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Creation 6(3):40–46, February 1984

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Editor’s note: As Creation magazine has been continuously published since 1978, we are publishing some of the articles from the archives for historical interest, such as this. For teaching and sharing purposes, readers are advised to supplement these historic articles with more up-to-date ones suggested in the Related Articles and Further Reading below.

The recent, rapid formation of the Mount Isa orebodies during Noah’s Flood

by

Abstract

Wikipedia.org Panorama_of_Mount_Isa_Queensland
Mount Isa orebodies show evidence of recent, rapid formation.

Mount Isa in north-west Queensland, Australia, is one of the world’s largest concordant base metal deposits, with both silver-lead-zinc and copper orebodies in the same Middle Proterozoic shale beds. Statistical analysis of the cyclicity and lateral zonation of the ore sulphides and host sediments in the silver-lead-zinc orebodies enables calculation of a deposition rate that could have produced the whole Mount Isa deposit in less than 20 days! Most geologists today agree that the orebodies at Mount Isa were originally deposited at the same time as, and as part of, the host shales, being deposited by submarine hot, salty, metal and sulphide-rich volcanic “springs”.

Since the host shales are also highly fossiliferous, it will be argued that they were laid down rapidly in the year-long global catastrophe of Noah’s Flood in situations similar to the biblically described “fountains of the deep”.

Introduction

Since its discovery in 1923 the major mineralisation at Mount Isa has been the subject of many studies and much controversy. Situated at lat. 20°44’ S, long. 139°29’ E (See Fig. 1) in Middle Proterozoic sediments of the Precambrian shield region of north-west Queensland, Mount Isa is one of the world’s largest concordant base metal deposits. Silver-lead-zinc and copper orebodies found in the same beds but spatially independent of each other, extend over a strike length of 4.5 km, a width of 1 km and a depth of 1.6 km with an average dip of 65°W (See Fig. 2). Since mining began in 1931 close to 80 million tonnes [as at February 1984] of ore averaging 3% copper, and another 55 million tonnes averaging 178 g/tonne silver, 7.4% lead and 5.8% zinc have been produced, but proven reserves are currently in excess of 140 million tonnes at 3% copper and 60 million tonnes of silver-lead-zinc ore.1

Geological setting

fig-1
Fig 1. Location of Mount Isa, Queensland, Australia.

The Mount Isa deposit lies in the western basin of an intensely deformed, multiply intruded and variably metamorphosed belt of Lower and Middle Proterozoic ‘age’ sediments. The basement to these sediments consists of acid volcanics complexly intruded by different phases of granite, and basic dykes, all of Lower Proterozoic (1800–2000 million A.G.Y.[arbitrary geological years]) ‘age’. Following a period of erosion of the basement, deposition of a variety of sediments (conglomerates, sandstones, siltstones, shales, tuffs, cherts and dolomites) and basaltic lavas began. Sedimentation appears to have been in two ‘waves’, the first ‘wave’ of sediments being dominantly quartz-rich sandstones, with some thick basalts, conglomerates, siltstones and other sandstones. In contrast, the subsequent Mount Isa Group sediments mark a change in sedimentation to a carbonate-rich black shale environment.1

The Mount Isa Group sediments themselves are estimated to be about 1650 million A.G.Y. old, and can be sub-divided into a sequence of dolomitic siltstones with minor dolomites, overlain by tuffaceous (that is, volcanic ash-bearing) dolomitic siltstones and shales. Within these tuffaceous upper Mount Isa Group sediments is the Urquhart Shale, the rock unit that contains all the known economic mineralisation at Mount Isa.1

Structural studies have revealed that following the cessation of sedimentation, the Mount Isa Group sediments were subjected to two episodes of deformation which tilted, folded and faulted the rock sequence. Accompanying metamorphism was extremely mild in the immediate area of Mount Isa and eastwards, but more severe to the west of a major fault zone which had developed just west of the Mount Isa deposit (the Mount Isa Fault).1

