EMPLACEMENT OF THE MAGGIE HAYS NICKEL DEPOSIT, LAKE JOHNSTON GREENSTONE BELT, YILGARN CRATON, WESTERN AUSTRALIA
The Maggie Hays mine block sequence is dominated by the Central Ultramafic Unit, a thick, extensively altered cumulate, which hosts disseminated Ni-sulfide mineralization in its lower portion and massive Ni-sulfide along the northern footwall contact. The Central Ultramafic unit is hosted in a sequence of felsic volcanics, metasediments, thin ultramafic and spinifex textured ultramafic units. Felsic volcanics are commonly porphyritic and variably sheared. Metasediments are characterized by dominant alternating chert and magnetite bands, and lesser massive exhalative sulfide. Thin ultramafic units are observed to be cumulate in texture with finer grained margins. Spinifex-textured units are observed immediately above the exhalative metasediments and are commonly thin and differentiated with well-developed B-zone cumulates and A-zone spinifex horizons. Locally, spinifex-textured units are overlain by thick flow top breccia sequences, up to 26m-thick.
We argue that the Central Ultramafic unit intrudes the felsic volcanics and metasediments acting as a magma conduit to the overlying thin ultramafic units and spinifex-textured sequences. Dominant Ni-sulfide accumulation occurs within the intrusive Central Ultramafic unit proximal to the interpreted feeder zone, whereas only minor amounts of Ni-sulfide occur in the overlying ultramafic lithologies. The volcano-stratigraphic architecture of the Maggie Hays setting displays similarities with the world-class Mount-Keith-Cliffs system in the Agnew-Wiluna greenstone belt of Western Australia.