in Relation to Crustal Evolution
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,
perhaps drift, 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.
the sake of simplicity, the process of crustal evolution is considered in six
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.
- Native copper and associated
sulfides as orthomagmatic disseminations and vesicular fillings Eg Solomon
- 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 the more important at this stage and the only important iron ore deposits
are of Lake Superior - Ontario -Quebec.
STAGE II: EARLY ALPINE TYPE ULTRAMAFIC
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.
STAGE III: DEVELOPING EUGEOSYNCLINAL STAGE
- Alpine type chromite deposits
in intrusives Eg Paleozoic to Tertiary "Serpentine Belts"
- 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.
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
- 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 &
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.
continues from the Developing Eugeosynclinal Stage (III) resulting in the
STAGE V: EARLY CONTINENTAL AND INTRUSIVE
- 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
- Some manganese deposits (Usinsk
Type) also develop in the eugeosynclines and miogeosynclines.
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
STAGE VI: SHELF AND SHIELD STAGE
- Cassiterite deposits as
disseminations in granites, contact metamorphic deposits and pegmatitic
- Quartz-cassiterite veins.
- Quartz-wolframite veins.
- Quartz-scheelite veins.
- Quartz-gold veins.
- Stibnite and stibnite-scheelite-gold
- 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
- 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
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.
- 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,
- 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
- "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.