Brian Marshall

Remobilization, syn-tectonic processes and massive sulphide deposits

Abstract Ductile remobilization by mechanical and chemical processes is most effective in the approximate temperature range of 350°–500°C under prograde and retrograde metamorphism. The relative behaviour of individual phases in mixed assemblages is expressed in terms of the viscosity contrast. A layer comprising a two-phase mixture has an effective viscosity between the limiting viscosities of the discrete phases. Where adjacent layers have different effective viscosities, the composite behaviour reflects the effective-viscosity contrast and obeys the principles of folding. Despite behavioural convergence in mixtures, substantial effective-viscosity contrasts exist between massive sulphide bodies, some carbonate rocks, and most silicate rocks; intense folding and remobilization manifest this. Internal and external ductile remobilization have the capacity to form oreshoots comprising enriched and/or thickened portions of mineralization within or external to other parts of the mineralized system. Internal shoots comprise hinge-zone concentrations, macroboudins and partitions, and elongation geometries. External oreshoots, including those arising from “transposition” and thrusting, fuel the debate on remobilization versus syntectonic emplacement of ore. A conceptual approach demonstrates that remobilization geometry will reflect the geometric relationships of the precursor mineralization and hostrocks, and the degree, extent and nature of the remobilization mechanisms. Dynamic interplay between ductile deformation and advective transfer can produce complex parageneses and microstructures which are indistinguishable from those described for syn-tectonic emplacement. The scope of this problem is illustrated with particular reference to deposits of the Cobar region, Australia. Guidelines are provided for discriminating between remobilized pre-tectonic and syn-tectonic deposits, but doing so is difficult, because a better understanding of the processes which give rise to many traditional pieces of evidence renders the evidence inconclusive. The best available technique requires thorough geometric and kinematic analysis on all scales of observation, coupled with a preparedness to accept polymodal genesis.

Patterns of folding and fold interference in oblique contraction of layered rocks of the inverted Cobar Basin, Australia

Abstract The inverted Cobar Basin, within the Lachlan Fold Belt of New South Wales, Australia, comprises a mid-Palaeozoic cover sequence, originally deposited in a NNW-trending basin. The pattern of F1 folding in the layered cover rocks changes from east to west; from tight well-cleaved folds parallel to the NNW-trending basin margin on the east, to open poorly cleaved en echelon folds at about 35° to the margin, further to the west. The change in fold trend and strain intensity has been repeatedly ascribed to the differing behaviour of discrete zones, decoupled across a north-trending strike-slip fault boundary. New field data show that the changes in orientation and strain intensity of F1 structures are progressively developed, that an abrupt boundary between discrete zones cannot be substantiated, and that interpretations involving decoupled blocks are not supported by the evidence. Conversely, the data require coherent behaviour across the basin, such that the overall pattern of F1 folding must be explained by strain compatible processes. This new interpretation of the F1 deformation pattern has been modelled and quantitatively analysed. Theoretical predictions of the orientation of structures in unlayered isotropic material undergoing oblique contraction are inapplicable to layered anisotropic material. The style of deformation in layered material will reflect the interaction of the bulk strain pattern due to convergence together with the influence of the layering anisotropy. The orientations of the finite strain axes inferred from the folding need not match those of the bulk deformation; the amount of strain recorded by folding may be unrepresentative of that developed in the deformed tract. Oblique contraction at a range of convergence angles was simulated by models employing layers of wet tissue paper. Quantitative analysis of the strain patterns in this layered anisotropic material showed consistent departures from the theoretical predictions for isotropic material. The orientations of the principal finite horizontal extension proximal to the margin yielded higher convergence angles than those which were imposed; the orientations distal from the margin yielded substantially lower apparent convergence angles. This is because the layering anisotropy results in tight folds dissipating the normal component of the oblique convergence vector close to the margin. Whereas more open structures further from the margin show orientations controlled by the progressively more dominant shear component of the vergence vector. Modelling of D1 the Cobar Basin shows that the F1 pattern is consistent with dextral oblique convergence at 60° to the eastern margin of the basin. The deformation patterns, in both the model and the Cobar Basin, yield higher proximal and substantially lower distal apparent convergence angles. This is as expected from theoretical considerations and quantitative analysis of oblique contraction over a range of convergence angles. The rheological anisotropy of the cover sequence of the basin is replicated by that of the layered wet tissue paper. Wet-tissue modelling of the superposition of the second period of deformation (D2) on F1 demonstrates the way in which the tightness and orientation of early folds influence the type of fold interference pattern. At the eastern margin of the Cobar Basin, where D1 was most intense, this resulted in major swings of the strike of bedding and cleavage, and of the trend of F1 folds. Further west, open basin and dome patterns developed where D1 was least intense. Principles developed in relation to the inversion of the Cobar Basin, are equally applicable to other basins in which layered cover rocks have undergone inversion by oblique contraction. Many basins in the Lachlan Fold Belt and in general would fall within this category.

