This post is a summary of the interesting paper on the geodynamic relationships  between Paleozoic arc development along the flanks of the interior (e.g. the Iapetus and Rheic) oceans and the exterior Paleopacific Ocean. Murphy, B., van Staal, C and Collins, W, A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens: Speculations on geodynamic correlations, Gondwana Research, 2011. doi:10.1016/j.gr.2010.11.019.

Summary

• This paper discusses the geodynamic relationships  between Paleozoic arc development along the flanks of the interior (e.g. the Iapetus and Rheic) oceans and the exterior Paleopacific Ocean.
• Paleozoic arcs in the Iapetus and Rheic oceanic realms are preserved in the Appalachian–Caledonide and Variscan orogens, and in the Paleopacific Ocean realm they are preserved in the Terra Australis Orogen.
• Paleocontinental reconstructions show Cambrian–Early Ordovician contraction of the exterior ocean as the interior oceans expanded and subsequent Paleozoic expansion of the exterior oceans while the interior oceans contracted.

• Subduction initiated in the eastern segment of Iapetus at ca. 515 Ma and Early to Middle Ordovician orogenesis along the flanks of this ocean is highlighted by arc–continent collisions and ophiolite obductions.

• Over a similar time interval, subduction and orogenesis took place in the exterior ocean and included formation of the Macquarie arc in the Tasmanides of Eastern Australia and the Famatina arc and correlatives in the periphery of the proto-Andean margin of Gondwana and correlatives in the periphery of the proto-Andean margin of Gondwana
• Major changes in the style of subduction (from retreating to advancing) in interior oceans occurred during the Silurian, following accretion of the peri-Gondwanan terranes and Baltica, and closure of the northeastern segment of Iapetus.

• During the same time interval, subduction in the Paleopacific Ocean was predominantly in a retreating mode, although intermittent episodes of contraction closed major marginal basins.
• Major disturbances in the Earth tectonic systems during the Ordovician, including an unprecedented rise in marine life diversity, as well as significant fluctuations in sea level, atmospheric CO2, and 87Sr/86Sr and 13C in marine strata carbonates.
• Stable and radiogenic isotopic data provide evidence for the addition of abundant mantle-derived magma, fluids and large mineral deposits that have a significant mantle-derived component.
• The authors speculate that the emergence of a superplume triggered by slab avalanche events within the Iapetus and Paleopacific oceans was associated with the establishment of a new geoid high within the Paleopacific tectonic regime.
• Closure of the interior Rheic Ocean and the amalgamation of Laurussia and Gondwana was a key event in the Late Carboniferous amalgamation of Pangea.

Introduction

In the classical Wilson cycle, oceanic crust generated during supercontinent breakup is consumed during subsequent amalgamation so that the supercontinent turns “inside in” (introversion). Alternatively, following supercontinent breakup, the exterior margins of the dispersing continental fragments collide during reassembly so that the super-continent turns “outside in” (extraversion).

Irrespective of whether a supercontinent forms by introversion or extroversion, orogenic activity attending the assembly and amalgamation of a supercontinent typically occurs by subduction-related then collisional orogenesis, with the resulting orogenic belts occurring in the interior of the supercontinent, known as interior orogens. Elimination of subduction zones between the colliding blocks results in their relocation to the periphery of the supercontinent, resulting in peripheral orogens, a general term to describe all types of activity along the periphery of supercontinents, including orogenesis due to subduction and accretionary processes.

The final breakup of the supercontinent Rodinia between 650 and 540 Ma is a manifestation of fundamental changes to global plate motions during the Late Neoproterozoic New oceanic lithosphere developed between the diverging plates leading to the development of interior oceans. At about the same time (Early Cambrian), Gondwana became completely amalgamated and subduction commenced in the exterior (paleo-Pacific) ocean, evidence of which is preserved in the 18,000 km long Terra Australis orogen (TAO).

