Paul Duuring

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

Abstract The generation of the Earths continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)-hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Structural evolution of the Agnew–Wiluna greenstone belt, Eastern Yilgarn Craton and implications for komatiite-hosted Ni sulfide exploration

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the worlds largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.

A mineral system approach to iron ore in Archaean and Palaeoproterozoic BIF of Western Australia

Abstract This review paper examines banded iron formation-hosted higher-grade (>58 wt% Fe) iron ore types present in the two main metallogenic districts of Western Australia, the Yilgarn Craton and the Hamersley Province. The principal iron ore deposits from both districts exhibit variation in ore properties and genesis within and across districts, but also striking similarities. There are five critical elements involved in iron ore formation and preservation: (a) BIF iron fertility defined by stratigraphic and geodynamic setting; (b) Si-dissolving fluid flow; (c) high permeability at a range of scales; (d) exhumation and supergene modification; and (e) preservation of BIF-hosted iron ore bodies by surficial modification, cover or structures (downdrop, overthrust). Several subsidiary or constituent processes are important for the formation of distinct iron ore types and have expressions as (mappable) targeting elements. Deposits in the Hamersley Province record the presence of basinal brines and meteoric fluids, whereas deposits in the Yilgarn Craton, while less well constrained, suggest the influence of metamorphic/magmatic and meteoric fluids. A scheme for BIF alteration related to ore formation in a crustal depth continuum is presented, which integrates pressure-/temperature-dependency of assemblages, fluid–rock ratios and Si-dissolution capability and is a conceptual guide to prospective zones for iron ore.

Hydrothermal alteration events controlling magnetite-rich iron ore at the Matthew Ridge prospect, Jack Hills greenstone belt, Yilgarn Craton

Archean Banded Iron Formation (BIF) hosts high-grade (>55% Fe) iron ore at the Matthew Ridge prospect, in the Jack Hills greenstone belt, of the Narryer terrane, Yilgarn Craton. The ca 3000 Ma (SHRIMP U–Pb on zircons) Algoma-type BIF contains magnetite–hematite ore zones that are a product of successive overprinting hydrothermal alteration events. Rock types in the prospect area include Archean gneiss, metasedimentary rocks and BIF that are interlayered with dolerite. All rocks record peak amphibolite metamorphic mineral assemblages that are variably replaced by greenschist facies metamorphic minerals. Hypogene iron ore formed owing to: (1) the replacement of primary silica-rich bands in the quartz–magnetite BIF by Stage 1 hypogene magnesite and magnetite alteration; and (2) subsequent removal of Stage 1 magnesite to concentrate residual Stage 1 magnetite. These magnetite-rich ore bodies define <50 m-long by 20 m-wide lenses that trend NE and coincide with the hinge zones of Z-shaped, tight F1 folds, which are most likely parasitic folds to the regional NE-trending, steeply NE-plunging anticline in the Jack Hills greenstone belt. Magnetite-rich ore zones are enriched in Fe and depleted in SiO2, with only minor changes in other major oxides and trace elements, compared with least-altered BIF. Magnetite-rich ore zones are locally cut by shear zone-hosted, talc–magnetite hydrothermal alteration overprinted by talc–microplaty hematite alteration. The shear zones and both talc mineral assemblages formed during a second deformation event that was characterised by NNE–SSE shortening. The third deformation event resulted in at least two generations of NE- to NW-trending extensional veins and strike-slip faults that locally cut the talc-rich shear zones. Supergene goethite–hematite replaces magnetite-rich ore bodies within 80 m of the present surface. The strong structural control and distinct hydrothermal alteration assemblages of the Matthew Ridge prospect are the best exploration indices for high-grade magnetite-rich ore in the region.

Genesis history of iron ore from Mesoarchean BIF at the Wodgina mine, Western Australia

ABSTRACT Several iron-ore deposits hosted within Mesoarchean banded iron formations (BIFs) are mined throughout the North Pilbara Craton, Western Australia. Among these, significant goethite±martite deposits (total resources >50 Mt at 55.8 wt% Fe) are distributed in the Wodgina district within 2 km of the world-class pegmatite-hosted, tantalum Wodgina deposits. In this study, we investigate the dominant controls on iron mineralisation at Wodgina and test the potential role of felsic magma-derived fluids in early alteration and upgrade of nearby BIF units. Camp-scale distribution and geochemistry of iron ore at Wodgina argue against any significant influence of identified felsic intrusions in the upgrade of BIF. Whereas, the formation of BIF-hosted goethite±martite iron ore at Wodgina involves: (i) early (ca 2950 Ma) metamorphism of BIF causing camp-scale recrystallisation of pre-existing iron oxides to form euhedral magnetite, with local enrichment to sub-economic grades (∼40 wt% Fe) within or proximal to metre-wide, bedding-parallel shear zones, and (ii) later supergene lateritic enrichment of the magnetite-bearing BIF and shear zones, forming near-surface goethite±martite ore. The supergene alteration sequence includes: (i) downward progression of the oxidation front and replacement of magnetite by martite, (ii) local development of silcrete at ∼40 m below the modern surface caused by the lowering of the water-table, (iii) intensive replacement of quartz by goethite, resulting in the goethite±martite ore bodies at Wodgina, and (iv) late formation of ferricrete and ochreous goethite. Goethitisation most likely took place within the hot and very wet climate that prevailed from the Paleocene to the mid-Eocene. Goethite precipitation was accompanied by the incorporation of trace elements P, Zn, As, Ni and Co, which were likely derived from supergene fluid interaction with nearby shales. Enrichment of these elements in goethite-rich ore indicates that they are potentially useful pathfinder elements for concealed ore bodies covered by trace element-depleted pedogenic silcrete and siliciclastic rocks located throughout the Wodgina mine.

Learning characteristic natural gamma shale marker signatures in iron ore deposits

Abstract Uncertainty in the location of stratigraphic boundaries in stratiform deposits has a direct impact on the uncertainty of resource estimates. The interpretation of stratigraphic boundaries in banded iron formation (BIF)-hosted deposits in the Hamersley province of Western Australia is made by recognizing shale markers which have characteristic signatures from natural gamma wireline logs. This paper presents a novel application of a probabilistic sequential model, named a continuous profile model, which is capable of jointly modelling the uncertainty in the amplitude and alignment of characteristic signatures. We demonstrate the accuracy of this approach by comparing three models that incorporate varying intensities of distortion and alignment in their ability to correctly identify a shale band of the West Angelas member of the Wittenoom Formation which overlies the Marra Mamba Iron Formation in the Hamersley Basin. Our experiments show that the proposed approach recovers 98.72% of interpreted shale band intervals and importantly quantifies the uncertainty in scale and alignment that contribute to probabilistic interpretations of stratigraphic boundaries.