Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: E:\banner.gif

 

Comparison between Earth's Evolutionary History and
Evolutionary Trends in Ore Deposits

 

 

 

Pre-stellar molecular clouds, from one of which the Earth and all other planets of the solar system formed, are made up of widely dispersed microscopic dust particles containing just about a dozen refractory minerals. These represent the starting point of planetary mineral evolution. Gravitational compression and consequent heating produced the primary refractory constituents of chondritic meteorites, including chondrules and calcium-aluminum inclusions, with about 60 different mineral phases. Subsequent aqueous and thermal alteration of chondrites, asteroidal accretion, differentiation, and consequent formation of achondrites resulted in a mineralogical assemblage of about 250 different minerals species found in unweathered meteorite samples.

Segregation of this material into planetary masses led to further physical and chemical processes. Mineralogy of the Earth has largely evolved as a consequence of physical, chemical, and biological processes following planetary accretion and differentiation.

The initial mineral evolution of Earth’s crust was most likely governed by processes that produced the first continents with their associated granitoids and pegmatites, metamorphic terrains hydrothermal deposits, evaporates and zones of surface weathering which resulted in an estimated 1500 different mineral species. These geochemical and petrologic processes included:

1)    Volcanism and degassing,

2)    Fractional crystallization,

3)    Crystal settling,

4)    Assimilation reactions,

5)    Regional and contact metamorphism,

6)    Plate tectonics, and

7)    Associated large-scale fluid-rock interactions.

Biological processes began to affect Earth’s surface mineralogy by the Eoarchean Era (~3.85–3.6 Ga). The Paleoproterozoic “Great Oxidation Event" (2.2 to 2.0 Ga), witnessed changes in the atmospheric and ocean chemistry and the formation of large-scale mineral deposits, including banded iron formations, when atmospheric oxygen may have risen to >1% of modern levels. The Neoproterozoic increase in atmospheric oxygen, which followed several major glaciation events, ultimately gave rise to multicellular life and skeletal biomineralization and irreversibly transformed Earth’s surface mineralogy. Biochemical processes may be responsible, directly or indirectly, for most of Earth’s 4500 known mineral species. Thus, different stages of mineral evolution arise from three primary mechanisms:

1)    The progressive separation and concentration of the elements from their original relatively uniform distribution in the pre-solar nebula;

2)    An increase in range of intensive variables such as pressure, temperature, and the activities of H2O, CO2, and O2; and

3)    The generation of inequilibrium conditions by living systems.

The sequential evolution of Earth's mineralogy from chondritic simplicity to Phanerozoic complexity introduces a temporal dimension to mineralogy. It provides a dynamic alternate approach to understanding the evolutionary trends of ore deposits. The mineralogical diversity now found at or near Earth’s surface at crustal depths less than about 3 km was not present for much of the planet’s history. Indeed, both the variety and relative abundances of near-surface minerals have changed dramatically over more than 4.5 billion years of Earth history through a variety of physical, chemical, and biological processes. The main objective here is to explore the diversity and distribution of ore deposits in juxtaposition with significant episodes of evolutionary change in the Earth's crust.

It is neither possible nor desirable here to consider crustal evolution in detail for the purpose in question. The continents are widely believed to have developed by accretion -- each has developed from a volcanic nucleus or nuclei, being joined and added to by peripheral volcanic nuclei. The process is aided by the accumulation of volcanic matter (pyroclastic material) and products of erosion. The pattern is complicated by later fractures and relative movement of various crustal segments. This evolutionary pattern seems to form a plausible framework for the succession of geologic environments which is parallelled by a similar pattern of evolution of ore types.

For the sake of simplicity, the process of crustal evolution is considered in six successive stages.

 

STAGE I: THE EARLY VOLCANIC STAGE

The beginning of crustal evolution is marked by the formation of broad swells on the basaltic ocean floor. These represent the early stages in the development of volcanic islands. Development of these swells leads to block faulting, extrusion of lavas, and their protrusion above the sea level to form volcanic islands (Eg Solomon Is in SW Pacific). The islands are composed of pillow lavas ( basalts poor in olivine with spilites and keratophyres). Pyroclastic material is absent, but products of erosion have started forming.