Implications of discrete strain compatibility in multilayer folding

Abstract Theoretical kinematic analysis of idealised folded multilayers shows that, throughout the fold, axial planar stretch of repeated points within the fold must be homogeneous. Layers are effectively pinned at fold limbs such that the rotation of competent layer limbs produces an axial planar stretch. To be strain compatible, this stretch (measured between discrete points) must be accommodated by an equivalent stretch in the fold hinges. Differing modes of accommodation of stretch in the hinge result in fold styles including saddle reef folds, Class 1b/3, Class 1c/3 and Class 2 multilayers. The process of accommodating stretch induced by limb rotation is termed compatible folding. Strain analysis of folds using the compatible folding model predicts significantly less shortening than for other fold flattening models. The model also predicts that Class 2 folds formed by compatible folding will redistribute a pre-existing lineation within a plane. Compatible folding is, therefore, one alternative to the shear folding model for explaining great-circle redistribution patterns of pre-existing linear elements.

Petrography and metamorphism of the Sääksjärvi ultramafic body, Southwest Finland

SummaryThe 1.89 Ga Sääksjärvi ultramafic body, comprising metaperidotite and subordinate amphibolite, is hosted by multiply deformed upper amphibolite to granulite facies rocks. Prograde peak metamorphic (M2) mineral assemblages in the metaperidotite comprise olivine + Ca-amphibole ± orthopyroxene ± Cr-spinel. Ca-amphibole exists as porphyroblastic, finer grained sub-idioblastic, and nematoblastic forms, the latter two overprinting and partly replacing the former. The porphyroblasts crystallized under P-T conditions of at least 650 °C and 5 ± 1 kb, based on solvus-closing in the Ca-amphibole system. On textural evidence, peak metamorphism crossed the Ca-amphibole + olivine (Fo76 – 82) = orthopyroxene (En77 – 81) + water reaction isograd at 700 °C and 4.5 to 5 kb. Metamorphism peaked between the “Mg-chlorite-out” reaction isograd at 720 °C and 5 kb and the slightly higher thermal stability limit of Al-rich “black-wall” chlorite. This accords with the absence of prograde Mg-chlorite and the presence of Al-rich chrome spinel (Al2O3 = 21 to 30 wt%) in the metaperidotite.The amphibolite comprises calcic amphibole with cummingtonite exsolution lamellae, minor plagioclase, quartz, albite and Fe-Mg cummingtonite. Upper greenschist to lower amphibolite facies retrograde conditions (M3) are recorded in amphibolite by: i) re-equilibration of plagioclase to quartz and albite (600 to 620 °C derived from the amphibole-plagioclase geothermometer); ii) 580 to 600 °C obtained from the phase relations of coexisting actinolitic hornblende and cummingtonite; and iii) 610 to 640 °C obtained from spinel-olivine Fe-Mg exchange data in metaperidotite. The range of temperatures is equally consistent with protracted reequilibration of the high-temperature M2, or with a discrete M3 event. In the metaperidotite, porphyroblastic amphibole (M2) partly recrystallized to matrix-grains and nematoblastic amphibole under “peak” retrograde conditions (M3).ZusammenfassungDer 1.89 Ga alte Sääksjärvi Ultramafit umfaßt Metaperidotite und untergeordnet Amphibolite in vielfach deformierten Metamorphiten der oberen Amphibolit- bis Granulit-Fazies. Prograde Mineralassoziationen des Metamorphose-Peaks M2 in den Metaperidotiten umfassen Olivin + Ca-Amphibol ± Orthopyroxen ± Cr-Spinel. Ca-Amphibole kommen als porphyroblastische, feinerkörnige subidioblastische und als nematoblastische Kristalle vor; die letzten beiden überprägen und verdrängen zum Teil die ersteren. Die Porphyroblasten kristallisierten bei P-T Bedingungen von mind. 650 °C und 5 ± 1 Kbar; diese Daten basieren auf der Schließung des Solvus im Ca-Amphibol-System. Texturelle Beobachtungen zeigen, daß die Peak-Metamorphose die Reaktionsisograde Ca-Amphibol + Olivin (Fo76 – 82) Orthopyroxen plus Wasser bei 700 °C und 4.5–5 Kbar überschritten hat. Der Höhepunkt der Metamorphose war zwischen der „Mg-Chlorit-Out” Reaktionsisograde bei 720 °C und 5 Kbar und der etwas höheren thermalen Stabilitätsgrenze von aluminiumreichen „black-wall” Chloriten erreicht. Dies stimmt gut mit dem Fehlen von Mg-Chlorit und der Anwesenheit von Al-reichem Chromspinell (Al2O3 = 21 bis 30 Gew.%) im Metaperidotit überein.Der Amphibolit umfaßt Ca-Amphibole mit Entmischungen von Cummingtonitlamellen, etwas Plagioklas, Quarz, Albit und Fe-Mg Cummingtonit. Hinweise auf retrograde Metamorphose (M3) in der oberen Grünschiefer- bis zur unteren Amphibolit-Fazies werden gegeben durch i) Reequilibrierung von Plagioklas zu Quarz und Albit (600–620 °C, auf Basis des Amphibol-Plagioklas Geothermometers) ii) 580–600 °C auf Basis der Phasenbeziehungen koexistierender Hornblende und Cummingtonit und iii) 610–640 °C auf Basis des Fe-Mg Austausches von Spinel und Olivin im Metaperidotit. Die Temperaturen stimmen entweder mit einer ausgedehnten Reequilibrierung des Hochtemperaturereignisses M2 oder mit einem selbstständigen M3 Metamorphoseereignis überein. Im Metaperidotit rekristallisierte porphyroblastischer Amphibol (M2) teilweise zu Matrixkörnern und zu nematoblastischem Amphibol unter „Peak” retrograden Bedingungen.

