<p> Ophiolites are generally regarded as fragments of ancient oceanic crust and upper mantle emplaced on the continents. Thetford Mines ophiolites probably formed in a marginal or back-arc basin by three separate but related igneous events. The first event involved the partial melting of a rising mantle diapir. The melt produced later underwent extensive fractional crystallization involving olivine, spinel, pyroxene and plagioclase to form a layered sequence of dunitic, pyroxenitic and gabbroic cumulate rocks. The residual magma fraction after this extensive fractional crystallization was later erupted as part of hypabyssal rocks and MgO, Cr and Ni poor lavas. During the second and third stages, the residual mantle material from the first episode of melting was remelted to produce melts from which low TiO2 lavas were formed. The low TiO2 lavas are also depleted in other incompatible elements such as Zr and Y which presumably were lost into the magma produced by the first stage melting of the mantle. The mantle residue after the multi-stage melting of the mantle is believed to be represented by the harzburgite occurring at the base of the ophiolite suite.</p> <p> The cumulates display wide variability in their major and trace as well as noble metal contents principally in response to fractional crystallization while the harzburgite displays uniform chemistry, compatible with its origin as a residue of extensive partial melting of the mantle. However, on the average, Thetford Mines plutonic rocks have similar noble metal contents to Mt. Albert pluton, Gaspe, but have higher PGE than the Troodos ophiolites. Thetford Mines rocks are also depleted in the noble metals in comparison to the stratiform layered complexes such as Bushveld and Stillwater. The volcanic rocks can be classified into three groups on the basis of their noble metal contents viz., i) low Ir, low Pd lavas, ii) low Ir, high Pd and iii) high Ir, high Pd lavas. These groups correspond to the lavas produced during the first, second and third igneous events respectively. The noble metal contents of the first group are similar to
most ocean floor basalts while the third group lavas are comparable only to oceanic island and intraplate basalts.</p> <p> During fractional crystallization of the parental magma of the
cumulates, Ir was strongly partitioned into early formed rocks such as the olivine-chromite cumulates while Pt and Pd were progressively enriched in the residual magma fraction. Au, however, was less sensitive to fractional crystallization than the PGE.</p> <p> After the formation of the ophiolites, the lavas and hypabyssal rocks were metamorphosed to greenschist facies by hydrothermal sea water, and the ultramafic rocks were serpentinized, first in the oceanic environment by sea water and later on the continent by meteoric water, Most of the major and trace elements were mobilized during the hydrothermal alteration of the lavas but the PGE were immobile. It is suggested that the redox potential of the hydrothermal solution was not high enough to oxidize the noble metals and so permit mobilization. Au, however, shows some degree of mobilization, and it is believed that some Au occurred along grain boundaries and was carried as particulate material (mechanically) by hydrothermal solution. The noble metals were only slightly mobilized during the continental serpentinization episode. Their inertness during the oceanic serpentinization episode is attributed to the reducing condition of the oceanic environment which prevented formation of soluble noble metal complex ions.</p> <p> Unlike many ophiolite occurrences, Thetford Mines rocks appear to have very little economic potential with respect to base and noble metals, principally because of lack of abundant sulfides to act as concentrators of the metals.</p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19851 |
| Date | 02 1900 |
| Creators | Oshin, Igbekele Oyeyemi |
| Contributors | Crocket, James H., Geology |
| Source Sets | McMaster University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
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