MINERAL DEPOSITS:

  • Native copper and associated sulfides as orthomagmatic disseminations and vesicular fillings Eg Solomon Islands.
  • Nickel and associated sulfides in volcanic sills Eg Kambalda, W Australia, Manitoba. iii) Precious metals viz. tellurides Eg Fiji.
  • Minor chemical sedimentation viz. manganese and iron with jasper. NOTE: Manganese in the more important at this stage and the only important iron ore deposits are of Lake Superior-Ontario-Quebec.
    NOTE: Manganese in more important at this stage and the only important iron ore deposits are of Lake Superior-Ontario-Quebec.

 

STAGE II: EARLY ALPINE TYPE ULTRAMAFIC STAGE

Vulcanism continues, but changes to basaltic andesite, gradually progressing to andesites, dacites and rhyolites. Pyroclastic activity becomes conspicuous with the onset of the andesitic stage and increases with the increase in the felsic nature of vulcanism. By this time the island is quite large ( 150 Km in length) and new swells are developing either in a linear or arcuate arrangement with respect to the earlier swells. The older swells are undergoing extensive erosion and much sedimentation. The sedimeents become folded, faulted and deformed due to near vertical block faulting and gravity collapse.

MINERAL DEPOSITS:

  • Alpine type chromite deposits in intrusives Eg Paleozoic to Tertiary "Serpentine Belts" everywhere.
  • Nickel sulfide concentrations.
    NOTE: Some of the nickel sulfide deposits associated with stage I may actually belong to this stage.
  • Nickel also occurs as Ni-rich olivine which may be concentrated to ore by later weathering.

 

STAGE III: DEVELOPING EUGEOSYNCLINAL STAGE

The volcanic islands are by now well established and are enlarged by bodily uplift, volcanic accretion and sedimentation. Volcanic products are becoming more felsic and pyroclastic material is becoming prominent. Earliest plutonic rocks of granitoid texture and dioritic- granodioritic composition appear (in pipe or stock form). These are of shallow subvolcanic nature. These may be products of magmatic differentiation or transformation of the deeply buried pyroclastic rocks.

MINERAL DEPOSITS:

  • Banded Iron Formations developed as volcanic chemical sediments as a result of sea-floor exhalative activity. Deposition of these took place in troughs (eugeosynclines) in inter-island regions of arcs.
  • Considerable jasper and some manganese Eg Guyana.
  • Modern analogues of these processes are operative in the Kuriles and the Solomon Islands.
  • Stratiforn Sulfide Deposits of marine and marine-volcanic affiliation. These include chemical sedimentation, sulfide pyroclastic concentrations and some lavas. All these are essentially sea floor volcanic accumulations. A few of these are
  • associated with basaltic and more mafic lavas Eg Cyprus and Japan, whereas a vast majority are associated with andesitic and dacitic rocks. Eg Base metals of Ontario, Mount Isa and McArthur River (Precambrian); Bathurst, New Brunswick and E. Australia (Paleozoic) and Japan (Tertiary).
  • Some Banded Iron Formations and minor manganese concentrations and barite are associated with the Stratiform Sulfide Deposits.
  • Most of these deposits are formed near the continental margins and biological activity (2500 y ago) is always indicated.
    NOTE: Related to these are the sulfide bearing subvolcanic intrusives, breccia plugs and related shallow igneous bodies of andesitic-rhyolitic composition -- these are the "Porphyry Coppers".

 

STAGE IV: ADVANCED EUGEOSYNCLINAL & MIOGEOSYNCLINAL STAGE

The oldest swells have become quite large (Java & Sumatra) with smaller intervening swells beginning to coalesce (Aleutian-Malaysian). Volcanic festoons isolated from continents, or extending outwards from these are characterized by igneous sedimentation (with formation of reefs in appropriate climates). There is a mixing of volcanic material with products of continental erosion around volcanic festoons bordering continents. There is, therefore, a hybrid sedimentation on the continental side of the arc (the miogeosyncline) while volcanic sedimentation progresses in the outward seaward trough (the eugeosyncline). Vulcanism at this stage becomes more felsic.

