A synvolcanic origin for magnetite-rich orebodies hosted by BIF in the Weld Range District, Western Australia

Abstract

Iron ore deposits hosted by Archean banded iron-formation (BIF) in the Weld Range greenstone belt are representative of most of the documented iron ore deposits in the Yilgarn Craton. They include nearsurface, supergene goethite-hematite orebodies that overlie and partly modify deeper occurrences of hypogene magnetite and specular hematite ores. The Cenozoic goethite-hematite-rich orebodies in these deposits are unequivocally the product of meteoric fluid alteration affecting BIF in the near-surface supergene environment; however, the deeper and likely older magnetite-and specular hematite-rich orebodies have a more contentious origin. This study is the first to present a fluid-alteration model for hypogene iron mineralization in the Yilgarn Craton that uses fluid inclusion and mineral chemistry data to constrain the physical-chemical characteristics and source of hypogene fluids responsible for mineralization. High-grade (>57 wt% Fe), magnetite-rich iron ore at the Beebyn deposit defines a discontinuous series of <80 m-thick by <1 km-long lenses that extend 3 km along strike in the BIF. These magnetite-rich lenses are surrounded by a broad carbonate alteration halo in the BIF and intense ferroan chlorite and talc alteration in nearby basalt, dolerite, and gabbro country rocks. Magnetite-rich lenses at Beebyn are the product of the replacement of primary quartz bands in the BIF by Stages 1 and 2 hypogene carbonate minerals, followed by their replacement by magnetite and minor ferroan dolomite. Fluid inclusion studies demonstrate that Stage 1 fluids were high-temperature (>440 °C) and CO2-rich. Paired O and C stable isotope data for Stage 1 ferroan dolomite suggest that these fluids had a magmatic source, while Stage 1 magnetite chemistry (e.g. enrichments in Mg, Mn, Ca, and Na) indicates chemical exchange took place between the fluids and mafic igneous rocks prior to crystallization of magnetite. The presence of monophase carbonic fluid inclusions in Stage 1 ferroan dolomite suggest that phase separation of a bicarbonate-rich aqueous fluid took place in deeper parts of the hydrothermal system, which led to the separation of the resultant volatile-rich and brine phases during transport of the Stage 1 fluid to shallower crustal levels. Cooling of the hydrothermal system during the Stage 2 fluid event involved (i) an early brine (>275 to 327 ºC; 36 to 40 equiv. wt% NaCl) with Cl/Br and C and O isotopes values that overlap the ranges 2 for magmatic fluids, with minor involvement of Archean seawater, and extensive chemical exchange with country rocks; followed by (ii) pulses of moderate-and lower-temperature Stage 2 brines (>125 to 260 ºC; 2 to 24 equiv. wt% CaCl2) with Cl/Br, O and C isotope, Na/Br, and Ca/Ca+Na signatures that suggest cooling of magmatic-derived fluids that mixed with Archean seawater and reacted with mafic igneous country rocks in areas more distal to fluid pathways. The last stage of formation of magnetite-rich ore at the Beebyn deposit involved the flow of Stage 3 fluids through the existing faul...

Fluid Inclusion and Oxygen Isotope Characteristics of Vein Quartz Associated with the Nabeba Iron Deposit, Republic of Congo: Implications for the Enrichment of Hypogene Ores

“…A comparison of the results from our study with other major iron ore deposits around the World reveals similarities with those of the Weld Range hypogene iron ore deposit in Australia [25], the Krivoy Rog iron ores in Ukraine [32], and the Serra Norte deposit in the province of Carajás, Brazil [7]. The general models for these deposits implied variations in the BIF-water interaction by early-stage high temperature magmatic ± metamorphic fluids, followed by mixing with low temperature basinal brines, seawater, or meteoric waters.…”

“…Plot of homogenization temperature versus calculated oxygen isotopic values of the oreforming fluid in equilibrium with vein quartz A and B in the Nabeba deposit. The compositions of primary magmatic water [23,24], metamorphic fluids [25], the Mesozoic meteoric water in the northern Xilamulun district, northeastern China [26], and seawater [27] are also shown for comparison. Figure 12.…”

“…For stage HO-1, a large variation in fluid inclusion salinity values can be interpreted as reflecting the entrapment of high temperature, higher salinity Type IIIa fluid inclusions; low to high temperature, low to moderate salinity fluid Type IIa and IIIb fluid inclusions; and then, moderate temperature, low salinity Type IIb fluid inclusions (Figures 10 and 15). These observed variations are best explained by the progressive trapping of multiple hydrothermal fluid pulses whose temperature and salinity values varied as a function of time and proximity to the central fluid source [25] and/or due to phase separation. In fact, most of the fluid inclusions in this stage correspond to Types IIa and IIIa, and display a positive correlation between homogenization temperatures and salinities.…”

Beneficiation of acid mine drainage (AMD): A viable option for the synthesis of goethite, hematite, magnetite, and gypsum – Gearing towards a circular economy concept

“…Furthermore, clean water can also be reclaimed for a wide array of defined uses, including drinking water as specified in different water quality guidelines, specifications and standards (Masindi et al, 2014, Masindi et al, 2019b. Goethite (FeO(OH)), hematite (Fe 2 O 3 ), and magnetite (Fe 3 O 4 ) are commercially valuable minerals which can be synthesized using Fe-rich salts or are formed naturally by geological processes (Akinwekomi et al, 2017, Duuring et al, 2018, Ali et al, 2016. These minerals are used in a number of fields (biomedicine, agriculture, and environment) such as chromatic science, ferro-fluid technology, information and energy storage, biomedicine, and controlled drug delivery, coal washing and as adsorbents.…”