The magnetite-apatite deposits ("Kiruna-type") and the iron oxide-Cu-Au deposits form end members of a continuum. In general the magnetite-apatite deposits form prior to the copper-bearing deposits in a particular district. While the magnetite-apatite deposits display remarkably similar styles of alteration and mineralization from district to district and throughout geologic time, the iron oxide-Cu-Au deposits are much more diverse. Deposits of this family are found in post-Archean rocks from the Early Proterozoic to the Pliocene. There appear to be three "end member" tectonic environments that account for the vast majority of these deposits: (A) intra-continental orogenic collapse; (B) intra-continental anorogenic magmatism; and (C) extension along a subduction-related continental margin. All of these environments have significant igneous activity probably related to mantle underplating, high heat flow, and source rocks (subaerial basalts, sediments, and/or magmas) that are relatively oxidized; many districts contain(ed) evaporites. While some of the magnetite-apatite deposits appear to be directly related to specific intrusions, iron oxide-Cu-Au deposits do not appear to have a direct spatial association with specific intrusions. Iron oxide-Cu-Au deposits are localized along high- to low-angle faults which are generally splays off major, crustal-scale faults. Iron oxide-Cu-Au deposits appear to have formed by: 1) significant cooling of a fluid similar to that responsible for precipitation of magnetite-apatite; 2) interaction of a fluid similar to that causing precipitation of magnetite-apatite with a cooler, copper-, gold-, and relatively sulfate-rich fluid of meteoric or "basinal" derivation; or 3) a fluid unrelated to that responsible for the magnetite-apatite systems but which is also oxidized and saline, though probably cooler and sulfate-bearing. The variability of potential ore fluids, together with the diverse rock types in which these deposits are located, results in the wide variety of deposit styles and mineralogies.
Recent work on the Olympic Dam Cu-U-Au-Ag deposit, South Australia,
the Wernecke Mountain breccias, Yukon, the Kiruna iron ore district,
Sweden, and the southeast Missouri iron ore district, and a review
of literature on other iron-rich mineral deposits in Proterozoic
rocks, suggest that these occurrences constitute a distinct class
of ore deposits characterized by low-titanium, iron-rich rocks
formed in extensional environ
ments.
Other examples of this class may include the mineral deposits
of the Great Bear magmatic zone of northwest Canada, the Bayan
Obo district of China, and perhaps the Redbank breccia pipes of
the Northern Territory, Australia. We designate this class of
deposits as Proterozoic iron oxide (Cu-U-Au-REE) deposits, and
propose that the ore deposits generally referred to as "Kiruna-type"
should be considered a subset of this larger class. Salient characteristics
of this class of deposits are as follows:
(1) Age. The majority of known deposits, particularly the larger examples, are found within Early to mid-Proterozoic host rocks (1.1 to 1.8 Ga).
(2) Tectonic setting. The deposits are located in areas that were cratonic or continental margin environments during the late Lower to Middle Proterozoic, and in many cases there is a definite spatial and temporal association with extensional tectonics. Most of the districts occur along major structural zones, and many of the deposits are elongated parallel to regional or local structural trends. The host rocks may be igneous or sedimentary; many of the deposits occur within silicic to intermediate igneous rocks of anorogenic type. However, mineralization in many deposits is not easily related to igneous activity at the structural level of mineralization.
(3) Mineralogy. The ores are generally dominated by iron oxides, either magnetite or hematite. Magnetite is found at deeper levels than hematite. CO3, Ba, P, or F minerals are common and often abundant. The deposits contain anomalous to potentially economic concentrations of REEs, either in apatite, or in distinct REE mineral phases.
(4) Alteration. The host rocks are generally intensely altered. The exact alteration mineralogy depends on host lithology and depth of formation, but there is a general trend from sodic alteration at deep levels, to potassic alteration at intermediate to shallow levels, to sericitic alteration and silicification at very shallow levels. In addition, the host rocks are locally intensely Fe-metasomatized.
In spite of these similarities, many variations occur between and within individual districts, particularly in deposit morphology. Individual deposits occur as strongly discordant veins and breccias to massive concordant bodies. Both the morphology and the extent of alteration and mineralization appear to be largely controlled by permeability along faults, shear zones and intrusive contacts, or by permeable horizons such as poorly welded tuffs. Thus, the variations of morphology are explicable in terms of local wall-rock and structural controls. Similarly, local variations in mineralogy and geochemistry may be largely attributable to wall-rock composition, and to P, T, and fo2 controls related to depth of formation.
 Breccia consisting of red hematitically altered siltstone in dark gray specular hematite plus chlorite matrix. Breccia formed by in situ alteration of siltstone. Wernecke-type breccia. Pagisteel prospect, Yukon Territory, Canada. |
We believe that these deposits formed primarily in shallow crustal environments (<4 to 6 km), and that they are expressions of deeper-seated, volatile-rich igneous-hydrothermal systems, tapped by deep crustal structures. The global occurrence of this type of deposit at approximately 1.8 to 1.4 Ga suggests a relation to global rifting events affecting continental crust, possibly the break-up of a Proterozoic supercontinent. Secular cooling of the Earth insured that subsequent rifting and mineralizing events might generate deposits similar in kind but smaller in magnitude.
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