. Such granites are enriched in the various elements found in IOCG deposits. It is peculated that fluids released from a granite intrusion would preferentially soak up, or partition, these elements and rise through the crust. As they neared the surface, the drop in temperature and pressure would cause the fluids to release these metals into the surrounding rocks.
Though such a model is plausible, it cannot explain the origin of IOCG deposits that are not located near anorogenic granite intrusions. Igneous activity resulting from underplating could create magmatic fluids, but they would have to travel from great depths. Another problem is why some anorogenic granites are associated with IOCG deposits, whereas others lack the signs of economic mineralization.
Another possibility are non-magmatic fluids such as a saline-rich brine stored in an overlying basin that was subsequently forced into a hydrothermal cell either by heat from igneous activity and/or tectonic compression and associated metamorphism.
Some researchers have suggested that such a brine could be partly derived from evaporites rich in various metals, yet the lack of reported evaporitic sequences in several major IOCG districts, including the Great Bear Magmatic Zone in the Northwest Territories, rules out this rock type as the sole source.
Various models invoking structural and stratigraphic traps, mixing, specialized host rocks and/or boiling have been used to explain how non-magmatic fluids could have created IOCG deposits. A persistent problem encountered in these models is finding a large enough fluid source to account for the huge alteration volumes found in IOCG districts.
Whatever the source, it is generally believed that the various zones found in an IOCG deposit, including the alteration envelopes, occurred intermittently over tens of millions of years.
Juniors
Numerous junior miners are exploring potential IOCG deposits. The following is a random selection of just a few of the smaller firms on the hunt for these potential treasure troves:
Alberta Star Development Corp [TSX-V: ASX)
Avalon Ventures [TSX-V: AVL]
Beowulf Mining [AIM: BEM]
Cardero Resource Corp. [TSX-V: CDU]
Crosshair Exploration & Mining [TSX-V: CXX]
Far West Mining [TSX-V: FWM]
Fortune Minerals [TSX: FT]
Fronteer Development Group [TSX-V: FRG]
Latitude Resources [OFEX: LAT]
Pathfinder Resources [TSX-V: PHR]
Trio Gold Corp. [TSX-V: TGK]
TORONTO (ResourceInvestor.com) -- The bull market in metals over the past few years has resulted in the exploration and promotion of the usual suspects among the juniors: porphyry copper and epithermal gold deposits.
Yet a difference in this cycle over previous ones is the focus on a relatively new type of deposit – the iron oxide-copper-gold (IOCG) deposit. Such systems have the potential for hosting copper, uranium and gold, as well as numerous other lesser-known metals.
Although geologists were aware of such deposits in the past, they received very little serious attention up until the last 15 years or so, following the discovery, and more importantly the appreciation of, the Olympic Dam deposit in southern Australia.
The mine, owned and operated by WMC, a subsidiary of BHP Billiton, [NYSE: BHP], is a true giant.
As of March 31, 2005, reserves stood at 650 million tonnes grading 1.5% copper, 0.5 kilogram per tonne (kg/t) U3O8, 0.5 gram per tonne (g/t) gold and 2.4 g/t silver. Resources were pegged at 3,980 million tonnes grading 1.1% copper, 0.4 kg/tonne U3O8, 0.5 g/t gold and 2.4 g/t silver. Production in 2004 totalled 8.886 million tonnes of ore from which 224,731 tonnes of copper and 4,404 tonnes of U3O8 were recovered.
It is a testament to geological cunning that the deposit was even discovered in 1975, considering it is blanketed by over 300 meters of sedimentary cover.
The complex itself consists of a hematite-quartz breccia flanked by zones of intermingled hematite-rich breccias and granitic breccias approximately 1 kilometer wide and up to 5 kilometers in length. Breccia is a rock in which angular fragments are surrounded by a mass of fine-grained minerals. Virtually all of the mineralization is hosted in the hematite-rich breccias
Despite the amount of studies carried out at Olympic Dam, and the discovery of other IOCG deposits, notably Phelps Dodge’s [NYSE: PD] Candelaria mine in Chile, only now is a comprehensive geological model beginning to be developed.
IOCG Geology
IOCG deposits are found throughout the globe, from the late Archean to the early Tertiary, though the majority are early to mid-Proterozoic (2.55 to 1.5 billion years old) in age. They occur as pods, veins and stockworks in various host rocks, extending both horizontally and vertically for kilometres with widths of metres to hundreds of metres.
