Banded iron formations, pyritic black shale, and gold deposits : a re-evaluation

Banded iron formations (BIF) are predominantly Precambrian sedimentary rocks composed of alternating layers of iron-rich minerals (commonly hematite and magnetite, but also siderite, chlorite, and grunerite) and silica-rich minerals (e.g., quartz or chert) in which the Fe content of the entire rock is at least 15 wt. %. Although typically thought of as strictly Fe resources, such as in the worldclass Archean-Proterozoic Pilbara Craton of NW Australia, some BIF are associated with non-Fe ore resources, including (but not limited to) gold deposits in greenstone terranes. Due to fluctuations in ocean and atmospheric chemistry, BIF are commonly interbedded with pyritic black shales, which are regarded as waste material in Fe deposits but nonetheless are of great scientific interest (as are BIF) and potentially play a role in the creation of some gold resources (recent research has suggested that black shales are a source of Au and As in sediment-hosted gold deposits). The source of gold in these BIF, or more accurately, in BIF that host gold deposits, is a contentious issue in historical and modern economic geology. Over the past 40 years, debate in the economic geology community has revolved around whether the gold now in such deposits was an original part of the iron formation, or if it was introduced from an external, distant source. Less attention has been paid to the sources of As, Ag, and Te in these deposits, but as they commonly co-exist with gold in ore zones, determining their provenance is of relevance in assessing the origin of sediment-hosted and greenstone-hosted gold deposits. In this thesis, black shales are investigated as the source of gold, arsenic, and tellurium at two BIF-hosted gold districts, Randalls (Australia) and Homestake (USA). The primary tools used in this investigation were petrography, textural analysis, whole-rock XRF, optical and scanning electron microscopy, and laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). In particular, sulfides associated with the BIF-gold ores (e.g., pyrite, pyrrhotite, and arsenopyrite) and 'background' sulfides in non-BIF rocks a short distance from the ore zones were analyzed using laser ablation imaging and spot methods to determine their trace element contents and zonation (especially their Au, As, Ag, and Te concentrations). Special attention was paid to fine-grained, carbonaceous and sulfidic (meta) sedimentary rocks, or black shales, as these are widely acknowledged to be excellent source material for sediment-hosted gold, nickel, and copper deposits. The Archean Randalls district (including Lucky Bay) is located in the southern Eastern Goldfields Superterrane, one of several crustal fragments that make up the Archean-Proterozoic Yilgarn Craton. The Yilgarn is one of the most gold-rich areas in the world, containing an estimated 9500 t (270 million ounces [Moz]) of Au. The Randalls district is located in the Belches Basin, one of several so-called 'Late-Stage Basins' in the Eastern Goldfields, and contains three BIF-hosted gold deposits (Cock-Eyed Bob, Maxwells, and Santa-Craze). Other styles of gold mineralization do occur in the area, such as the Daisy Milano deposit (quartz-vein lodes in altered basalts) and the Imperial-Majestic deposits (possible intrusion-related gold systems). The >40 Moz (1300 t Au) Paleoproterozoic Homestake BIF-hosted deposit (the largest of its kind in the world) is located in the Black Hills dome of western South Dakota, which consists of an Archean- Proterozoic metasedimentary/metaigneous core flanked by Phanerozoic sediments; the current geologic architecture of the Black Hills was created during the 80‚Äö-40 Ma Laramide orogeny, which formed the Rocky Mountains. Numerous felsic and alkalic intrusions were emplaced in all Precambrian and Phanerozoic units during this time, some of which are present in the Homestake mine area. These are commonly pyrite-bearing. At Randalls, the ore host BIF is enveloped by km-thick quartz- and feldspar-rich turbidites, with very little shale (and virtually no carbonaceous black shale). However, 10 km west of Cock-Eyed Bob is the Lucky Bay prospect, which contains abundant carbonaceous, fine-grained, and sulfidic (meta) black shale. This unit contains several types of pyrite, including pyrite nodules that are orders-of-magnitude more enriched in Au (0.1‚Äö-2 ppm), As (500‚Äö-10,000 ppm), Ag (1‚Äö-100 ppm), and Te (0.5‚Äö-50 ppm) than average crustal background levels. Other elements enriched in the Lucky Bay nodules are Co (500‚Äö-1000 ppm), Ni (500‚Äö-1000 ppm), Cu (100‚Äö-500 ppm), Zn (50 ppm), Se (10‚Äö-50 ppm), Mo (1‚Äö-5 ppm), Sb (100‚Äö-200 ppm), Hg (1000‚Äö-2000 counts per second), Tl (0.5‚Äö-10 ppm), Pb (500‚Äö-1000 ppm), and Bi (20-50 ppm). Structural relationships observed in drill core and under the microscope suggest that these nodules are pre-metamorphic and pre-deformation, as evidenced by the 'wrapping' of