By the end of the Early Cambrian (ca. 515 Ma), subduction had commenced in the interior Iapetus Ocean along both of its margins. Although punctuated by obduction of oceanic lithosphere and several episodes of terrane accretion, subduction continued more-or-less continuously in the interior oceans throughout the remainder of the Paleozoic, culminating in Late Paleozoic terminal collision and the amalgamation of Pangea. Similarly, the TAO preserves evidence of subduction adjacent to the flanks of the exterior paleo-Pacific ocean. Beginning locally at ca. 580 Ma, subduction was established along the entire length of the TAO by 550 Ma and continued until ca. 230 Ma

In the classical Wilson cycle, oceanic crust generated during supercontinent breakup is consumed during subsequent amalgamation so that the supercontinent turns “inside in” Alternatively, following supercontinent breakup, the exterior margins of the dispersing continental fragments collide during reassembly so that the super-continent turns “outside in” Irrespective of whether a supercontinent forms by introversion or extroversion, orogenic activity attending the assembly and amalgamation of a supercontinent typically occurs by subduction-related then collisional orogenesis, with the resulting orogenic belts occurring in the interior of the supercontinent, known as interior orogens. Elimination of subduction zones between the colliding blocks results in their relocation to the periphery of the supercontinent, resulting in peripheral orogens, a general term to describe all types of activity along the periphery of supercontinents, including orogenesis due to subduction and accretionary processes.

The final breakup of the supercontinent Rodinia between 650 and 540 Ma is a manifestation of fundamental changes to global plate motions during the Late Neoproterozoic New oceanic lithosphere developed between the diverging plates leading to the development of interior oceans. At about the same time (Early Cambrian), Gondwana became completely amalgamated and subduction commenced in the exterior (paleo-Pacific) ocean, evidence of which is preserved in the 18,000 km long Terra Australis orogen.

By the end of the Early Cambrian (ca. 515 Ma), subduction had commenced in the interior Iapetus Ocean along both of its margins. Although punctuated by obduction of oceanic lithosphere and several episodes of terrane accretion, subduction continued more-or-less continuously in the interior oceans throughout the remainder of the Paleozoic, culminating in Late Paleozoic terminal collision and the amalgamation of Pangea. Similarly, the TAO preserves evidence of subduction adjacent to the flanks of the exterior paleo-Pacific ocean. Beginning locally at ca. 580 Ma, subduction was established along the entire length of the TAO by 550 Ma and continued until ca. 230 Ma.

References

Murphy, B., van Staal, C and Collins, W, A comparison of the evolution of arc complexes in Paleozoic interior and peripheral orogens: Speculations on geodynamic correlations, Gondwana Research, 2011. doi:10.1016/j.gr.2010.11.019.

High-sulphidation deposits are formed by the interaction of gases from magma with ground water. They are frequently closely related to or even overlap large mineralized intrusive bodies.

The interaction with ground water creates strong acids which dissolve the surrounding rock, leaving behind a sponge-like formation of porous vuggy silica. Gold and copper-rich brines can then precipitate their metals within the permeable silica bodies. The shape of the deposits is determined by the distribution of silica, which can be widespread if the acids interact with a permeable rock unit like a volcanic breccia or tuff.  If the host rocks have low natural permeability then the deposits may be restricted to or close to the structure providing access for fluids from depth.

The acidic fluids are progressively neutralized (buffered) by the host rock. The rocks in turn are altered by the fluids into progressively more stable minerals the further away from the fault. As a result, definable zones of alteration minerals.  Typically the sequence is to move from vuggy silica close to the main structure progressing through quartz-alunite to kaolinite-dickite,  to illite and to chlorite rich rock.  Alunite, kaolinite, dickite and illite are generally whitish to yellowish in color. The clay and sulphate alteration can cover hundreds of square kilometres.

We have compiled a collection of images of vuggy silica from high-sulphidation deposits around the world. These images are not well attributed and we will seek to redress this with your help. If you would like to contribute to this compendium with images from your deposits we would be most happy to provide full attribution and your co-operation will be greatly appreciated by your geoscience colleagues.

This post is a review and summary of an excellent paper: Bradley Cave, Richard Lilly, Alexander Simpson, Lucy McGee, A revised model for the George Fisher and Hilton Zn-Pb-Ag deposits, NW Queensland: Insights from the geology, age and alteration of the local dolerite dykes, Ore Geology Reviews 154 (2023) 105311. https://doi.org/10.1016/j.oregeorev.2023.105311

