|dc.description.abstract||Presently non-commercial occurrences of Mississippi Valley-type ore
assemblages in the Middle Silurian strata of the Niagara Peninsula have
been studied. Based on this detailed study, a new poly-stage genetic
model is proposed which relates ore mineralization in carbonate environments
to the evolution of the sedimentary basin. Sulphide ore mineralization
occurred during two episodes:
1. During the late diagenesis stage, which is characterized by
compaction-maturation of the sediments, the initial mineralization took
place by upward and outward movement of connate waters. Metals were
probably supplied from all the sediments regardless of their specific
lithologies. However, clay minerals were possibly the main contributors.
The possible source of sulphur was from petroleum-type hydrocarbons
presently mixed with the sediments at the site of ore deposition. Evidence
for this is the fact that the greatest abundance of ore minerals is in
petroliferous carbonates. The hydrocarbons probably represent liquids
remaining after upward migration to the overlying Guelph-Salina reservoirs.
The majority of sphalerite and galena formed during this period, as well as
accessory pyrite, marcasite, chalcopyrite, chalcocite, arsenopyrite, and
pyrrhotite; and secondary dolomite, calcite, celestite, and gypsum.
2. During the presently ongoing surface erosion and weathering
phase, which is marked by the downward movement of groundwater, preexisting
sulphides were probably remobilized, and trace amounts of lead
and zinc were leached from the host material, by groundwaters. Metal
sulphides precipitated at, or below, the water table, or where atmospheric
oxygen could raise the Eh of groundwaters to the point where soluble metal
complexes are unstable and native sulphur co-precipitates with sphalerite
and galena. This process, which can be observed today, also results in the transport and deposition of the host rock material. Breakdown of
pre-existing sulphide and sulphate, as well as hydrocarbon present in
the host rock, provided sulphur necessary for sulphide precipitation.
The galena and sphalerite are accompanied by dolomite, calcite, gypsum,
anglesite, native sulphur and possibly zincite.||en_US