Trace elements in sulphide minerals and their genesis in the south Pennine orefield.

The genesis of ore mineralisation in the south Pennine orefield have been studied in light of mineralogy, paragenesis and trace element geochemistry of principal sulphide minerals. Field observations of 138 ore bodies, hand specimen examinations of more than 4100 mineralised samples collected from these deposits and microscopic studies of 527 polished sections prepared from selected samples were used in mineralogical and paragenetic studies. The generalised regional paragenetic sequences indicate the following episodes of ore mineralisation: early calcite-baryte-fluorite followed by the main phase of galena-sphalerite-iron and copper sulphide mineralisation and was followed by the late calcite- baryte-fluorite precipitation. The main controls of ore mineralisation were structural, stratigraphical and lithological. Two hundred and eighteen galena and 41 sphalerite samples were analysed for Cu, Fe, Ni , Co, Ag, Cr, Mn, Mg, Cd, Sr and Ca. In addition, the concentration of Zn in the galenas and Pb in the sphalerites were determined and 118 of the galena samples and 16 of the sphalerite samples were also analysed for Bi and Sb. Cu, Fe, Ni, Co, Mn, Cd, Ca and Bi show preference to incorporate mainly in sphalerite whereas Ag, Cr, Sr and Sb have higher concentration in galena. The co-existence of galena and sphalerite does not have any apparent effect on the partition of the trace elements The distribution of the trace elements in galena and sphalerite is controlled by temperature, type of admixed mineral inclusions, original composition of the ore fluids responsible for precipitation of these sulphides, and local variation in the geochemistry of these fluids. Based on these studies, a poly-stage genetic model is proposed which relates ore mineralisation in carbonate environments to the evolution of sedimentary basins. Sulphide deposition may have occurred during three stages ; 1- Minor metal sulphides could have formed during the early diagenesis phase; sulphur being supplied by decomposition of organic matters and by sulphur-reducing bacteria, and metals by pore waters. 2- During the late diagenesis stage, the main phase of ore mineralisation took place by upward and outward circulation of heated saline connate brines. The ore fluids were generated by release of formation waters during compaction of sediments and by membrane filtration and clay dehydration. The ore minerals precipitated behind a "front" of hydrocarbons over a temperature range of 50 to 150C. Metal and sulphur were derived from more than one source. 3- During the uplift and weathering phase, some minerals from diagenetically-formed ore deposits were transported by meteoric waters and re-deposited in residual sediments of caves. In addition, supergene formation of sulphides may have been responsible for deposition, or significant enrichment, of exploitable ore deposits in the orefield.