The mineralisation

The Mount Isa deposit consists of copper and silver-lead-zinc orebodies which are spatially separated within the Urquhart Shale (see Fig. 2). Copper ore is restricted to irregular masses of brecciated siliceous and dolomitic rocks locally called ‘silica dolomite’ that are broadly transgressive to the bedding in the enclosing Urquhart Shale, and terminated at depth by the green-schist basement.1,2,3

The mineralisation consists of cross-cutting veinlets and blebs of chalcopyrite (CuFeS2) within the ‘silica dolomite’ masses.3 Up dip from the green-schist basement, lobes of ‘silica dolomite’ interdigitate with zones of finely laminated and bedded silver-bearing galena (PbS)-sphalerite (ZnS)-pyrrhotite(FeS)-pyrite(FeS2) ore.3 These silver-lead-zinc orebodies occur in generally slightly recrystallised pyritic tuffaceous shales, are strictly conformable to the shales, and consist of alternating bands of shale and sulphide-rich shale.2,3 Ore boundaries are normally defined by economic limits rather than by limits of mineralisation.2 Between the orebodies the Urquhart Shale consists of barren tuffaceous, carbonaceous dolomitic shale and siltstone.1

Theories of ore formation

Picture: superiorresources.com.au Fig 2a cross sections mount isa
Fig. 2 (a) Typical cross-section through the southern mining area of the Mount Isa mine.

Early investigators concluded that the silver-lead-zinc orebodies were epigenetic, that is, the mineralisation had been introduced by hydrothermal fluids (that is, hot waters) into the structurally prepared host rocks subsequent to the latter’s deposition.1 Galena, sphalerite and pyrite were said to have been deposited by hydrothermal replacement of selected beds within the shales, despite the theoretical necessity for complex alternation of deformation and introduction of fluids to explain mineral textures.1,2

In the past 25 years careful research has resulted in most geologists favouring a syngenetic origin for the silver-lead-zinc orebodies, that is, the sulphides were deposited contemporaneously with the sediments which now form the enclosing rock.1–8 That this is now established beyond doubt9 may be seen by comparison with probable modern analogues in the depths of the Red Sea, Gulf of California, and East Pacific Rise (at 21°N) rift zones10–14 where metal and sulphide-rich hot salty volcanic waters are being spewed out, sometimes as ‘black smokers’,15 resulting in adjacent deposition of black carbonaceous metal sulphide muds. Substantial precipitation of the metal sulphides from the columns of hot salty water may be further achieved by the work of myriads of bacteria which swarm around these vents.16,17 At Mount Isa subsequent post-depositional deformation and metamorphism has recrystallised, folded and partly re-distributed the metal sulphides, but bedding features such as particle size grading are still evident.

Controversy regarding the formation of the Mount Isa copper orebodies in the ‘silica dolomite’ has been a little more difficult to settle, but in spite of some dissenters18,19,20 the current consensus amongst most geologists favours a syngenetic and sedimentary-exhalative origin, like the silver-lead-zinc orebodies.1,7,21

Picture: superiorresources.com.au Fig 2b cross sections mount isa
Fig. 2 (b) Typical cross-section through the northern mining area of the Mount Isa mine.

It is now considered that conditions necessary for deposition of all sulphides began with the development, through penecontemporaneous faulting, of a restricted basin of sedimentation. Active volcanism present during deposition of the Urquhart Shale triggered release of silver-lead-zinc-copper bearing brines into an already hypersaline environment on the shallow sea floor.21 The copper was deposited in a chert-dolomite facies near the submarine fault scarp, and silver-lead-zinc in the black shales further into the basin.1,7 Subsequently the copper ores underwent greater reconstitution than the silver-lead-zinc ores during diagenesis, deformation and metamorphism, the copper being remobilised from the original sediment into veins, while the host sediments were recrystallised and brecciated.1

Evidence for recent rapid formation during Noah’s Flood

Evidence for recent rapid formation of the Mount Isa orebodies during Noah’s Flood falls into two categories:

  1. Evidence for recent formation during Noah’s Flood—the presence of fossils and carbonaceous (organic) matter; and
  2. Evidence for rapid formation—analysis of the cyclicity in the sulphides and sediments, leading to calculation of the deposition rate.