SUMMARY

1. The George Fisher and Hilton Zn-Pb-Ag deposits are located approximately 20 km north of Mount Isa.
2. A dolerite dyke has been discovered at the George Fisher Zn-Pb-Ag deposit and the dolerite dyke have a large spike in TiO2 and V values.
3. In drill core, the dolerite dykes occur as a light grey to grey-brown coloured rock that is commonly overprinted along its margins by Zn-Pb-Ag mineralisation.
4. In-situ U-Pb geochronology performed on igneous apatite produce a lower intercept age of 1611 +/- 21 Ma and 1619 +/- 22 Ma for the dolerite dykes at the George Fisher and Hilton deposits, respectively.
5. The dolerite dykes have experienced multiple stages of post-emplacement hydrothermal alteration/veining.
6. Monazite from a quartz-albite-K-feldspar vein in the Hilton dyke produces a lower intercept age of 1513 +/- 16 Ma.
7. To assess the timing of alteration in the adjacent George Fisher Zn-Pb-Ag deposit, in-situ Lu-Hf geochronology was performed on pre-mineralisation calcite from a section of stratabound Zn-Pb-Ag mineralisation, and a paragenetically late cross-cutting sphalerite-calcite vein.
8. Calcite from the pre-mineralisation alteration assemblage produces a Lu-Hf age of 1501 +/- 32 Ma.
9. Calcite from a cross-cutting vein that post-dates Zn-Pb-Ag deposition produced a Lu-Hf age of 1289 +/- 26 Ma. The 1289 Ma age is associated with late faulting and movement along the adjacent Mount Isa Fault (B. Cave pers comms 2023).
10. The paragenetic equivalents of the hydrothermal alteration/veining in the dolerite dykes are found in the adjacent Zn-Pb-Ag orebodies.
11. The maximum age of alteration within the dolerite dykes is constrained by the monazite age of 1513 +/- 16 Ma, and the maximum age of stratabound Zn-Pb-Ag mineralisation is constrained by the Lu-Hf age of 1501 +/- 32 Ma.
12. The dolerite dykes intruded during the early Isan Orogeny at ca. 1620 Ma, and experienced subsequent hydrothermal alteration during D3 of the Isan Orogeny coeval with Zn-Pb-Ag mineralisation. Post mineralisation faulting occurred during D2 of the Isan Orogeny, at ca. 1290 Ma.

Implications

The change of metallogenic model for the George Fisher and Hilton Zn-Pb-Ag deposits has significant implications to local and regional-scale exploration. Previously the emphasis had been on rocks of a highly carbonaceous nature adjacent to a major crustal structure. Brad Cave et. al. would suggest that structure is more important than host lithology, however I would disagree. The dominance of major metal deposits within the Urquhart Shale suggests that the reductive nature of these highly carbonaceous rocks played an important role in the evolution of these mineral deposits. If you combine a major crustal structure for fluid egress and a reactive lithology you must enhance the opportunity for creation of a major metal deposit – we sure see it elsewhere. Is it possible that the reason that the the Mt Isa-George Fisher deposits have no associated magnetitie, hematite or related magnetic anomalies is that the fluids were buffered by the carbonaceous shales?

Brad Cave (pers comms, 2023) commented, “Magnetite is not reported in these deposits often, but there is a fair amount there. Adjacent the hanging wall at Hilton there is a lot of magnetite associated with high Cu and Pb grades. At Isa, there are large zones associated with the presence of magnetite and biotite. Similarly at Mount Novit, there is also magnetite adjacent to Zn-Pb-Ag ore. However, we haven’t seen any at the George Fisher deposit yet”. 

In the Mt Isa region (see the image below) the Upper Soldier Cap Group to the east of the Mt Margaret Fault consists of strongly carbonaceous sediments and mafic volcanics and intrusives of the Toole Creek Volcanics. These rocks were deposited in a deep water environment and are the same age as the Urquhart Shale.

The Continental Copper acreage, shown below, covers 50km of this geology and is coincident with:

• A pronounced magnetotelluric anomaly which has been modelled as a pipe-line body extending into the mid-crust. This appears to be coincident with the Gidyea Suture, the Proterozoic collision zone between the North Australian Craton to the west and the Laurentia Craton to the east,
• Very high Cu-Pb and Zn geochemistry within a regional Eh anomaly along the eastern side of the Mt Margaret Fault,
• Lead isotope geochemistry indicating a Proterozoic lead-source and,
• A strong untested EM conductor.

Introduction

Over time there has been considerable controversy regarding the timing of mineralization at Mt Isa.  The earliest geological investigators concluded that the mineralization was generated during deformation and subsequent to deposition of the host Urquhart Shale.  Then, despite the overwhelming geological and observational evidence, the syngeneticists arrived. Modern geochemistry, geochronology and geology has conclusively supported the earliest geologist’s contention.  Bradley Cave and colleagues have published conclusive data on the large George Fisher and Hilton deposits confirming similar timing of Zn-Pb-Ag mineralization to that at Mt Isa.

