Feldspar mineralogy and rare-earth element (re)mobilization in iron-oxide copper gold systems from South Australia: a nanoscale study

Nanoscale characterization (TEM on FIB-SEM-prepared foils) was undertaken on feldspars undergoing transformation from early post-magmatic (deuteric) to hydrothermal stages in granites hosting the Olympic Dam Cu-U-Au-Ag deposit, and from the Cu-Au skarn at Hillside within the same iron-oxide copper-gold (IOCG) province, South Australia. These include complex perthitic textures, anomalously Ba-, Fe-, or REE-rich compositions, andREE-flourocarbonate + molybdenite assemblages which pseudomorph pre-existing feldspars. Epitaxial orientations between cryptoperthite (magmatic), patch perthite (dueteric) and replacive albite (hydrothermal) within vein perthite support interface-mediated reactions between pre-existing alkali-feldspars and pervading fluid, irrespective of micro-scale crystal morphology. Such observations are consistent with a coupled dissolution-reprecipitation reaction mechanism, which assists in grain-scale element remobilization via the generation of transient interconnected microporosity. Micro-scale aggregates of hydrothermal hyalophane (Ba-rich K-feldspar), crystallizing within previously albitized areas of andesine, reveal a complex assemblage of calc-silicate, As-bearing fluorapatite and Fe oxides along reaction boundaries in the enclosing albite-sericite assemblage typical of deuteric alteration. Such inclusions are good REE repositories and their presence supports REE remobilization at the grain-scale during early hydrothermal alteration. Iron-metasomatism is recognized by nanoscale maghemite inclusions within ‘red-stained’ orthoclase, as well as by hematite in REE-fluorocarbonates, which reflect broader-scale zonation patterns typical for IOCG systems. Potassium-feldspar from the contact between alkali-granite and skarn at Hillside is characterized by 100–1000 ppm REE, attributable to pervasive nanoscale inclusions of calc-silicates, concentrated along microfractures, or pore-attached. Feldspar replacement by REE-fluorcarbonates at Olympic Dam and nanoscale calc-silicate inclusions in feldspar at Hillside are both strong evidence for the role of feldspars in concentrating REE during intense metasomatism. Differences in mineralogical expression are due to the availability of associated elements. Lattice-scale intergrowths of assemblages indicative of Fe-metasomatism, REE-enrichment and sulfide deposition at Olympic Dam are evidence for a spatial and temporal relationship between these processes

The evolution of apatite in iron-oxide-copper-gold mineralization of the Olympic Cu-Au Province: unraveling magmatic and hydrothermal histories through changes in morphology and trace element chemistry

