Black shale may provide unfathomed mineral wealth
It may appear paradoxical, and surprising that many professionals engaged in mineral development do not appreciate the great potential of Black Shale with respect to hosting very valuable mineral resources. In some cases, this may be due to the fact that several minerals being exploited around the globe were discovered by chance or by traditional prospecting. Genetic models in such cases may thus become irrelevant.
Also, some geologists regard black shales merely as indicators of changes in the environment of deposition of sedimentary sequences. The result is that substantial mineral resources associated with black shales are yet to be discovered and possibly exploited.
Minerals associated with black shales include uranium, vanadium, lead, zinc, copper, gold, manganese as well as petroleum.
Black shale is commonly papery thinly laminated and carbonaceous and exceptionally rich in organic matter. They are formed in marine environment depleted of oxygen and a variety of Paleocene and water depth.
Black shale have a fairly wide distribution North Africa, Italy, Great Britain, Germany, Poland (other parts of Europe),USA, parts of West Africa among other localities.
Black shales may be associated with porphyries which are often metal rich source rocks.
Genesis and major features
Black shales indicate periods of depletion of oxygen in marine environments. The paleoceanographic setting and water depths are very varied. For example, the Kupferschiefer Formation which extends from North England to Poland was deposited during the rapid transgression of the Zeichstein Sea while the La Luna Formation (Venezuela and Colombia) was deposited in warm shallow sea associated with carbonates. In general most black shales are formed by partial decay of organic matter in quiet water under reducing conditions such as stagnant basin characterized by slow deposition.
The Kupferschiefer Formation which extends from northern England to Poland is a fascinating stratigraphic unit. During my study visit to the formation in northern England and Poland I noticed that carbonates are major features from the base sandstones appear to be minor constituents.
However, in Sierozowice, southern part of the fore sudetic monocline sandstones underlie the black shale. In this area, the base of the Kupferschiefer comprises carbonate-rich shales underlain by organic rich shales.
Copper mineralization is concentrated in the dolomites overlying the black shale and in the black shale.
The Kupferschiefer is notable for the rich economic copper deposits. Other black shales contain considerable amounts of organic matter with variable occurrences of carbonates.
Mineral potential of black shales
As noted earlier, some black shales are rich in metals, metallic mineral deposits and some host valuable industrial minerals. In addition black shales could also be source rocks for oil and gas. Besides the Cu-rich Kupferschiefer, there are examples in many parts of the world. Black shales that host valuable minerals have a wide distribution in time and space.
A few examples will suffice to emphasize this great potential.
Cambrian barite deposits in Black shales in China
- Uranium-rich black shale (Late Devonian) Arkansas, USA
- Cretaceous and Tertiary source rocks in the Gulf of Mexico
- Lower Silurian and Upper Devonian of the Saharan platform black shale source rocks
- Columbia Emeralds in black shales
- Nigeria uranium, vanadium, manganese and other metallic mineralization in the black shales of the Dange Formation.
- Paleocene, Sokoto Basin
The foregoing indicate the varied nature of mineral occurrences/deposits associated with black shales petroleum, metallic minerals, industrial minerals, gemstones, among others.
Mineral resources associated with black shales may be placed in two broad groups;
i) Black shale hosted mineral resources e.g. uranium, copper, uranium emeralds, barites etc.
ii) Black shales as indicators of occurrences of mineral resources e.g. black shale association with rock suites as guide to Phosphogenesis (Phosphorite deposits)
iii) Manganese deposits with trangressive stratigraphic sequences near or at black shale pitchouts.
iv) Placer deposits originating from black shales e.g. emerald.
Gold in black shales
Gold occurrences in black shales is generating a great deal of economic and academic interest. I was fascinated by the occurrences of black shales in the black shale of the Dange Formation, Paleocene, Sokoto Basin, Nigeria.
There are reports of similar occurrences in other parts of the world.
Gold mineralization in black shales may be indicated by:
a) Increased FeO content
b) Increased values of FeO/Fe2O3, Corg and S.
c) Proportions of Na compared to Ca and Ti
d) Higher content of Na and much lower values for Ca and Ti .
Most studies carried on black shales have been for academic purposes environment of deposition, paleoceanographic settings, contained fossils e.g. vertebrates.
Notable examples are:
Black shales, Newfoundland and Labrador
Several black shale formation at the Cenomanian/Turonian boundary worldwide including studies of carbonate platform drowning, oceanic and anoxic events.
Toarcian black shale, Togno (Bergamo, Italy)
Upper carboniferous and lower Jurassic black shales in England.
Detailed reports of genetic/exploration models for mineralization are few.
Useful reports include Sawolicz (1993) on copper mineralization in the Kupferschiefer Formation, Poland. Copper mineralization resulted from accumulating clay carbonate muds rich in organic matter which generated hydrogen sulphide and produced a reducing environment. The amount of metals concentrated during deposition account for the geochemical anomaly typical of black shales. Multi-stage processes for the metals in the Kupferschiefer (Sawolicz, in the Dange Formation, Sokoto Basin, Nigeria (Nwabufo-Ene 1995, 2004,among others) provide a useful guide in establishing exploration models for metal rich black shales elsewhere.
Black shale hosted manganese deposits are of great value in establishing genetic/exploration models for other metals.
Typical depositional and geological settings include transgressive stratigraphic sequences near or at black shale pinch outs. Shallow marine and sedimentary rocks such as dolomites and sponge spicule clays which occur with black shales in starved basins are good guides.
Exploration models for black shale hosted mineral resources should embrace paleogeographical, paleoceanographic and stratigraphic studies. In addition geochemical sampling and testing must be contemplated. The value of geochemical studies is demonstrated by the genetic model
a) Manganese Increase in Eh and /or pH of Mn rich water could produce Mn- rich, Fe depleted chemical sediments.
b) In exploration for gold hosted black shales geochemical studies will reduce the proportions of FeO, FeO/Fe2O3 c.org and S as well as Na, Mg and Ti.
Exploration for mineral resources associated with black shales ought to adopt the scientific method which includes appropriate interpretation of the mineral deposit model and ore deposit model. The mineral deposit model is: A systematically arranged body of information describing the essential attributes (properties) of a class of mineral deposits while the ore deposit model includes two end members the empirical model and the genetic model.
A multidisciplinary approach to the said exploration will save costs and produce good results.
The mineral resources associated with black shales have been reviewed in retrospect. Limited attention paid to black shales as sources for varied mineral wealth may be due to the fact that investors and mineral explorationists are unaware of the great potential in this regard. Perhaps the future will be different just as porphyry copper deposits has become a major source of the world's supply of gold.
Nwabufo-Ene K. E. 1995 The mineral resources of the Black Shales, Dange Formation, Paleocene Sokoto Basin Nigeria. Proc. Of Symposium and Field Workshop on the minerals industry in Nigeria. Enugu, Nigeria PP20 -24
Nwabufo-Ene K. E. In Press (a) The Realm of Stratigraphy Trammel Universal Publishers 289p
Nwabufo-Ene K. E. In press (b) Economic equations in the minerals industry Trammel Universal Publishers 276p
Salowicz. Z 1992 Primary sulphide mineralization the Cu- Fe S zones of the Kupferschiefer fore sudetic monocline, Poland. Trans. Inst. Min. Metall. Vol. 101 B1-9