An integrated geochemical, petrologic, geochronological, and metallogenic study of the Powder Horn Intrusive Suite and the associated Lodestar Prospect : a magmatic-hydrothermal auriferous breccia zone that links epithermal and porphyry systems, Northern Burin Peninsula, Newfoundland

Hinchey, John Glenn (2001) An integrated geochemical, petrologic, geochronological, and metallogenic study of the Powder Horn Intrusive Suite and the associated Lodestar Prospect : a magmatic-hydrothermal auriferous breccia zone that links epithermal and porphyry systems, Northern Burin Peninsula, Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Lodestar Prospect is associated with the Powder Horn Intrusive Suite and comprises auriferous sulphide-oxide mineralized (polylithic breccias which have a magmatic-hydrothermal or phreatomagmatic origin. The best grades obtained by exploration companies from chip-channel sampling of the breccia were 4.98 g/t Au and 14.8 g/t Ag over 15.9 m, along with one area that returned an assay of approximately 2.0 % Zn. Earlier assays derived values of 6.13 g/t Au over 4.7 m and 4.91 g/t Au over 3 m. More recent channel samples 5.6 g/t Au over 8.5 m including 12.6 g/t Au over 1 m. -- The Powder Horn Intrusive Suite is a multi-phase intrusive suite, with both pre- and post-brecciation (mineralization) phases. Pre-mineralization rocks consist of (1) the late Neoproterozoic Musgravetown Group sedimentary rocks, which are hornfelsed close to the intrusion. (2) medium grained gabbro/diorite, and (3) minor felsic material; the gabbro phase has been U-Pb zircon dated at 603+/- 2 Ma. Post-mineralization phases include (1) fine grained gabbro/diorite, (2) felsic phases, and (3) diabase dykes; a felsic dyke has been U-Pb zircon dated at 605 +/- 5 Ma. -- The main mineralized breccia, which occurs at the contact between sedimentary rocks and the pre-breccia gabbro/diorite, is exposed over approximately 25 meters and contains Au associated with Cu, As, and Zn. Arsenopyrite geothermometry suggests a crystallization temperature of approximately 425 +/- 50ﾟC for the sulphides. The mineralization occurs dominantly as the matrix to the breccia, whereas unmineralized sections of the breccia contain actinolite/chlorite as the matrix. The breccia clasts consist of host sedimentary material, pre-breccia gabbro/diorite, quartz-feldspar porphyry (QFP), diabase, and minor fine-grained granite. The sedimentary and gabbro/diorite clasts were locally derived from the wall-rock, however, the QFP clasts have not been observed as outcrop in the area of the showing and have REE patterns unlike felsic intrusive rocks in the vicinity of the prospect. Thus the QFP are assumed to be derived from depth. In places, these QFP clasts contain pre-breccia sulphide mineralization. They also appear to be finely comminuted, producing a rock-flour matrix in portions of the breccia which probably indicates transportation from depth. Many characteristics of the Lodestar Prospect suggest a magmatic-hydrothermal origin for the breccia produced by fluid exsolution from an auriferous intrusive system. Furthermore, the presence of exotic mineralized QFP clasts within the breccia suggests that the mineralization may be related to a deep Cu-Au porphyry system. If so, the prospect may have formed in a transitional environment between a deeper porphyry system and a shallow epithermal system. The Lodestar Prospect is, therefore, very significant because it indicates the possibility for porphyry-style mineralization within the Powder Horn Intrusive Suite, as well as elsewhere on the Burin Peninsula.

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