Emplacement and implications of peridotite‐hosted leucocratic dykes, Vammala Mine, Finland

Abstract Leucocratic dykes cut the lowermost layer of the Stormi ultramafic complex that hosts the Vammala Ni‐Cu‐Fe sulphide mine. The dykes and associated wallrock alteration zones formed when granitic melt, derived from migmatites during high‐grade amphibolite facies metamorphism, invaded a fracture system within the peridotitic host. The geometry of the fracture system reflects the gross shape of the peridotite body. It is consistent with a mode of formation in which granite‐magma‐driven initiation and propagation were facilitated by contractional cooling. Lack of a secondary fabric overprinting the peridotite and wallrock alteration zones, the high‐grade metamorphic mineralogy in the zones, and the dynamic recrystallization textures of plagioclase in the dykes, are all consistent with peridotite emplacement and leucocratic dyke‐development after formation of the regional gneissosity (in D2) and before or during the ensuing regional event (D3). The same age relationships probably apply to other nickel‐...

A genetic model for the Woodcutters PbZnAg orebodies, Northern Territory, Australia

The Woodcutters PbZnAg base-metal deposit is hosted by an inverted intracontinental-basin sequence of Early Proterozoic age, near Darwin, Australia. The 1800 Ma inversion event (D1) resulted in horizontal upright folds (F1), axial plane cleavage (S1) and extension lineation (Le), and lowermost greenschist facies metamorphism. Northwest-trending left-lateral faults of Late Proterozoic to Cambrian age offset the folds. A mine sequence of five dolomitic carbonate units in dominant slate defines the “3” and “5” anticlines, with the “3” and “5” faults on their eastern limbs. A lamprophyre dyke within the “5” fault is pre-S1; a second is oblique and post-dates S1. The deposit has two main distributions of sulphide: banded PbZnFe massive sulphides which have saddle, inverted wishbone and poddy-shaped forms in the hinge zones of the “3” and “5” anticlines; and laminated (crack-seal) pyrite-rich, vein-type mineralization which partly occupies the “3” and “5” faults. The banded (layered) ore is mainly stratiform and stratigraphically controlled, having mimetically replaced bedding in carbonate-rich units. The fault-hosted mineralization comprises the “3” and “5” “feeder” channels, vertically linking the banded sulphide orebodies. Mine-, meso-, and micro-scale relationships between the sulphides and the dykes, faults and D1 structures, such as folds (F1), S1Le and boudinage, strongly suggest that the ore was emplaced pre- to early syn-D1. There is a compelling case for the ore having been folded, overprinted by cleavage, and locally remobilized. Post-D1 emplacement of ore is disproved, but the possibility of syn-deformational genesis (that is, progressive emplacement of ore during folding and cleavage development) remains. Substantially after syn-S1 remobilization, the ore underwent left-lateral faulting and fault-related remobilization, followed by intrusion of lamprophyre and intrusion-related remobilization. The genetic model involves basin dewatering and channelled flow up deep-reaching faults reactivated by the onset of basin inversion. The ore formed by volume-for-volume replacement of carbonate-rich units and was progressively folded and remobilized during overprinting by cleavage. Woodcutters is a deformed epidiagenetic deposit comprising replacement-ore and attendant “feeder” mineralization.