Iron-oxide-copper-gold (IOCG) mineralization is expressed in various forms across some 700 km of the eastern Gawler Craton throughout the N-S striking Olympic Cu-Au Province. In all instances, IOCG mineralisation and the rocks that host it contain variable concentrations of apatite with varying morphological and chemical characteristics. A large body of work has demonstrated apatite's ability to chemically reflect the physiochemical conditions under which it formed and act as tracers of magmatic and hydrothermal processes. This is confirmed throughout the IOCG deposits and prospects studies as part of this work. Magmatic apatite hosted within the Roxby Downs Granite, the dominant host to the Olympic Dam deposit displays characteristics indicative of a complex magmatic history. Namely, nano-scale, oriented inclusions of pyrrhotite and fluorite within the cores of apatite closely associated with mafic enclaves are indicative of the granites protracted interaction with mafic melts. Their chondrite-normalized rare earth element (REE) fractionation trends are light REE (LREE) enriched and vary when altered by hydrothermal fluids along with the concentrations of several other elements. Magmatic apatite hosted in other intrusives displays similar behaviour when altered, but contains higher concentrations of Cl, Sr, and lower Mn which vary systematically with regards to bulk rock basicity. Many of the deposits and prospects within the Olympic Cu-Au Province exhibit a chemical and mineralogical zoning grading from early, reduced and later, oxidized hydrothermal assemblages as evidenced by changes in the dominant Fe-oxide, Cu-Fe-sulphide species and as we report herein, changes in apatite. Within the early, reduced, high-temperature expressions of IOCG mineralisation throughout the Province, apatite is abundant, making up, alongside magnetite, the bulk of the mineralisation. Such apatite is dominantly near end-member fluorapatite characterized by LREE-enriched chondrite-normalized signatures and variable but measurable concentrations of S and Cl. Overprinting of the magnetite-dominant reduced assemblages by later oxidised hematite-sericite altering fluids results in LREE-loss within the early, hydrothermal and magmatic apatite. Such hematite-sericite altered zones along with newly formed apatite display middle REE (MREE) enriched signatures and are devoid of many of the other trace elements present in magmatic and early hydrothermal apatite, such as S and Cl. This behaviour is observed within the Olympic Dam deposit, as well as the Wirrda Well and Acropolis prospects. Late apatite hosted within the high-grade massive bornite mineralisation of Olympic Dam and within chalcopyrite-barite-rich zones of the Acropolis prospects displays extreme MREE-enriched REE-signatures with positive Eu-anomalies. The latter characteristic is unique amongst all other apatite examined as part of this study and highly anomalous globally. Numerical modeling shows that the evolution in apatite trace elements, and in particular REE signatures is the direct result of fluid evolution within IOCG systems. Given this association, the various assemblages within IOCG systems are classified according to REE-signature and the use of apatite in mineral exploration and as a petrogenetic tool is discussed in detail. The modeling of REE behaviour in hydrothermal fluids typical of IOCG mineralised systems has offered important insights into the transport and deposition of REE within Olympic Dam and possibly other IOCG systems. Specifically, REE are transported primarily as REE-Cl species and deposited under conditions of suppressed REE-Cl activity. The propensity of the LREE to occur as Cl-complexes explains both their significantly greater enrichment in IOCG systems when compared to the HREE, as well as their preferential depletion in apatite during hematite-sericite alteration.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Physical Sciences, 2017

Feldspar mineralogy and rare-earth element (re)mobilization in iron-oxide copper gold systems from South Australia: a nanoscale study

Nanoscale characterization (TEM on FIB-SEM-prepared foils) was undertaken on feldspars undergoing transformation from early post-magmatic (deuteric) to hydrothermal stages in granites hosting the Olympic Dam Cu-U-Au-Ag deposit, and from the Cu-Au skarn at Hillside within the same iron-oxide copper-gold (IOCG) province, South Australia. These include complex perthitic textures, anomalously Ba-, Fe-, or REE-rich compositions, andREE-flourocarbonate + molybdenite assemblages which pseudomorph pre-existing feldspars. Epitaxial orientations between cryptoperthite (magmatic), patch perthite (dueteric) and replacive albite (hydrothermal) within vein perthite support interface-mediated reactions between pre-existing alkali-feldspars and pervading fluid, irrespective of micro-scale crystal morphology. Such observations are consistent with a coupled dissolution-reprecipitation reaction mechanism, which assists in grain-scale element remobilization via the generation of transient interconnected microporosity. Micro-scale aggregates of hydrothermal hyalophane (Ba-rich K-feldspar), crystallizing within previously albitized areas of andesine, reveal a complex assemblage of calc-silicate, As-bearing fluorapatite and Fe oxides along reaction boundaries in the enclosing albite-sericite assemblage typical of deuteric alteration. Such inclusions are good REE repositories and their presence supports REE remobilization at the grain-scale during early hydrothermal alteration. Iron-metasomatism is recognized by nanoscale maghemite inclusions within ‘red-stained’ orthoclase, as well as by hematite in REE-fluorocarbonates, which reflect broader-scale zonation patterns typical for IOCG systems. Potassium-feldspar from the contact between alkali-granite and skarn at Hillside is characterized by 100–1000 ppm REE, attributable to pervasive nanoscale inclusions of calc-silicates, concentrated along microfractures, or pore-attached. Feldspar replacement by REE-fluorcarbonates at Olympic Dam and nanoscale calc-silicate inclusions in feldspar at Hillside are both strong evidence for the role of feldspars in concentrating REE during intense metasomatism. Differences in mineralogical expression are due to the availability of associated elements. Lattice-scale intergrowths of assemblages indicative of Fe-metasomatism, REE-enrichment and sulfide deposition at Olympic Dam are evidence for a spatial and temporal relationship between these processes