(A) Evidence for recent formation

In the Biblical framework of creationist geology, Noah’s Flood (approximately 4,300 years ago) was the event responsible for most of the fossils in the earth’s crust. Thus, as argued by Snelling,22 wherever fossils or organic matter representing fossil remains are found in the geological column the rocks containing the fossils were deposited either by or after Noah’s Flood regardless of their assumed evolutionary geological age.

At Mount Isa both fossils and organic matter have been found in abundance in the Urquhart Shale which contains all the known economic mineralisation. Saxby and Stephens23 noted that carbonaceous (organic) matter, which constitutes on average up to 1% by weight of the Mount Isa sulphide ores, occurs both as relatively large discrete flakes and as dispersed films. Chemical analyses of the isolated organic matter revealed a carbon content of approximately 93% for the Mount Isa ore, while both transmission electron microscope and electron diffraction examination of the crystalline structure suggests that the Mount Isa organic matter is more akin to graphite than anthracite. Thus Saxby24 places the Mount Isa organic matter well along the coalification series towards graphite, confirming that it was probably derived from plant material such as algae and bacteria.

Mathias and Clark1 suggested that the possibility of some of the ‘silica dolomite’ being original algal material cannot be dismissed, while Bennett25 had earlier maintained that he had discovered algae-like bodies in the ‘silica dolomite’. Love and Zimmerman26 and Love27 were, in fact, the first to describe the possibility of microfossils in the Urquhart Shale at Mount Isa. They were particularly interested in trying to assess if pyrite in the shales had a microbial origin, and described a number of forms, some of which represented the organic framework from which they had dissolved the pyrite. Other forms appeared to be double-walled cells, commonly infilled with pyrite, which Love and Zimmerman26 believed to be the remains of organisms.

Because of the ensuing debate as to the origin of the pyrite grains, Love and Amstutz28 decided that the Mount Isa organic structures were not microfossils. More recent re-examination of organic matter from the silver-lead-zinc orebodies by Muir29, however, has demonstrated conclusively that the host Urquhart Shale contains abundant organic remains of micro-organisms. These microfossils are brownish-grey to black in colour as a result of the mild metamorphism of the Mount Isa Group sediments, but are reasonably well-preserved in other respects apart from occasional distortion of cells as a result of internal growth of pyrite or other minerals. Love and Zimmerman26 had found both single-and double-walled microfossils that correspond with the morphologies encountered by Muir29 (see Fig. 3). While the microfossils observed by Love and Zimmerman26 were mainly isolated forms with occasional clusters, Muir29 found large masses of both double-and single-walled cells resembling colonies.

She concluded that the greatly abundant microfossils were the remains of blue-green algae (cyanobacteria).

Even more recently Neudert and Russell30 have reported their discovery of stromatolites, the layered structures formed as a result of the accretion of fine grains of sediment by matted colonies of micro-organisms, principally blue-green algae (cyanobacteria), in sediments closely associated with the Mount Isa orebodies. They found these algal stromatolites both immediately below the ore-bearing Urquhart Shale in the Native Bee Siltstone, and above in the Kennedy-Spear Siltstone. They described them and concluded that their shape and internal fabric closely resemble algal structures that occur today in shallow marine environments such as at Hamelin Pool, Shark Bay, Western Australia, and the Persian Gulf, while similar stromatolites are also known on the shores of the Great Salt Lake, Utah, and in the Coorong area, South Australia.

The fossiliferous character of the ore-bearing Urquhart Shale and upper Mount Isa Group sediments has thus been established beyond doubt. When it is realised that the algae whose remains are found fossilised in these so-called 1650 million A.G.Y. old rocks at Mount Isa have identical counterparts alive today, it is more than reasonable to conclude that these fossils were deposited recently and contemporaneously with both the Urquhart Shale and the Mount Isa ore deposit during Noah’s Flood.