MINERAL DEPOSITS:

Deposition continues from the Developing Eugeosynclinal Stage (III) resulting in the formation of:

  • Stratiform or non-stratiform marine volcanic sulfide ores.
  • Banded Iron Formations in the eugeosynclines (for some reason major iron formations are not associated with basemetal sulfide deposits).
  • Volcanic basemetals are contributed to the reef and off-reef environments on both sides of the arc (particularly on the miogeosynclinal side). These are eventually concentrated by sedimentary, diagenetic and later processes to form Limestone-Lead-Zinc Deposits.
  • Some manganese deposits (Usinsk Type) also develop in the eugeosynclines and miogeosynclines.

 

STAGE V: EARLY CONTINENTAL AND INTRUSIVE STAGE

The volcanic islands are by now welded to each other and also to the continental margins. Intense fault movement, compressional folding, more felsic plutonic intrusions, waning of vulcanism and rapid erosion are characteristic of this stage of crustal evolution. Plutonic rocks are granitic and pegmatitic in composition.

MINERAL DEPOSITS:

  • Cassiterite deposits as disseminations in granites, contact metamorphic deposits and pegmatitic deposits.
  • Quartz-cassiterite veins.
  • Quartz-wolframite veins.
  • Quartz-scheelite veins.
  • Quartz-gold veins.
  • Stibnite and stibnite-scheelite-gold veins.
  • Basemetal sulfide veins with arsenopyrite and increasing proportions of Pb & Zn as compared to Cu.  NOTE: Some basemetal veins may result from the remobilization (destruction) of earlier eugeosynclinal stratiform ores (Stages III & IV).
  • Plutonic type anorthositic iron-titanium oxides.
  • In the more mafic parts of these intrusions chromite is segregated. Eg Bushveld Complex.
  • Sometimes ilmenite, magnetite and minor Fe-Ti-O.
  • Major nickel-copper sulfides (Eg Sudbury) are also identified with this stage (?).
  • Chromite deposits formed in ultramafic rocks during the Alpine Type Ultramafic Stage (II) are well serpentinized by this time and move along large fault systems (along which they formed) and take up new lithological and structural positions in the evolving crust.

 

STAGE VI: SHELF AND SHIELD STAGE

Continuing folding, faulting and varying degrees of metamorphism leads to the formation of a "Continental Shield". This stage is marked by outpouring of flood basalts and stabilization of the crustal segment. Erosion and peneplanation lead to the development of broad flat areas susceptible to inundation during sea-level rises resulting in extensive deposits of detrital sediments, reefs (carbonate), evaporites and chemical sediments. Movement of the shoreline in response to sea-level fluctuations leads to the interfingering of shallow water marine and fluviatile sediments, particularly in the lower reaches of braided streams, deltas and outwash fans. Mineral deposits forming at this stage include a variety of igneous and sedimentary ores, and deposits formed during the earlier stages undergo substantial metamorphism.

MINERAL DEPOSITS:

  • There are few mineral deposits associated with flood basalts. Exceptions are the copper bearing lavas of the Keweenaw  Peninsula, Lake Superior.
  • Limestone-lead-zinc deposits often associated with oil bearing strata and evaporites, Eg Pine Point, Canada.
  • Non-volcanic sedimentary manganese deposits (orthoquartzite-glauconite-clay association), Eg Nikopol, USSR, and Morocco.
  • Ironstones of the Clinton, Lorraine and English type associated with near-shore, estuarine or lagoonal sedimentation.
  • "Sandstone Type" Cu-U-V ores formed in coarse sediments of outwash fans, near-shore braided streams and deltas, Eg Colorado.
  • Gold-uranium deposits of Witwatersrand-Bhind River- Jacobina Type in coarse conglomerates and grits of braided stream channels.
  • Basemetal sulfide deposits of non-volcanic association occurring with evaporites, Eg Kupferschiefer Marl Slate of Europe and England and the Copperbelt of Zambia. viii) A very minor category of iron ores -- the bog or marsh iron ores such as those of the present northern hemisphere.

Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: Description: E:\vine1.gif

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University


This website is hosted by

S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150

email: farooq.amu@gmail.com