The Hilton (12.2 Mt @ 6.4 % Zn, 4.6 % Pb and 102 g/t Ag) and George Fisher (51 Mt @ 7.4 % Zn, 3.4 % Pb and 55 g/t Ag) Zn-Pb-Ag deposits are located in the Western Fold Belt of the Mount Isa Inlier, 20 km north of the Mount Isa Cu-Zn-Pb-Ag deposit.

The Syn-sedimentary Model

Earlier works proposed a syn-sedimentary metallogenic model for Zn-Pb-Ag mineralisation, with mineralisation occurring during, or slightly after the deposition of the host Urquhart Shale at ca. 1655 Ma. If a syn-sedimentary metallogenic model is used as a basis for exploration, reduced Proterozoic shales of the Isa Superbasin that are located adjacent to major syn-sedimentary normal or strike-slip faults are deemed the most suitable host for economic Zn-Pb-Ag mineralisation.

The Syn-deformational Model

In contrast, a syn-deformation metallogenic model has also been proposed for these deposits, which favors formation of Zn-Pb-Ag mineralisation during late-stage deformation (ca. 1520 Ma) of the Isan Orogeny. If a syn-deformational metallogenic model is favored, sites of increased structural heterogeneity with localised dilation and fracturing that were active, or formed during the late stages of the (1620–1500 Ma) Isan Orogeny are deemed the most suitable locations to host economic Zn-Pb-Ag mineralisation.

At the Hilton Zn-Pb-Ag deposit, a N–S trending dolerite dyke has previously been described and occurs along the deposit-scale Dyke Trace Fault. A previous investigation into the dolerite dyke concluded that it intruded during D2 deformation of the Isan Orogeny, and was interpreted to post-date the formation of Zn-Pb-Ag mineralisation. Cave et al provide the geological, geochemical and isotope data to support this contention. 

Here is an excellent paper from Richard Henley et al that provides support for the argument that potassic alteration is largely isochemcial and and not an introduced component. Henley has elequently defined the porphyry copper environment as that of a dynamic, internally and externally stressed, gas phase reactor where repetitive fracturing generates high permeability flow paths for expansion of the magmatic gas phase from source to surface.

SUMMARY of the Abstract

• Potassium silicate alteration is a hallmark of porphyry copper deposits, which provide two thirds of the world’s annual copper demand
• These deposits form in the cores of calc-alkaline to alkaline volcanic systems from the flux of magmatic gas that transports copper and other metals from source to surface
• The Grasberg Cu-Au deposit is a giant deposit that formed within a maar-diatreme complex following a resurgence in magmatism
• The defined resources of this deposit occur from a few hundred meters depth to 1.7 km below the paleosurface, which is partially preserved as a section of maar tuffs
• Potassium silicate alteration is commonly interpreted as the result of the addition of potassium to the host rocks via interaction with a potassium-rich brine of magmatic origin
• Data show that alteration at the deposit scale is isochemical with respect to major rock-forming components and that only sulfur and economic metals are added by flux of reactive magmatic gas
• Silicate solubilities are low, so only a minor fraction of alkalis in the host rock are extracted by alteration reactions and discharged at the paleosurface
• Reaction of magmatic gas phase with plagioclase results in the coupled deposition of anhydrite and disproportionation of SO2 to release H2S
• In-situ release of H2S immediately scavenges Cu and other chalcophile metals from the continuing magmatic gas flux to form the Cu-Fe-sulphides that make up the economic reserve
• Sequestration of Ca into anhydrite and deposition of silica into early quartz veins increases concentration of K2O, Na2O, MgO, etc. in remaining silicate assemblage in porous host rock
• This results in the development of intermingled potassium-enriched silicate and sulphur-rich sub-assemblages that constitute the mineralized phyllic
• Understanding of the alteration processes provides insights into gas-solid reactions processes inside active magmatic arc volcanoes but the magnitude of copper mineralisation is dependent on the original metal content of the source of the magmatic gas phase.

Introduction

Approximately two thirds of the world’s annual copper production is sourced from ‘porphyry copper’ deposits, which are hydrothermal ore deposits that developed in the cores of volcanic systems in magmatic arcs throughout the Phanerozoic. The defining geochemical characteristic of these deposits is the presence of economic grades of sulphide mineralisation (averaging > ~ 0.5wt% copper), along with potassium silicate rock alteration. This alteration is usually divided into ‘potassic’ assemblages (dominated by potassium feldspar and micas) and ‘phyllic’ assemblages (characterised by ‘white’ mica and quartz).