(B) Evidence for rapid formation

Examination of diamond drill cores from the Mount Isa silver-lead-zinc orebodies reveals that, although the ores and sediments are finely laminated, the ores commonly form zones several centimetres thick separated by sulphide-deficient sediment. On a cut surface of a hand specimen of these mineralisation zones it can be seen that they are made up of one or more readily identified simple lithologies:

  1. galena-rich usually containing pyrrhotite,
  2. sphalerite-rich usually containing pyrrhotite,
  3. mixed pyrite-sphalerite usually as very thin (mm or so) alternating laminae, and
  4. fine-grained pyrite-rich.
fig-3
Fig. 3 Double-walled cells as part of a large algal colony, Urquhart Shale, Mount Isa. (Scale bar = 5/µm)

The sulphide layers are separated by intervals of fine-grained dolomitic Urquhart Shale, the thicknesses of the individual units being extremely variable.

Finlow-Bates3 carefully studied two long sequences of drill cores through the silver-lead-zinc orebodies, one sequence being close to, and interdigitating with, lobes of the ‘silica dolomite’ and the other sequence being up-dip and distal to the ‘silica dolomite’ (refer to Fig. 2). He investigated the extent and nature of ordering and cyclicity in the orebodies by applying statistical concepts known as entropy (randomness), a difference matrix and an embedded Markov probability matrix all drawn up for the sequence of sediments under study. Simply put, the idea is that if the presence of one lithology (A) in some way favours the formation of a second lithology (B) above it, then throughout the sedimentary sequence the probability of B following A will exceed that expected by chance alone. Thus a ‘tally matrix’ was constructed for the whole of both sequences showing the number of times each lithology is followed by every other lithology. This was then turned into a probability matrix, giving a measure of the degree of upward ordering in the succession. It was thus found, for example, that there is a 0.66 probability that a sphalerite layer will follow a galena layer. Further analysis via an independent trials matrix established that the Mount Isa sediments were deposited by a nonrandom process that did not vary with time.

Finlow-Bates3 then turned his attention to answering the question: what are the elements responsible for the nonrandom deposition process? He subtracted the probability matrix from the independent trials matrix to produce a difference matrix in which each positive entry represents a transition that occurs more often than expected by chance. Thus, for instance, his results show that even though galena is found a disproportionate number of times above a sphalerite bed, given that a sphalerite bed is found, then dolomitic shale is still the commonest following lithology.

Mathias and Clark1 provide details of variations in lead-zinc abundances with distance from the ‘silica dolomite’ for a 3 metre unit in the footwall of the No. 7 orebody. Although this represents an average of a number of sulphide cycles the progressive differentiation of lead and zinc is strikingly apparent. Values of 14% lead and 10% zinc at 80–100 m from the copper ores give way to 3½% lead and 8% zinc at 500 m. They also observed that the isogrades rake to the north, pyrite shows the greatest lateral extent of all the sulphides, and the metal abundance trends are common to all the silver-lead-zinc orebodies. Finlow-Bates3 thus concluded that these observations are all compatible with the view that the source of the lead, zinc, and iron is closely related to the present site of the copper ores with waning metal deposition away from this source with differential deposition of lead, then zinc and finally iron. He also went on to suggest that we may, therefore, reasonably conclude that the transitions upwards dolomitic shale → sphalerite → galena are more abundant towards the copper-bearing ‘silica dolomite’; and dolomitic shale → sphalerite/pyrite and dolomitic shale → pyrite away from it. It also seems reasonable to conclude that the different minerals were deposited from pulses of the same parent solution. Differentiation of this solution during mixing with sea water to produce zonation is much more plausible than a separate supply of a lead solution, a zinc solution and an iron solution. In this case the vertical cyclicity of the ores and sediments revealed by the difference matrix analysis is a function of the lateral zonation of the individual sulphide beds. Finlow-Bates3 thus found that these conclusions were consistent with his probability matrices and so was able to construct a hypothetical sulphide unit (see Fig. 4) that fits all the data—laterally and vertically (upwards and downwards transitions). These then were the elements responsible for the nonrandom deposition process.

fig-4
Fig. 4 Diagram of a hypothetical sulphide unit, silver-lead-zinc orebodies, Mount Isa.

