Tin- and tantalum-bearing LCT-type granitic pegmatites occur in a 45 km long belt between Eskdale and Mount Wills in north-eastern Victoria. Near Mount Wills, several compositionally zoned rare-element pegmatites contain complex assemblages of primary and secondary phosphate minerals, many of which are rare and previously unrecorded in Victoria. The phosphate assemblages can be divided into Al-rich and Fe–Mn-rich suites, in addition to ubiquitous fluorapatite. The Al-rich phosphate suite includes montebrasite, scorzalite, bertossaite and brazilianite. The Fe‒Mn phosphate suite includes heterosite, phosphoferrite, wolfeite, alluaudite (sp.), arrojadite (sp.) and jahnsite (sp.), derived from the metasomatic alteration of primary triplite. Further hydrothermal alteration of this assemblage has resulted in a secondary suite of strengite, rockbridgeite, phosphosiderite, whiteite, jahnsite and whitmoreite forming in etch cavities and fractures. A Late Silurian age of 420±4 Ma was obtained from one of the dykes via CHIME radiometric dating of monazite, suggesting a similar age for the adjacent Mount Wills Granite, which has not been reliably dated. This highly fractionated, peraluminous granite is presumed to be the source of the rare-element pegmatites based on their close spatial relationship.
Integrated petrological and geochemical examination of metasedimentary assemblages associated with mesothermal lode gold mineralization in five major centers of past and present production (Ballarat, Percydale, Tarnagulla, Maldon, Fosterville) in central Victoria, Australia, reveal that hydrothermal alteration in the hosting Cambro-Ordovician turbidites is much more pronounced and extensive than has previously been recognized. Characteristically, the auriferous reef structures are surrounded by bleached zones up to several tens of meters wide. Sericitization, carbonatization and carbonate spotting, and pyrite and arsenopyrite porphyroblasts—within and beyond zones of visible bleaching—are the most obvious features of alteration. Quantitative XRD analysis illustrates that these marked bleached halos are due to the replacement of albitized plagioclase clasts by white mica and the breakdown of metamorphic chlorite to form secondary carbonates, and to a lesser extent, kaolin. The negative correlation between metamorphic chlorite and hydrothermal carbonate phases provides a reliable mineralogical alteration index useful to exploration. Geochemical reconnaissance profiles also indicate the development of systematic trends in response to hydrothermal alteration involving mainly SiO2, K2O, Na2O, CO2, As, Au, and S at constant bulk rock volume. Calculation of a geochemical alteration index given by the ratio K2O + CO2 / K2O + CO2 + Na2O + Al2O3 suggests that this can be a reliable indicator of alteration, in particular where concentrations of individual invariable oxides such as Al2O3 vary too much in response to lithological variations. Saturation indices that are based on ratios of 3K/Al and CO2/(Mg + Fe + Ca) are less reliable, because muscovite and carbonate are not always the dominant alteration mineral species of relevance.
The alteration assemblages, as well as mineralogical and geochemical trends at the five ore systems investigated herein, are common to many slate belt-hosted gold deposits. The results of this investigation show that, although the detection of such systematic spatial variations relative to mineralization can be complicated, for example by postore modification of the wall rocks due to contact metamorphism (e.g., at Maldon) or where mineralization is not associated with massive to laminated quartz veins (e.g., at Fosterville), recognition of extensive, though not always conspicuous, alteration halos around mesothermal lode gold mineralization represents a potentially powerful tool for exploration in slate belts.
Gold mineralisation in the Lachlan Orogen of western Victoria, is generally hosted in turbidites with very low-grade metamorphic assemblages. Metamorphic data from these turbidites are relatively rare because of the fine-grained nature of the pelitic component and lack of suitable assemblages for thermobarometric estimates. In this study, 'illite crystallinity' (Kübler Index) and b-lattice spacing measurements were carried out on white micas in metapelites, collected from near the inferred western margin of the Selwyn Block, as well as three exploration targets, in an attempt to relate thermal and barometric conditions to mineralisation. Higher-grade (epizone) metamorphic conditions are recorded in sequences west of the Whitelaw Fault and lower-grade (anchizone) metamorphic conditions to the east of the fault. The change from epizonal to anchizonal grade is abrupt, resolved to a distance of a few hundred metres. The b-spacing values change adjacent to the Muckleford Fault. This is due to rocks to the east being exhumed as the edge of the Selwyn Block moved westward during the Middle Devonian Tabberabberan Orogeny at ∼380 Ma. We propose that the juxtaposition of rocks with contrasting thermal and barometric histories represents expression of the upper crustal location of the western margin of the Selwyn Block at the time of peak deformation, at about 440 Ma, and this crustal structure controlled the distribution of the major quartz-vein-type gold deposits. The Middle Devonian orogenic activity (∼380 Ma) was accompanied by the formation of disseminated gold deposits such as Fosterville. This represents a mineralising event that overprints the earlier gold deposits in a corridor at least 50 km wide and to the west of the Whitelaw Fault, that parallels the margin of the Selwyn Block. The correlation between gold assays and 'illite crystallinity' results, from X-ray diffraction and from short-wave infrared-reflectance field-spectroscopy data, were ambiguous. Kübler Indices are not found to be effective in targeting of mineralisation as the values obtained from the alteration and the host-rock assemblages were similar and reflected the ambient P–T conditions at the time of mineralisation.
Differences in oblique overprinting, along‐strike complexity as well as structural, metamorphic and timing constraints suggest that the boundary between the western and central subprovinces of the Lachlan Orogen, currently designated by the Governor Fault, cannot be a single structure. Previously limited data on the nature and kinematics of the fault/shear systems defining the boundary have led to varying scenarios for the tectonic evolution of the Lachlan Orogen. These scenarios either involve large‐scale strike‐slip displacement along the boundary with subsequent overthrusting or convergence of oppositely vergent thrust‐systems with limited strike‐slip translation. Geometrical constraints, fabric chronology and kinematic indicators in both the Mt Wellington (Melbourne Zone) and Governor (Tabberabbera Zone) Fault Zones indicate that maximum displacements relate to thrusting and duplex formation, followed by minor strike‐slip faulting perhaps in response to slightly oblique collision of the Melbourne and Tabberabbera structural zones. Collision of these zones took place between ca 400 and 390 Ma. At Howqua, structural relationships indicate that collision involved northeast‐directed thrusting of the Melbourne Zone (Mt Wellington Fault Zone) over the Tabberabbera Zone (Governor Fault Zone), and was followed by regional, northwest‐trending, open folding. These structures overprint the dominant fabrics and metamorphic assemblages that are interpreted to relate to disruption and underthrusting of Cambrian oceanic/arc crust during closure of a marginal basin. Major deformation in the Tabberabbera Zone took place from ca 445 Ma and was associated with mélange formation, underplating and imbrication or duplexing (Governor Fault Zone, East Howqua segment). At slightly higher crustal levels, and following deposition of Upper Ordovician black shale and chert sequences ( ca 440 Ma), Tabberabbera Zone evolution included offscraping of a serpentinite body (Dolodrook segment) that may have been either a Marianas‐style seamount or transform fault zone within the Cambrian oceanic/arc crust. Major thrusting in the Mt Wellington Fault Zone was underway sometime after ca 420 Ma, and in contrast to the Governor Fault Zone, no mélange or broken formation was produced, metamorphism was at slightly higher temperatures and deformation probably occurred under higher strain states.
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