bedding around the nodules. Pressure shadows containing quartz, mica, and a second generation of pyrite (plus sphalerite and chalcopyrite) also point to pre-deformational (likely syn-sedimentary or early diagenetic) growth. Likewise, fine-grained (<0.5 mm diameter) anhedral pyrite and larger (up to 5 mm), partially recrystallized pyrite nodules in etamorphosed black shale at Homestake also have high amounts of Au (up to 0.5 ppm, average 0.3 ppm), As (up to 10,000 ppm), Ag (~10 ppm), Te (~10 ppm), and the other ore-associated elements noted above for pyrite nodules at Lucky Bay, though not quite at the same level as those nodules. This is likely due to the greater intensity of metamorphism (lower-middle amphibolite facies) experienced by lithologies at the Homestake deposit and surrounds, which recrystallized and partially converted the pyrite to pyrrhotite; pyrite at Lucky Bay (lower-middle greenschist facies) was spared this same fate. Furthermore, whole-rock analyses of the pyrite-bearing lithologies at Homestake show that the unit contains levels of V, Ag, Mo, Cu, and Zn on par with anomalous carbonaceous black shale (as published in the literature). Lead isotopes of the pyrite nodules and the diagenetic pyrites from both localities appear to reinforce the claim that these pyrites did indeed form early in the history of the sedimentary units that host them. However, whereas Pb-Pb model ages of the Lucky Bay nodules are within error of the U-Pb depositional age, those from Homestake are hundreds of millions of years younger than the established intrusive, crystallization and depositional ages for all Proterozoic rock types in the Black Hills, suggesting that Pb loss and isotopic resetting had a major effect on minerals and rocks in and around Homestake. Futhermore, Pb isotope systematics of the eastern Yilgarn Craton are complex, such that two pyrites of otherwise different generations (i.e., sedimentary vs. ore-stage pyrite) can have Pb isotope compositions within error of each other. These aspects of the pyrite Pb isotope data from Lucky Bay and Homestake are here presented as caveats, not insurmountable obstacles, to the study and understanding of Pb isotope systematics in sulfides. The sulfides within ore zones at both deposits do not contain the same high levels of trace elements, nor (with rare exceptions) do the amounts of the trace elements they have rival the levels of concentration seen in the various sedimentary pyrites outside the ore zone. At Randalls, pyrrhotite and arsenopyrite (the two major ore sulfides) are enriched in Co (up to 2000 ppm), Ni (up to 250 ppm), Se (up to 60 ppm), Mo (up to 50 ppm), Ag (up to 15 ppm), Sb (up to 200 ppm), and Te (up to 500 ppm). Certain elements, such as Mo and Te, are enriched only in arsenopyrite, while the rest are enriched at similar levels in both sulfides (furthermore, Te and Mo are more highly enriched in ore-stage arsenopyrite at Randalls than any other sulfide studied in this work). Arsenopyrite and pyrrhotite from Homestake (again the dominant ore zone sulfides) contain Co (50‚Äö-1000 ppm), Ni (50‚Äö-500 ppm), Se (100‚Äö-200 ppm), Mo (10‚Äö-100 ppm), Sb (50‚Äö-500 ppm), Pb (1‚Äö-100 ppm), and Bi (0.1‚Äö-100 ppm). Many of the trace elements in the Randalls sulfides are strongly zoned, whereas the Homestake sulfide trace elements are more (but not completely) homogenized; as with the sedimentary pyrite above, this is interpreted to reflect higher metamorphic conditions attained at Homestake (lower-middle amphibolite facies) compared to Randalls (upper greenschist facies), which would lead to the expulsion of most trace impurities, or at least the erasure of original zonation. This work demonstrates that certain trace elements, including those most commonly associated with sediment-hosted and greenstone-hosted gold deposits, were concentrated in upper-crustal siliciclastics during sedimentation or early diagenesis (that is, prior to later events that gave rise to sediment-hosted and greenstone-hosted gold deposits), and particularly in syn-sedimentary to early diagenetic pyrite. This research also has the potential to address metal source issues in some very large orogenic gold resources around the world: for example, an extension of the work completed here was conducted at the world-class, ~60 Moz dolerite-hosted Golden Mile Au district in Kalgoorlie, ~60 km NW of Lucky Bay and Randalls. Despite the overwhelming abundance of basalt, komatiite, and dolerite, three separate black shale layers are present in the district as interflow sediments between the mafic/ultramafic volcanic flows. All three units (Kapai Slate, Oroya Shale, and Black Flag Group shale) contain pyrite nodules with very similar internal and external textures to those at Lucky Bay. The likenesses between these nodules' textures and the Lucky Bay nodules' (including deformed bedding around the nodules) indicate that those at the Golden Mile are diagenetic, not hydrothermal. LA-ICP-MS geochemical studies of the Golden M...