In discussing his results, Finlow-Bates3 argued that the metals and some sulphur were carried in a fluid that entered the sea as a buoyant plume or sea-floor-hugging current. All the evidence from the carbonate-hosted lead-zinc deposits worldwide indicates that lead is precipitated preferentially before zinc and that iron continues to precipitate after lead and zinc have been removed.

If the rate of metal deposition was slow compared to the rate of solution supply, then the vertical sequence dolomitic shale → galena → sphalerite → pyrite → dolomitic shale would be expected across the entire orebody resulting in a very different matrix analysis. If the precipitation was slow, lead would reach the far limits of the depositional basin. To the contrary, the observed lateral zonation lead → zinc → iron, is depicted in Fig. 4, implies that the rate of metal deposition must have kept pace with the solution supply in a controlled manner. Because mixing of a plume with sea water would have caused rapid temperature drops and thus uncontrolled instantaneous sulphide dumping, Finlow-Bates3 concluded that the Mount Isa deposits were formed from a sea-floor-hugging current rather than a plume.

The consequences of this model on sulphide deposition rates are somewhat startling. Backer31 has calculated a deposition rate of 1 metre/1,000 years for the Red Sea deposits referred to earlier. Finlow-Bates3 argues for a more rapid rate as his following hypothetical calculation shows. Consider a layer of dense ore solution, a metre deep, flowing on the sea floor at a rather sluggish rate of 1 metre/minute carrying 50 ppm lead all of which is to be deposited within a distance of 1,000 m. A galena bed carrying 25% lead with an average thickness of 1 cm would form in just under 5 weeks, a rate greater than 1 metre/10 years!
Thus he argues that discrete ore beds represent weeks rather than years of deposition if deposited from sea-floor-hugging currents of dense brine.

It is not difficult to see the implications of these calculations. If we make some appropriate and reasonable changes to Finlow-Bates’ parameters and then recalculate the deposition rate the result is even more startling. Consider, then, a layer of dense ore solution, 15 metres deep, flowing on the sea floor at a rate of 500 metres/minute (30 km/hour, still relatively slow) carrying 1000 ppm lead all of which is to be deposited within a distance of 1,000 m. (It should be noted that these figures are reasonable even in present day terms: the Red Sea brine pools are up to 250 metres deep;32 dense turbidity currents are known to have travelled thousands of kilometres down the continental slope and across the deep ocean floor at speeds up to between 65 and 80 km/hour;33 and concentrations of metals such as lead carried by ore-forming solutions are by consensus stated to be in the range X0–X,000 ppm, where X = 1, 2, … ,34 and by analysis of residual fluid inclusions in ore and ore-related minerals have been measured as up to 10,000 ppm.35) A galena bed carrying 25% lead with an average thickness of 1 cm would then form in only about 20 seconds, a rate of about 1 metre/30 minutes!!

Discussion

The combination of the prominent lateral zonation, the pulsed nature of the sulphides, and the high grade of the individual sulphide layers is compatible with the hypothesis that the hydro-thermal fluid was supplied in relatively rapid bursts and deposition occurred relatively rapidly.3 Injection of the majority of the fluid through fractures by major earth movements36 is the most attractive model to account for this periodicity. Smith37 argued that many of the faults in the Mount Isa area were active during deposition of the Urquhart Shale, and there seems little doubt that the nearby Mount Isa fault was still moving.