“Pseudostratigraphy” and thrusting in relation to the structural evolution of the Joma ore-body, North Trøndelag, Norway

Abstract Joma Mine exploits a stratiform CuZn volcanic-associated massive sulphide deposit within the Norwegian Caledonides. The ore layer overlies an inferred feeder system hosted by mafic metavolcanites within the grossly inverted stratigraphy of the Leipik Nappe. Mine-scale inversion is supported by orebody zonation and the relative position of ore layer and feeder system. Mesoscale structural data from regionally distributed quartz-rich phyllites require two major periods (D 2 and D 3 ) and one minor period (D 4 ) of deformation. D 2 generated a transposition foliation containing an elongation lineation essentially co-linear with F 2 hinge lines. D 3 formed a NW -dipping crenulation foliation as hinge surface to SW -plunging F 3 folds. Before F 3 , D 2 linear data probably trended NW in the shallowly W -dipping transposition foliation. D 4 formed minor kinkbands. Mesoscale data from the ore layer affirm the deformation sequence, but also suggest a D 1 thrusting and folding event. F 2 vergence and facing at regional and mine scales further support D 1 . The ore-layer sequence varies between and, less commonly, within layers, and lacks consistent polarity. This variable sequence is ascribed to D 1 and D 2 thrusting, folding and sliding, and is herein termed “pseudostratigraphy”. The existence of level-scale pseudostratigraphy contrasts with preservation of gross relationships at mine scale, and places limitations on the dimensions and consequences of D 1 and D 2 structures. At the end of D 1 , pseudostratigraphy of the thrust-disrupted ore system occupied the lower limb of an E-closing regionally developed antiformal anticline. Folding and sliding during D 2 resulted in the further disrupted ore system being inverted and occupying the lower limb of a large-scale, E-closing, F 2 antiform. In D 3 , the inverted ore layer occupied and was slightly S of the broad hinge zone of the regionally developed NE-closing Joma Synform; its present arcuate geometry reflects this. D 4 had little influence on the structural evolution of the orebody.

Petrogenetic and Tectonic Interpretations of Ultramafic Bodies in the Vammala Nickel Belt, Southwestern Finland, from the Study of the Sääksjärvi Ultramafic-Mafic Complex

Ultramafic/mafic complexes hosting Fe-Ni-Cu mineralization occur as small, lensoidal bodies within the Svecofennian, molasse-like metasedimentary rocks of the Vammala Nickel Belt (VNB) in southwestern Finland. One of them, the Saaksjarvi metaperidotitemetagabbro complex, has been studied to gain a better understanding of their petrogenesis and timing of emplacement. These ultramafic rocks were emplaced before the regional upper-amphibolite-facies metamorphism of the Svecofennian orogeny. They recrystallized to amphibole-dominated assemblages comprising: (1) in metaperidotiteolivine + magnesian hornblende ± chromite ± enstatite ± augite ± phlogopite; (2) in hornblendite-actinolitic hornblende ± augite ± plagioclase ± Fe-Ti oxides; and (3) in metagabbro-actinolitic hornblende + plagioclase ± Fe-Ti oxides ± biotite. The recrystallization was accompanied by changes that involved the formation of a lattice-preferred orientation in olivine and porphyroclastic, poikiloblastic, and equigranular textures. Geochemi...