The Dorchap Dyke Swarm hosts the first recorded occurrence of lithium–caesium–tantalum (LCT) pegmatites in Victoria, Australia. Syn-orogenic emplacement of pegmatite dykes occurred along a northwest-trending shear system during the Benambran Orogeny. Pegmatites are derived from fractionated melt associated with the Mount Wills Granite, which is an S-type, peraluminous granite originating from supracrustal melting of Ordovician sedimentary sequences. A distinct, eastward-oriented fractionation trend across the Dorchap Dyke Swarm has highlighted a 20 × 8 km highly fractionated zone in the northeastern Dorchap Range, which includes spodumene- and petalite-bearing pegmatites. A distinct pattern of elemental enrichment (P > Cs > Be > Nb ≥ Ta > Li > Sn) is observed across the strongly fractionated zone of the Dorchap Dyke Swarm. Subsequent metasomatic fluid movements and hydrothermal overprinting have resulted in redistribution of mobile elements in the Dorchap Range, either as hydrothermal alteration species or in some instances as the development of exomorphic halos. Additionally, a regional alteration overprint, likely associated with subsequent metamorphism of pegmatite dykes has resulted in alteration of primary petalite to spodumene plus quartz, and primary spodumene to cookeite. Bulk rock geochemical data from the Dorchap Dyke Swarm suggest a syn-collisional setting for dyke intrusion, consistent with the inferred tectonic setting of the central Lachlan Fold Belt at the time of pegmatite emplacement. Pegmatite dykes locally contain an overprinted structural foliation, which is consistent with the primary structural trend of deformed metasediments and may indicate that dyke emplacement was syngenetic with regional folding and compression of the surrounding Omeo Metamorphic Complex during the Benambran Orogeny. Subsequent hydrothermal alteration of some dykes likely occurred immediately following the Bindian Orogeny.KEY POINTSDorchap Dyke Swarm are the first recorded Li–Cs–Ta pegmatites in Victoria, Australia.Pegmatites were emplaced synchronous with or immediately following the Benambran Orogeny.Dorchap Dyke Swarm pegmatites are geochemically correlated with the Mount Wills Granite. Dorchap Dyke Swarm are the first recorded Li–Cs–Ta pegmatites in Victoria, Australia. Pegmatites were emplaced synchronous with or immediately following the Benambran Orogeny. Dorchap Dyke Swarm pegmatites are geochemically correlated with the Mount Wills Granite.
In tropical climates, postdrilling oxidation of sulfide-rich core can severely degrade drill core, producing low-temperature iron oxyhydroxides, sulfates, and clays. Variable growth of these secondary minerals in exposed drill core, combined with the hydration and degradation of primary hydrothermal minerals, may lead to the production of spurious results in near-infrared (NIR) spectroscopic studies. However, the NIR technique can remain an effective tool in assessing hydrothermal alteration, even in extremely degraded core. We have assessed the usefulness of the NIR technique on degraded core at the Ladolam gold deposit, Papua New Guinea. Here, we seek to determine whether the primary alteration mineralogy had been significantly transformed by postdrilling oxidation over several years of weathering. In doing so, the study tested whether NIR analysis can be an effective tool in the discrimination of primary hydrothermal minerals in degraded core. Our study was made possible using semiquantitative X-ray diffraction (QXRD) analyses of a drill hole in 2004, where samples were collected at 50-m intervals. We subsequently repeated NIR and QXRD analyses on the same drill core in 2012. After nine years of storage, the drill core had degraded considerably, with the growth of jarosite and other sulfates. Despite this, XRD results from 2004 and 2012 show no major differences in the primary alteration mineralogy. Closely spaced NIR analyses were conducted at 1-m intervals to increase the chance of obtaining a spectrum of the primary mineralogy and to exclude secondary oxidation minerals. The drill core, where possible, was broken immediately prior to analysis to obtain a fresh surface. On average, over a 10-m interval, approximately 25% of the NIR spectra did not contain secondary minerals and relict primary alteration minerals could be detected. The remaining spectra were affected by the occurrence of secondary jarosite, gypsum, and/or residual water, but in most cases, the primary alteration mineralogy could be determined. We conclude that NIR analyses remain an effective tool in the construction of geological deposit models when logging degraded historic core, even for sulfide-rich core that has degraded in tropical environments.
Turbidite-hosted orogenic Au deposits are commonly enriched in W, along with a variety of other trace elements. A mineralogical source for W has recently been shown in the Otago Schist of southern New Zealand (Cave et al. 2016), with detrital rutile in the metasedimentary rocks recrystallizing to metamorphic titanite and making W available to be mobilized from the rock mass. In this study, we investigate the availability of W through prograde metamorphic mineral recrystallization in two additional turbidite-hosted orogenic Au provinces, one containing orogenic Au mineralization with associated subordinate W (Meguma Terrane, Canada), and the other containing orogenic Au mineralization without associated W (Bendigo-Ballarat Terrane, Australia). This was undertaken to assess whether W availability during prograde metamorphism is a key process in controlling the presence of W in turbidite-hosted orogenic Au mineralization. Like the Otago Schist, in both terranes detrital rutile is identified as being the most important host mineral for W in the lowest metamorphic grade rocks, and its prograde metamorphic recrystallization (to ilmenite) makes significant amounts of W available for mobilization (0.65 and 1.85 g of W per tonne of rock from the Goldenville and Halifax groups of the Meguma Terrane, respectively, and 0.16 g of W per tonne of rock from the Castlemaine Group of the Bendigo-Ballarat Terrane). This release of W in the Meguma Terrane is likely the source of W in these orogenic Au deposits. The lack of W in the orogenic Au deposits of the Bendigo-Ballarat Terrane suggests that W availability is not the only process controlling the presence of W minerals in turbidite-hosted orogenic Au mineralization. Alternatively, it might reflect a lower greenschist facies metasedimentary (Castlemaine Group) source for these deposits (i.e., a lower metamorphic grade source than the rutile to ilmenite conversion), as has been previously suggested.