Thus fault movements propelled the fluids into the sedimentary site (possibly through the ‘silica dolomite’ breccia) to produce the main sulphide bands. For the time that zinc remained soluble, galena only was precipitated from the solution and so galena overlies dolomitic shale. Occurrence of pyrrhotite in the mineralisation closer to the source may well be a function of higher solution temperatures and/or sulphur deficiencies since pyrrhotite’s occurrence is known to be controlled by the availability of sulphur.38 Thus the formation of pyrite would have required addition of some sulphur from the sea water’s dissolved sulphate ions. Greater degrees of sea water mixing expected as the solution reaches the outer margins of the orebody would both increase the availability of sea-water sulphate and lower temperatures. Both trends favour pyrite over pyrrhotite.

The transgressive and regressive character of the system is most reasonably interpreted as corresponding to the waxing and waning of the metal supply, and hence perhaps also the solution supply. The evidence suggests that the waxing phase was faster than the waning phase, which in reality is quite logical. The dolomitic shale content of the sulphide beds is usually less than 30%,3 which would tend to suggest that sulphide deposition was significantly faster than the basin carbonate and shale deposition.

However, we cannot be certain that the metal solution did not carry the wherewithal to make dolomitic shale as well. In fact, Finlow-Bates3 cites evidence suggesting that in the sulphide layers most of the silica and some of the carbonate almost certainly had its origin in the hydrothermal fluid. Galena is not preferentially present in the thick sulphide units, but is still found in some quite thin sulphide layers, suggesting that the lead:zinc ratio may have changed in different ore pulses.3 But at the fine lamination scale considerable mixing of sulphides obviously occurred so that despite an apparently uniform overall trend short term perturbations were common.

There is other evidence to suggest that formation of the Mount Isa orebodies was a rapid process—lead isotope evidence. Richards39 reports that the silver-lead-zinc orebodies at Mount Isa show marked lead isotopic uniformity. Richards analysed a suite of ore samples, collected by Mount Isa Mines Ltd geologists, that represented the full stratigraphic range across the deposit and extended the lateral spread along strike. A few other random samples, including poly-sulphide veins in-filling fractures and strike faults within the ore host rocks, were also included in the suite.

Richards reported that the lead isotopic homogeneity at Mount Isa was ‘impressive’.39 He found that this lead isotopic homogeneity extended along strike from Mount Isa for almost 20 km northwards to the Hilton silver-lead-zinc deposit and for at least another 15 km southwards, making the total lateral extent at least 35 km. Furthermore, this isotopic homogeneity also extends in depth through the full stratigraphic span of the whole silver-lead-zinc ore sequence, including lead ore that has been remobilised into fractures. One obvious explanation for this isotopic homogeneity (Richards did not attempt an explanation) is that the lead in the silver-lead-zinc orebodies, and hence the orebodies themselves, was deposited rapidly from a single uniform source. If ore deposition had taken millions of years then isotopic homogeneity over such impressive areal dimensions could hardly be anticipated. On the other hand, deposition of the entire sequence of orebodies in less than 20 days would, of virtual necessity, be expected to produce the observed isotopic homogeneity.

Numerous attempts have been made to identify the source of the metals that were deposited to form the Mount Isa copper and silver-lead-zinc orebodies. Again lead isotopes offer potential as geochemical tracers. An isotopic match between lead-in-ore and the lead in a geologically suitable source unit lends credence to a proposed genetic relationship. Conversely and perhaps more strongly, an isotopic mis-match contributes by the process of elimination to the narrowing of alternatives.

Wiki commons black-smoker
A 'black smoker'

Farquharson and Richards40 used this technique in their search for a genetic relationship between the lead in the Mount Isa silver-lead-zinc orebodies and igneous rocks in the region. The choice of the igneous rocks as likely candidates for being the source rocks of the lead is based on the obvious role of volcanic activity during ore deposition, as evidenced by the thin tuff units interbedded with the orebodies and by the tuffaceous character of the dolomitic shales that host the sulphide minerals themselves. Their approach involved derivation of the lead isotope initial ratios by the whole-rock isochron method, and comparison of the initial ratios (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) with the isotopic composition of the ore lead. On the basis of their results, Farquharson and Richards40 concluded that both the Eastern Creek Volcanics and the Kalkadoon Granite could be related to the orebodies, possibly through the mechanism of weathering and erosion. They went on to suggest that an examination of the source of the ore-related tuff units, which argue very strongly for a contemporaneous volcanic source of the metals, could even yet lead to the possibility that Kalkadoon-like material beneath the pile of sediments and volcanics was the source of magma generation, volcanism and mineralisation.