Their distinguishing characteristic is large concentrations of low-titanium, iron oxide minerals, mainly magnetite and hematite, as opposed to iron sulphides typical of porphyry copper-gold systems. The large amount of iron oxides can impart IOCG deposits with high magnetic and gravity signatures, making geophysical surveys an important facet of exploration.
The high levels of iron oxides have led some researchers to speculate that IOCG systems and ‘Kiruna-type’ magnetite-apatite deposits are end-members of continuum, though the former are geologically more diverse. Unfortunately, outside of the abundance of copper and gold (and uranium in some IOCG deposits), there is no set of geologic features that distinguishes one deposit from the other. Whether this reflects a fundamental difference in these systems remains to be seen.
Potassium feldspar, albite, sericite, biotite and chlorite, along with copper sulphides and pyrite, are generally present. Bulk rock analyses show varying enrichment in gold, silver, cobalt, bismuth, barium, fluorine, phosphorus, rare earth elements, uranium and thorium.
Economic mineralization consists of copper sulphides (chalcopyrite, bornite, chalcoite) scattered throughout iron oxides; hematite at shallow depths, magnetite at deeper levels. Ores may be localized along both high- and low-angle faults that may be traced back to major crustal-scale structures. Brecciated rocks are common.
Host rocks occur within hydrothermal alteration envelopes that can range from tens to hundreds of square kilometers in size. The exact alteration mineralogy depends on the host rocks and depth of formation, but there is a general trend from sodic alteration at deep levels to potassic alteration at more shallow levels. Both sericitic alteration and silicification are present at very shallow levels, though these zones are usually only a few kilometers in extent. Iron metasomatism may be locally pervasive.
Tectonic Setting
IOCG deposits are located in areas that are thought to represent intra-cratonic or continental margin environments. In many cases there is a definite spatial and temporal association with extensional tectonics, such as intra-continental orogenic collapse and extension along a subduction-related continental margin.
All of these environments were subjected igneous activity, high heat flow and contained rocks that could be the ultimate source of the metals (e.g., subaerial basalts, granitic magmas and evaporites)
Fluid source
IOCG systems require saline-rich, sulfide-poor, relatively oxidized fluids to account for the abundant iron oxides and sparse sulfides. But the data is unclear on the source(s) of these fluids and their metals, and the processes responsible for depositing the valuable minerals.
One candidate is magmatic fluid. Various IOCG deposits are found near anorogenic granites; granites produced far from orogenic, or mountain-building, zones, generally in an intra-cratonic environment. Such granites are enriched in the various elements found in IOCG deposits. It is peculated that fluids released from a granite intrusion would preferentially soak up, or partition, these elements and rise through the crust. As they neared the surface, the drop in temperature and pressure would cause the fluids to release these metals into the surrounding rocks.
Though such a model is plausible, it cannot explain the origin of IOCG deposits that are not located near anorogenic granite intrusions. Igneous activity resulting from underplating could create magmatic fluids, but they would have to travel from great depths. Another problem is why some anorogenic granites are associated with IOCG deposits, whereas others lack the signs of economic mineralization.
Another possibility are non-magmatic fluids such as a saline-rich brine stored in an overlying basin that was subsequently forced into a hydrothermal cell either by heat from igneous activity and/or tectonic compression and associated metamorphism.
Some researchers have suggested that such a brine could be partly derived from evaporites rich in various metals, yet the lack of reported evaporitic sequences in several major IOCG districts, including the Great Bear Magmatic Zone in the Northwest Territories, rules out this rock type as the sole source.
Various models invoking structural and stratigraphic traps, mixing, specialized host rocks and/or boiling have been used to explain how non-magmatic fluids could have created IOCG deposits. A persistent problem encountered in these models is finding a large enough fluid source to account for the huge alteration volumes found in IOCG districts.
Whatever the source, it is generally believed that the various zones found in an IOCG deposit, including the alteration envelopes, occurred intermittently over tens of millions of years.
Juniors
Numerous junior miners are exploring potential IOCG deposits. The following is a random selection of just a few of the smaller firms on the hunt for these potential treasure troves:
Alberta Star Development Corp [TSX-V: ASX)
Avalon Ventures [TSX-V: AVL]
Beowulf Mining [AIM: BEM]
Cardero Resource Corp. [TSX-V: CDU]
Crosshair Exploration & Mining [TSX-V: CXX]
Far West Mining [TSX-V: FWM]
Fortune Minerals [TSX: FT]
Fronteer Development Group [TSX-V: FRG]
Latitude Resources [OFEX: LAT]
Pathfinder Resources [TSX-V: PHR]
Trio Gold Corp. [TSX-V: TGK]
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