John Logothetis

Field, mineralogical and geochemical studies were applied to selected alteration-mineralization phenomena from occurrences in the New Ross-Vaughan Complex (NRVC) of the South Mountain Batholith. The following types of late-stage alteration phenomena were found to be spatially related to fractures and shear zones within the roof and/or border endocontacts of granitoid bodies:

(1) greisens (mainly quartz + phengite varieties and associated Sn+W+Zn+Cu+U mineralization) as overlapping pods and greisen-bordered veins;

(2) episyenites (albite + K-feldspar + chlorite + phengite assemblages with U phosphate mineralization) as with porphyry dyke bodies;

(3) Fluoritised granitoid rocks, characterized by the presence of fluorite + quartz + hematite + sulphide veins, as irregular zones in granitoid rocks affected by fracturing and/or brecciation; and

(4) Hematised granitoid rocks, characterized by the presence of secondary hematite in microfractures and as inclusions in feldspars and micas.

Field and petrographic investigations of the greisen and episyenite zones indicate that the alteration mineral assemblages are complexly zoned - both spatially and paragenetically.

The major element chemistry of the main secondary minerals (i.e., phengite, chlorite, K-feldspar, albite, tourmaline and apatite) from the main alteration zones were studied in detail. The composition of chlorites is strongly influenced by the composition of the biotite precursor. Although phengites exhibited a wide degree of compositional variation, the high Fe and Ti contents of some of the phengites are related to the effects of Ti - Fe metasomatism due to greisenisation. Discriminant analysis was found to be successful in separating greisen-hosted phengites and chlorites from those hosted in episyenites.

The results of chemical analyses (31 samples) and mass transfer calculations reveal that greisenisation and episyenitisation involved the addition and subtraction of major (i.e., Si, K, Na, Ca, Fe, Mn, Ti and F) and trace elements (i.e., Li, Rb, Cs, Ba, Sr, Sn, W, Mo, Cu and U). Volume changes (as much as + 35%) associated with chemical changes reflect largely the leaching and/or addition of silica. Combined with petrographic and modal analysis, it was possible to assess the chemical changes associated with the constituent types of alteration (e.g., chloritisation, rutulisation, albitisation, fluoritisation, hematisation, silicification, etc.).

This study provides new evidence in support of the role of F-bearing hydrothermal fluids in the mobilisation of REE`s during fluid-rock interaction. In particular, greisenisation in the muscovite-rich zones resulted in the depletion of all REE`s; rutilisation and Fe-chloritisation associated with the late-stages of greisenisation were accompanied by the enrichment of light and HREE`s; and episyenitisation produced depletions in light and MREE`s (especially Ce).

The economic potential of the NRVC was assessed using various field, mineralogical and geochemical criteria. The granitoid rocks constituting the NRVC satisfy many criteria of granitoid bodies associated with Sn-W-Mo-U mineralization: (i) presence of highly differentiated granitoid rocks, especially muscovitic leucomonzogranites, (ii) abundant and widespread evidence of hydrothermally altered granitoid rocks, (iii) its structural setting within the roof and/or border zones of the batholith, and (iv) the presence of geochemically specialized granitoid bodies. Polymetallic tin mineralization is genetically and spatially associated with specialized leucomonzogranite bodies characterized by relatively low FeO and K2O contents and very high concentrations of Sn, F, Li, Rb and Cs. These findings may be used in assessing the mineralization potential of granitoid bodies in the South Mountain Batholith.

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Supervisors: D. Barrie Clarke / Gunter Muecke