The significance of their results lies in the location of the igneous rocks they concluded could be related to the orebodies. The Eastern Creek Volcanics, in fact, are in fault contact with the overlying Mount Isa Group sediments beneath the Mount Isa orebodies (see Fig. 2) where alteration and metamorphism have changed what were thick basalts into greenschists or greenstones.

The Kalkadoon Granite outcrops in a north-south linear belt to the east of the Mount Isa orebodies, but its lateral extent at depth is unknown. It is part of the crystalline basement complex and is said to have intruded the basement volcanics between 1800 and 1930 million A.G.Y. ago, with a later phase at 1785 million A.G.Y., and possibly an even younger phase.1 The Eastern Creek Volcanics are said to be about 1800 million A.G.Y. old, whereas the orebodies are estimated to have been deposited at about 1650 million A.G.Y. ago, the same approximate ‘age’ as the main phase of the Sybella Granite which outcrops to the west of the Mount Isa orebodies.40

Farquharson and Richards40 suggested that the lead to form the silver-lead-zinc orebodies may well have been derived via weathering (that is, oxidation) from both the Kalkadoon Granite and the Eastern Creek Volcanics. Many recent studies41,42 (and references therein) have focused on the hydrothermal alteration of basalts on the ocean floor by sea water and have endeavoured to reproduce the process in the laboratory so as to assess the effects. These studies have shown conclusively that significant amounts of iron, zinc and copper are readily leached from basalts under conditions of hydrothermal circulation of sea water.

Haynes43 made a study of the geochemistry of altered basalts and associated copper deposits which related the formation of strata-bound sedimentary copper deposits to solutions that had acquired copper during oxidative alteration of basalts. The study showed that altered basalt lavas occur beneath many such copper deposits, that altered basalt lavas are depleted in copper during the alteration episode, and that the amount of copper ‘released’ during alteration is sufficient to provide all of the copper found within the overlying copper deposits. Altered basalt lavas were thus concluded to be the source rocks for these copper deposits. It was this study and its application to the Stuart Shelf area of South Australia which resulted in Western Mining Corporation geologists finding the huge Olympic Dam copper-uranium-gold-silver deposit at Roxby Downs in 1975.44

It is thus highly significant that the Mount Isa deposit is underlain due to faulting by the Eastern Creek Volcanics which consist of up to 5,000 m of ‘flood’ basalts with minor tuffs that have been altered to greenschists or greenstones, largely by metamorphism, beneath the orebodies. Furthermore, veins of copper and lead sulphides are known to occur in the Eastern Creek Volcanics, for example, in relatively unaltered basalts just east of Mount Isa.39 The Eastern Creek Volcanics also contain lenses of quartzites (essentially rocks composed of interlocking grains of pure silica, that is, quartz) intercalated with the basalts, and so it may also be significant that the highest grades of copper in the copper orebodies correlate with high silica content of the host ‘silica dolomite’,1 silica that could have been derived, like the copper, from the Eastern Creek Volcanics.

Conclusions—a creationist interpretation

It has been clearly shown that the rocks which host the Mount Isa copper and silver-lead-zinc orebodies are highly fossiliferous and thus in the creationist’s Biblically-derived historical framework these rocks were laid down rapidly in the year-long global catastrophe known as Noah’s Flood. The statistical analysis of the cyclicity and lateral zonation of the sulphides and sediments in the silver-lead-zinc orebodies enables calculation of the deposition rate. All the silver-lead-zinc orebodies and their host fossiliferous dolomitic shales could thus have been deposited in less than 20 days.

