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 80–40 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 (0.1–2 ppm), As (500–10,000 ppm), Ag (1–100 ppm), and Te (0.5–50 ppm) than average crustal background levels. Other elements enriched in the Lucky Bay nodules are Co (500–1000 ppm), Ni (500–1000 ppm), Cu (100–500 ppm), Zn (50 ppm), Se (10–50 ppm), Mo (1–5 ppm), Sb (100–200 ppm), Hg (1000–2000 counts per second), Tl (0.5–10 ppm), Pb (500–1000 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 (