The stratigraphy and deformation history of the Daisy Milano gold deposit, Mt Monger, Western Australia
thesisposted on 2017-02-23, 00:31 authored by Cohalan, Louis
This thesis aims to understand the stratigraphy and deformation history of the Mt Monger Goldfield as well as the timing of its high-grade gold deposits. Whole rock geochemical analyses were carried out to precisely differentiate between the mafic and ultramafic units using REE spidergrams and La and Th v TiO2 ratios in particular. These ratios are compared to values established in the literature for regional stratigraphic units. It was found that each of the three mafic units near Daisy Milano have compositional equivalents in the Kambalda Sequence of the Kalgoorlie Terrane, which hosts many major gold deposits. The Lower Basalt, Wombola Dolerite and Mt Monger Sills near Daisy Milano are compositionally very similar to the Lunnon Basalt, Devon Consols Basalt and Paringa Basalt of the Kambalda Sequence, respectively, and correlate to Eastern Goldfields-wide basaltic groups. In contrast, the Daisy Milano Komatiite is compositionally distinct from the Kambalda Komatiite. Furthermore, the felsic to intermediate stratified clastic unit at Daisy Milano, below and intercalated with the komatiite unit, is stratigraphically lower than are stratified clastic units in the Kambalda Sequence. Structural analysis from surface exposures and oriented drillcore reveal evidence for an early N-S shortening event (D1) giving rise to low-angle thrust planes verging north preceding the Eastern Goldfields-wide ENE-WSW shortening event (D2) which produced the dominant NNW-trending steeply-dipping foliation at Daisy Milano. This was followed by a later dextral strike-slip deformation event along N-S oriented structures (D3). In the Wombola structural domain (west part of the Daisy Milano area), gold occurs in NE- to ENE-trending vertical quartz veins cross-cutting dolerite. These veins and the gold mineralisation are interpreted to be associated to the main ENE-WSW shortening event (D2). In the Daisy Milano structural domain (east part of the Daisy Milano area), gold occurs in N-S to NNW-SSE structures, usually along the contacts of felsic porphyritic dykes intruding these structures, and especially just below the stratigraphic level of the komatiite units. These mineralised structures are interpreted to have developed parallel to the axial planes of D2 folds resulting from ENE-WSW shortening and to have been reactivated during the later dextral strike-slip event (D3). Gold mineralisation in these structures is interpreted to be contemporaneous to, and controlled by, the later strike-slip event (D3) and possibly also by D2. The felsic porphyritic dykes were dated ~2676 and ~2687 Ma using zircon U-Pb geochronology by LA-ICP-MS. The orientation and structures of these dykes suggest that their emplacement was pre- and syn-D2. The combination of stratigraphic, geochronological and structural investigation has shed new light on the geologic controls of the deposits. It places them in a regional context allowing for easier comparison to other known deposits in the Eastern Goldfields which can lead to new exploration strategies.