It has long been assumed that the potassium silicate alteration is the result of infiltration of saline liquid-phase brines from intrusives. However, research into the Grasberg porphyry copper-gold deposit has indicated that gas-solid reactions within the sub-volcanic environment likely played a role, with the gas phase composition of high temperature gas mixtures released by fumaroles in modern volcanoes dominating. This raises the question of how such large-scale potassium enrichment occurs without the addition of high salinity brines. To answer this, this paper analyses micro to mine scale petrographic and geochemical data using a range of techniques including high resolution tomography to measure microporosity, based on samples from the Grasberg deposit, which contains over 32 Mt. of copper.

SUMMARY of the Conclusions

• Porphyry copper-gold deposits form within magmatic vapour plumes.
• These plumes contain highly reactive SO2 and HCl, along with Cu and other metals.
• The permeability of these plumes is maintained through their evolution by time variant internal and regional deviatoric stress.
• Porphyry copper deposits are characterized by potassium silicate alteration at the scale of several km3.
• This alteration is directly associated with Cu-Fe-S mineralisation and occurs in large deposits such as the Grasberg deposit.
• Fluid inclusions in such assemblages are commonly assumed to be a potassium-enriched magmatic vapour.
• However, new data from Grasberg show alteration at the deposit scale of several km3 was essentially isochemical for major rock forming elements.
• There is no evidence for the involvement of a K-rich brine or K-enriched magmatic gas phase.
• Apparent potassium enrichment is the result of sequestration of calcium into anhydrite.
• This alteration process continually evolves intergranular porosity during mineral replacement with anhydrite and sulphides.
• This process maintains reactive gas flux in the altering host rock and sustains higher flux of magmatic gas through developing fracture veins.
• A quasi-isochemical hypothesis for potassic alteration in porphyry copper deposits is proposed.
• This alteration process occurs to various extents in evolving magmatic arc volcanoes due to sub-surface gas-solid reactions.
• Porphyry copper formation is correlated with processes inside active volcanoes.
• The relative economic potential of these volcanoes arises primarily from the relative fertility of the magmatic source regime.

We have compiled the landmark 4,400km² Magnetotelluric survey conducted by the GSQ north of Cloncurry.  Depth slices from 40km to near-surface are compiled with other data in this presentation. For more information: Here

Continental Copper has six exploration permits covering 980 km² in the sparsely explored terrane north of Ernest Henry (245 Mt at 1.2% Cu and 0.6g/t Au) and east of Dugald River (60Mt at 12% Zn and 1.6% Pb) and Little Eva (306Mt at 0.42% Cu, 0.23 g/t Au).  This area is the most prospective Cu-Au and Zn-Pb-Ag target terrane in the Mt Isa region.

Continental projects are covered by an 11,000km² water-bore geochemical survey.  The highest Cu-Pb-Zn bore-water geochemistry outside of the immediate mine areas is located in multiple samples within the Continental Copper licences.  Lead isotope geochemistry confirms the prospectivity of the anomalies. 

A 4,400 km² Magnetotelluric (MT) geophysical survey has generated prominent and unexplained conductivity anomalies within the Continental Copper licences coincident with the Gidyea Suture and the Mt Margaret Fault system.  A very strong and undrilled EM conductor coincides with the geochemistry and the MT conductor.  This EM anomaly has been modelled as three separate conductors with strikes of 3km to a depth of 1.5km.

What can be seen in this compilation is that there is a regionally extensive flat-lying structure in the mid crust which evolves towards the surface as a number of variably continuous vertical conductors. At Olympic Dam in South Australia, MT has defined a similar low resistivity zone in the mid-crust with vertical and continuous conductive zones linked to the known areas of mineralization with the most significant being the huge Olympic Dam deposit.

These modelled low resistivity zones are likely graphite and/or sulphides associated with the magmatic and alteration systems which generated metallic mineralization nearer to the surface.

Within the Continental Copper Maureen-Lola property, a strong SQUITEM anomaly has been modelled as three conductors extending from near surface to 1,500 metres depth and with a strike of 3km. This undrilled SQUITEM anomaly is coincident with the trend of the MT conductor, strongly anomalous Cu, Pb and Zn hydrogeochemistry and at shallow depth a likely erosional ridge in Palaeo-Proterozoic basement. Isotope data from the hydrogeochemistry survey confirms that these samples contain lead of a likely Palaeo-Proterozoic source in basement.