Since Noah’s Flood occurred approximately 4,300 years ago according to the Biblical chronology, the evolutionary ages for the rocks and ores at Mount Isa have to be discarded. That this discarding of radiometric dates can be done with confidence is due to the work of Slusher,45 Setterfield,46 Matthews,47 and others, who have shown that radioactivity is unreliable as a means for dating rocks. Nonetheless, field relationships between rock units still need to be recognised as indicating a sequence of events, albeit a much more rapid sequence of events. That there is still a systematic pattern to the radiometric dates coinciding with the observed sequence of rock units has been explained by Setterfield46 as due to the decay in the speed of light and consequently a more rapid radioactive decay rate in the past, as well as his later work48 on systematic isotope variation in successively deeper magma zones from this same c decay effect in the early history of the earth.

Having thus eliminated the great age differences between the granites, basalts, dolomitic shales and the orebodies at Mount Isa, it is now possible to reinterpret the data within a Flood model. Thus it is conceivable that the source of the ore metals was both the Eastern Creek Volcanics and the later phases of the Kalkadoon Granite. Deep circulation of sea water through the thick cooling Eastern Creek basalts, deposited only days or weeks earlier, was responsible for the leaching of copper, lead, zinc, and other metals, and silica, magnesium, calcium, carbonate and other ions. These waters were heated by the cooling basalts, by the increasing depth of burial due to the ‘waves’ of Mount Isa Group sediments, and algae, being washed in and deposited on top of the basalts, and by heating from beneath during intrusion of the Kalkadoon Granite into the basement rocks below. Volcanism and the release of metal-laden hydrothermal fluids associated with the granite intrusion were triggered by the deep penetration of faulting and fault movements. Explosive volcanism produced showers of tuff that mingled with the sediments of the upper Mount Isa Group resulting, for example, in the tuffaceous character of the Urquhart Shale, and sometimes producing distinct tuff beds, as seen by the tuff marker beds in the Mount Isa deposit. Mixing of hydrothermal fluids and the hot deep circulating sea water produced a dense metal-laden hot brine that was ‘pumped’ in pulses to the sea floor surface by the fault movements. Such pulses propelled the metal-laden brines across the sea floor as sea-floor-hugging currents depositing their contained metals in layers, interspersed with the sediments being washed in, in the manner described earlier. The silica, magnesium, calcium and carbonate contained by the brines were deposited with the metals, particularly as the dolomite component of the domomitic shales, as chert units, and as the ‘silica dolomite’ closer to the faults (for example, the Mount Isa Fault). Brecciation of the ‘silica dolomite’ was produced by these same fault movements. And all this happened in a matter of a few weeks reminiscent of the opening of the ‘fountains of the deep’, rather than over countless millions of years.

At the cessation of these fault movements and the associated cycle of deposition, folding and renewed faulting deformed and tilted the rocks, resulting in mild metamorphism and some minor redistribution of metals, particularly the copper. This deformation and metamorphism was largely terminated by uplift of the area that resulted in the commencement of a new sedimentation cycle as the turbulent Flood waters scoured and eroded the rising land mass, and shifted their acquired sediment load to new deposition sites in adjacent subsiding areas. Final uplift in the closing stages of the Flood year resulted in further erosion by the retreating waters draining off the continent, leaving behind the present land surface.

Posted on homepage: 31 October 2014

References and notes

  1. Mathias, B.V., and Clark, G.J., Mount Isa copper and silver-lead-zinc orebodies- Isa and Hilton Mines. In: Knight, C.L. (Ed.) Economic Geology of Australia and Papua New Guinea, The Australasian Institute of Mining and Metallurgy, Parkville, Australia, pp. 351–372, 1975. Return to text
  2. McDonald, J.A., Economic Geology 65(3):273–298, 1970. Return to text.
  3. Finlow-Bates, T., Economic Geology 74(6):1408–1419, 1979. Return to text.
  4. Stanton, R.L., Transactions, Institution of Mining and Metallurgy 72:69–124, 1962. Return to text.
  5. Stanton, R.L., Proceedings, Australasian Institute of Mining and Metallurgy 205:131–153, 1963. Return to text.
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