Peter J. Pollard

Contrasting evolution of fluorine- and boron-rich tin systems

Individual Sn provinces or regions within provinces are sometimes enriched in fluorine or boron, giving rise to fluorine-rich and boron-rich environments. The structural styles of mineralisation within these environments are similar except that hydrothermal intrusive breccia pipes are more common in boron-rich environments and apogranite/massive greisen systems are more common in fluorine-rich environments. The increased solubility of H2O in B-bearing magmas compared to F-bearing magmas may play a role in the structural evolution of the mineralising systems. The greater mechanical energy produced during crystallisation of B-rich magmas provides a mechanism for breccia pipe and stockwork formation, while the more passive crystallisation of F-rich magmas often results in the formation of disseminated mineralisation. The partitioning of boron toward the aqueous fluid phase and the enhanced solubility of silica in the fluid phase frequently results in tourmalinisation and silicification of the wall-rocks in B-rich environments. In contrast, feldspathic and sericitic alterations usually predominate in F-rich environments.

Australian Proterozoic Iron Oxide-Cu-Au Deposits: An Overview with New Metallogenic and Exploration Data from the Cloncurry District, Northwest Queensland

Enigmatic hydrothermal vein/breccia/replacement Cu-Au deposits with magnetite and/or hematite are well-represented in Australian 1850 Ma to 1500 Ma terrains and associated with different-aged synorogenic intrusions in the Tennant Creek Block (ca. 1850 Ma); the Gawler-Curnamona region (1640 Ma to 1590 Ma); and the Cloncurry district (Mount Isa Eastern Fold Belt, 1540 Ma to 1500 Ma with a possible earlier event at ca. 1600 Ma). No deposits are known to be coeval with various 1780 Ma to 1610 Ma anorogenic intrusions. Deposits are hosted by many different rock-types with varying metamorphic grade including granites and various supracrustal rocks. Depth of mineralization varied from many kilometers in semiductile crust (e.g., Cloncurry deposits) to very shallow (e.g., Olympic Dam). Ore deposition near Cloncurry occurred in brittle-ductile shear zones from geochemically variable and complex, CO2-rich, 300°C to 500°C, high salinity fluids with magmatic stable isotopic signatures. Recently published studies of a giant granitoidhosted magnetite vein complex at the Lightning Creek prospect (>1000 Mt magnetite) suggest it is a product of internal differentiation and endogenous Fe and Cu-rich hydrous-carbonic fluid phase generation within a quartz monzodiorite-monzogranite intrusion. Coupled with other field relationships, this points to a possible genetic relationship with intermediate (55 to 65 wt% SiO2) members of an alkaline and partly shoshonitic granitoid supersuite which appears to have both mantle and crustal source components from eNd evidence. In constrast, main-phase mineralization at Olympic Dam in the Gawler Craton is distinguished by hematite-phyllosilicate alteration and chalcopyrite-bornite-chalcocite zoning, reflecting fluid mixing in a high level (<250°C) system with a probable large component of meteoric water. Early high-temperature parageneses and fluid inclusions imply that the extensive hematitic breccias overprinted an older magnetite system which may have had similarities with those at Lightning Creek and Ernest Henry in the Cloncurry district. Deposits of this family are inherently difficult to find and evaluate as even within a single district, there is no reliable relationship between the location of ore and any specific combination of geophysical characteristics. Diverse alteration assemblages, geochemistry and physical characteristics suggest the deposits reflect the interaction of multi-sourced fluids with different host rocks in a wide range of geological environments. Recent discoveries and research in the Cloncurry district have extended the range of deposit models available and aid the development of a rudimentary classification in which economic and exploration characteristics can be linked to variations in the mechanisms and environments of ore formation.

Petrology and textural evolution of granites associated with tin and rare-metals mineralization at the Pitinga mine, Amazonas, Brazil

Abstract The Agua Boa and Madeira igneous complexes at the Pitinga mine were emplaced into acid volcanic rocks of the Paleoproterozoic Iricoume Group, and host major tin, rare-metal (Zr, Nb, Ta, Y, REE) and cryolite mineralization. The igneous complexes are elongate NE–SW and each is composed of three major facies that, in order of emplacement, include porphyritic and equigranular rapakivi granite and biotite granite in both igneous complexes, followed by topaz granite in the Agua Boa igneous complex (ABIC) and albite granite in the Madeira igneous complex (MIC). Rapakivi, porphyritic and granophyric textures observed in the granites are interpreted to reflect multiple stages of crystallization at different pressures (depths). Decompression during ascent shifted the magmas into the plagioclase stability field, causing partial resorption of quartz, with subsequent growth at lower pressure. Fluid saturation and separation probably occurred after final emplacement at shallow levels. Temperature and pressure estimates based on phase relations and zircon concentrations range from a maximum of 930 °C and 5 kbar for the rapakivi granites to below 650 °C and 1 kbar for the peralkaline albite granite. This suggests initial crystallization of early intrusive phases at around 15 km depth, with final emplacement of more volatile-rich crystal-mush at a depth of 0.5–1 km. Accessory minerals, including zircon, thorite, monazite, columbite–tantalite, cassiterite, bastnaesite and xenotime are present in almost all facies of the Agua Boa and Madeira igneous complexes, attesting to the highly evolved character of the magmas. The presence of magnetite and/or primary cassiterite indicate crystallization under oxidizing conditions above the NNO buffer. The evolutionary sequence and Nd isotope characteristics ( T DM =2.2–2.4 Ga) of the Pitinga granites are similar to those of other Proterozoic rapakivi granites. However, petrographic, geochemical and Nd isotopic data ( e Nd initial =−2.1 to +0.5) suggest that the different facies of the Pitinga granites were derived from different crustal sources with variable input of a mantle component.

