Murray W. Hitzman

Geological characteristics and tectonic setting of proterozoic iron oxide (CuUAuREE) deposits

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 tectonic environments. 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. (1) Age. The majority of known deposits, particularly the larger examples, are found within Early to mid-Proterozoic host rocks (1.1–1.8 Ga). 2. (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. (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. (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. We believe that these deposits formed primarily in shallow crustal environments (<4–6 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 a global rifting events effecting 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.

Discussion: “Age of the Zambian Copperbelt” by Sillitoe et al. (2017) Mineralium Deposita

The recent paper by Sillitoe et al. (2017) nicely confirms through Re-Os dating the results of earlier geochronological studies that molybdenite and, by extension, spatially associated (but generally not intergrown) copper sulfide minerals in some Zambian Copperbelt deposits were precipitated in a 50myr (~ 540–490 Ma) time span during the later stages of the Lufilian collisional orogeny. However, the paper does not support the authors’ hypothesis that earlier mineralization events did not take place in the Zambian Copperbelt. The paper’s sweeping conclusion ignores decades of careful geological and more recent geochronological work in the Zambian and adjacent Congolese Copperbelt that provides strong evidence of a prolonged period of mineralization. Sillitoe et al. (2017) dated 15 samples from 7 deposits in the Zambian Copperbelt and the Northwest Province of Zambia that contained examples of both disseminated molybdenite and molybdenite in veins. The primary problem with their approach is that it focused only on molybdenite-bearing assemblages. Over 50 years of detailed geological research in the Zambian Copperbelt has demonstrated from careful petrographic studies of the paragenetic sequence of sulfide mineral precipitation that the earliest stage of mineralization resulted in formation of disseminated iron, copper, and cobalt sulfides with similar mineral assemblages in pre-folding veins (Darnley 1960; Mendelsohn 1961; Fleischer et al. 1976; Annels 1989; Sweeney and Binda 1989; Selley et al. 2005; Hitzman et al., 2012). The types of textural evidence utilized to suggest a pre-Lufilian mineralization event occurred include: (1) ore sulfides typically occupying interstitial sites between detrital and secondary (authigenic) phases suggesting either infill of porosity, replacement of diagenetic cement, or hydrocarbons; (2) sulfides commonly displaying the same grain size as adjacent detrital grains suggesting replacement (Selley et al. 2005); (3) a spatial association of ore sulfides with heavy mineral bands in sedimentary structures (Fleischer et al. 1976); (4) foliated and deformed sulfides are present at many deposits and are cut by younger mineralized veins dated as synto post-Lufilian by Re-Os geochronology on molybdenite (Mendelsohn 1961; Selley et al. 2005; Torrealday et al. 2000). At many deposits, there is also a clear association of ore with syn-rift faults and related distribution of lithofacies, and an asymmetric sulfide zonation intimately associated with a diagenetic redox boundary (Brown 1997; Selley et al. 2005). In several deposits, there is isotopic evidence that reduction of hydrothermal fluids was due to interaction with hydrocarbons which would not be expected to be preserved following the Lufilian deformational and thermal event (Selley et al. 2005). Geochronological evidence from multiple dating techniques also provides strong direct evidence for pre-Lufilian stages of mineralization. Early disseminated and prefolding chalcopyrite and bornite from a diagenetic evaporitic nodule assemblage lacking molybdenite from the hangingwall of the Konkola Ore Shale deposit have been dated by the Re-Os method and yielded a six-point isochron with a slope age of 816 ± 62Ma (Barra et al. 2004). Pb isotopic dating of disseminated ore sulfide at the Musoshi Ore Shale deposit (northwestern satellite of the Konkola deposit) returned an age of 645 ± 15 Ma (Richards et al. 1988a). A similar age of 652 ± 7.3Ma is obtained for early Umineralization at multiple deposits in DRC (Decrée et al. 2011). At Musoshi, this early Editorial handling: B. Lehmann

Mineral resource geology in academia: An impending crisis?

AUGUST 2009, GSA TODAY Murray Hitzman, Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado 80401, USA, mhitzman@mines.edu; John Dilles, Dept. of Geosciences, Oregon State University, 104 Wilkinson Hall, Corvallis, Oregon 97331-5506, USA, dillesj@geo.oregonstate. edu; Mark Barton, Dept. of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E. 4th Street, Tucson, Arizona 85721, USA, barton@geo.arizona.edu; and Maeve Boland, Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado, 80401-1885, USA, mboland@mines.edu