Possible Role of Organic Matter within Albo-Vraconnian and Cenomano-Turonian Black Shales of Slata-Guern Halfaya in the Genesis of Pb-Zn Ore Deposits in the NW Tunisian Diapiric Zone

Rddad, Larbi

The geochemical analysis of Albo-Vraconian facies of Slata and Cenomano-Turonian facies of Guern-Halfaya indicates that these facies are rich in organic matter with a Total Organic Carbon varying from 0.2% to 1.4% and from 0.62% to 5.46% respectively in Slata and Guern-Halfaya. These black shales are also rich in trace elements (Pb, Zn, Cd, Ni). The pyrolysis rock Eval analysis reveals that the organic matter is mature (oil window, 431°C < Tmax. < 447°C) in Albo-Vraconian facies and immature in Cenomano-Turonian facies (423°C < Tmax.< 433°C).

The correlations between TOC and organic content (pyrolysable hydrocarbons and residual organic matter) in one hand and trace elements (Pb, Zn, Cd, and Ni) in black shales on the other are positive in Guern-Halfaya and negative to absent in Slata. These correlations' trends are best explained by the degree of organic matter's maturity. In Slata, subsidence of the Albo-Vraconian black shales causes thermal cracking of kerogen, destabilization of organo-metal ligands and expulsion of metals and oil. In Guern-Halfaya, however, organic matter is immature and the organic and metallic content is preserved.

In Slata, the oil and metals, expulsed from the Albo-Vraconian black shales, might have contributed in the genesis of the nearby Pb-Zn ore deposits (e.g Sidi Amor) hosted in the Aptian carbonates. The Cenomano-Turonian black shales rich in organic matter and metals might have also played a role in the genesis of ore deposits in areas where organic matter is mature.

To further understand the relationship organic mater-ore in this region, further geochemical studies in black shales and ore deposits are needed.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013