Deformation history of the Naraku Batholith, Mt Isa Inlier, Australia: implications for pluton ages and geometries from structural study of the Dipvale Granodiorite and Levian Granite

Plutons of the Naraku Batholith were emplaced into Proterozoic metasediments of the northern portion of the Eastern Fold Belt of the Mt Isa Inlier during two intrusive episodes approximately 200 million years apart. Structural relationships and geochronological data suggest that the older plutons (ca 1750 Ma) are contemporaneous with granites of the Wonga Batholith to the west. The Dipvale Granodiorite and the Levian Granite represent these older intrusive phases of the Naraku Batholith, and both contain an intense tectonic foliation, S1, which is interpreted to have formed during the north‐south shortening associated with D1 of the Isan Orogeny. The geometry of S1 form surfaces at the southern end of the Dipvale Granodiorite, and of the previously unrecognised sheeted contact, defines a macroscopic, steeply south‐southwest‐plunging antiform, which was produced by the regional D2 of the Isan Orogeny. S1 form surfaces in the Levian Granite define open F2 folds with wavelengths of several hundred metres. The structural age of emplacement of the Dipvale Granodiorite and the Levian Granite is interpreted to be pre‐ or syn‐ the regional D1. An intense foliation present in some of the younger (ca 1505 Ma) granites that comprise the bulk of the Naraku Batholith is interpreted to represent S3 of the Isan Orogeny. Foliations commonly have similar styles and orientations in both the pre‐D1 and younger plutons. This emphasises the simplicity with which regional fabrics can be, and probably have been, miscorrelated in the Eastern Fold Belt, and that the classification of granites in general on the basis of structural and geometric criteria alone is fraught with danger.

Fluid mixing versus unmixing as an ore-forming process in theCloncurry Fe-oxide-CuAu District, NW Queensland, Australia: evidence from fluid inclusions

Abstract Fluid mixing and/or unmixing (including boiling) are thought to be important mechanisms of mineralisation in copper-golddeposits. Detailed fluid-inclusion studies of regional sodic (-calcic) alteration and local mineralisation in the Cloncurry Fe-oxide-Cu Au District, NW Queensland, suggest that both fluid mixing and unmixing occurred in these giant mineralised hydrothermal systems. In some cases, the primary character of coexisting multisolid, hypersaline brine inclusions and CO 2 - or vapour-rich inclusions, the latter crosscut by late Ca- and Na-rich fluid inclusions, indicate that fluid mixing probably occurred subsequent to fluid unmixing and finally resulted in Cu Au mineralisation. However, the relationship between hypersaline brines and CO 2 , which was believed to result from an unmixing of a magma-derived H 2 O CO 2 NaCl ± CaCl 2 fluid (see [Miner. Depos. 36 (2001) 93] and references therein), is rather complex as some hypersaline brine inclusions obviously predate CO 2 inclusions.

Petrogenesis of the Squirrel Hills granite and associated magnetite-rich sill and vein complex: Lightning creek prospect, Cloncurry district, Northwest Queensland

The Lightning creek Fe-oxide Cu–Au prospect is hosted within a Mesoproterozoic granitoid batholith, composed dominantly of high-K, calc-alkaline, porphyritic quartz monzodiorite. The quartz monzodiorite contains enclaves of quartz diorite and has been intruded by more felsic granitoids (monzogranite and alkali-feldspar granite) and a series of subhorizontal sills. The quartz monzodiorite crystallised at crustal depths in excess of 10 km (Pnot, vert, similar4 kb, T=800–850°C, melt XH2O=>4 wt.%, fO2not, vert, similarNNO buffer), and was probably fluid-saturated (XH2O:XCO2 of >0.5). As might be expected, the more fractionated granitoids crystallised at progressively lower temperatures (quartz monzonite not, vert, similar800°C, alkali-feldspar granite 2.5 kb) and temperatures greater than 500°C. Phase separation occurred in the fluid phase, producing in a CO2-rich vapour and a metal-rich (Fe, Cu) brine.

Textural evidence for quartz and feldspar dissolution as a mechanism of formation for Maggs Pipe, Zaaiplaats tin mine, South Africa

Shallowly plunging and branching pipe systems in Lease and Bobbejaankop Granite at the Zaaiplaats mine are host to major tin mineralization. Detailed textural study of Maggs Pipe indicates that dissolution of the granite was a major process in the formation of open space which provided permeability for the passage of hydrothermal fluids, and sites for the precipitation of ore and gangue minerals. The pipe formation process initiates with the dissolution of granite quartz and subsequently extends to feldspar dissolution, particularly in the central portion of Maggs Pipe. Spaces created by mineral dissolution are filled by hydrothermal phases and the relict feldspar matrix becomes progressively more altered toward the centre of the pipe. The distribution of alteration and infill minerals defines a zoning pattern which, from the outer margin to the central core, includes calcite-quartz, chlorite (± cassiterite, albite, fluorite) and synchisite-calcite zones. It is postulated that quartz and feldspar dissolution resulted from interaction between the granite and hydrothermal fluids containing alkali-chloride, -fluoride or -carbonate complexes which had separated from the granite magma during crystallization. Preliminary observations on several other pipes at Zaaiplaats indicate that quartz and feldspar dissolution was a major procress in forming the pipe systems.