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.
The Dorchap Dyke Swarm hosts the first recorded occurrence of lithium–caesium–tantalum (LCT) pegmatites in Victoria, Australia. Syn-orogenic emplacement of pegmatite dykes occurred along a northwest-trending shear system during the Benambran Orogeny. Pegmatites are derived from fractionated melt associated with the Mount Wills Granite, which is an S-type, peraluminous granite originating from supracrustal melting of Ordovician sedimentary sequences. A distinct, eastward-oriented fractionation trend across the Dorchap Dyke Swarm has highlighted a 20 × 8 km highly fractionated zone in the northeastern Dorchap Range, which includes spodumene- and petalite-bearing pegmatites. A distinct pattern of elemental enrichment (P > Cs > Be > Nb ≥ Ta > Li > Sn) is observed across the strongly fractionated zone of the Dorchap Dyke Swarm. Subsequent metasomatic fluid movements and hydrothermal overprinting have resulted in redistribution of mobile elements in the Dorchap Range, either as hydrothermal alteration species or in some instances as the development of exomorphic halos. Additionally, a regional alteration overprint, likely associated with subsequent metamorphism of pegmatite dykes has resulted in alteration of primary petalite to spodumene plus quartz, and primary spodumene to cookeite. Bulk rock geochemical data from the Dorchap Dyke Swarm suggest a syn-collisional setting for dyke intrusion, consistent with the inferred tectonic setting of the central Lachlan Fold Belt at the time of pegmatite emplacement. Pegmatite dykes locally contain an overprinted structural foliation, which is consistent with the primary structural trend of deformed metasediments and may indicate that dyke emplacement was syngenetic with regional folding and compression of the surrounding Omeo Metamorphic Complex during the Benambran Orogeny. Subsequent hydrothermal alteration of some dykes likely occurred immediately following the Bindian Orogeny.KEY POINTSDorchap Dyke Swarm are the first recorded Li–Cs–Ta pegmatites in Victoria, Australia.Pegmatites were emplaced synchronous with or immediately following the Benambran Orogeny.Dorchap Dyke Swarm pegmatites are geochemically correlated with the Mount Wills Granite.
IC and b spacing values of white micas in slates from the Tabberabbera Zone have been used to assess the P and T conditions of metamorphism. IC values range from 0.19 to 0.34, with upper anchizone conditions (T = 250–300° C) over much of the zone, and a belt of epizone rocks (T > 300° C) along the eastern edge of the zone. Average b spacing is 9.023 ± 0.006 ¬, with no discernible trends across the zone. This value is typical of medium-P metamorphic belts, with a geothermal gradient of about 30° C/km. For a temperature of 300° C, this equates to a pressure of ~0.3 GPa, and a depth of about 10 km, the same as migmatites of the Omeo Zone, adjacent to the east. The medium-P metamorphism is difficult to fit to an accretionary prism origin for the zone, unless an active spreading ridge was subducted under it.
Detailed b lattice parameter and illite crystallinity (IC) studies of K‐white micas in slates from the Stawell and Ballarat‐Bendigo Zones (SZ, BBZ) in the western Lachlan Fold Belt of Victoria, Australia, reveal a metamorphic pattern characterized by regional metamorphism associated with crustal thickening and younger contact metamorphism accompanied by deformation. The IC data indicate that rocks regionally metamorphosed prior to the intrusion of the Early and Late Devonian granitoids, vary in grade from epizonal (greenschist facies) to diagenetic (zeolite facies) and that most are of epizonal to anchizonal (prehnite–pumpellyite facies) grade. In the BBZ, a decrease in grade from west to east occurs. Across fault zones, IC values show little change, indicating that limited vertical displacement has occurred. This is in accord with the thin skinned deformation model proposed for the western Lachlan Fold Belt. The b lattice parameters (x=9.022 Å; n =137; σ n =0.009) indicate baric conditions intermediate between those of New Hampshire ( P =Al 2 SiO 5 triple point) and Otago (intermediate P ). Thus, a moderately low geothermal gradient existed 450–430 Ma ago, when these rocks were deformed. K D Fe/Mg (actinolite)/Fe/Mg (chlorite) values (0.52–0.70) obtained from coexisting actinolite and chlorite in metabasites from fault zones support the moderately high‐ P ( c . 4 kbar) metamorphism suggested by the b cell parameter values. The metamorphic conditions indicated by these data are contrary to the low‐ P /high ‐T conditions proposed by previous authors, who inferred an intimate association between deformation, granitoid intrusion and gold mineralization. The b lattice parameter of white micas in slates adjacent to Early Devonian ( c . 400 Ma) granitoids with schist bearing aureoles in the north‐eastern part of the BBZ (x=9.002 Å; n=27; σ n =0.007), indicate pressures in the order of c . 2.5 kbar which are in accord with those obtained from andalusite–cordierite and zoisite–garnet bearing assemblages observed in the higher grade metapelitic and calcareous rocks. This contrasts with the higher pressure ( c . 4 kbar) existing during regional metamorphism and implies that c . 6.5–8 km of metasedimentary rocks in the BBZ were removed before the emplacement of the Early Devonian granitoids. Metamorphic assemblages in hornfelses associated with Late Devonian granitoids indicate a further 5–6 km of metasediment were removed in the next 40 Ma prior to their emplacement. This study shows the value of white mica studies in elucidating the tectonothermal history of a low‐grade metamorphic terrane dominated by metapelitic rocks.
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