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Ore textures and possible sulphide partial melting at Broken Hill, Aggeneys, South Africa I: Petrography

Department pf Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa, Present address: Paleoproterozoic Mineralisation Research Group (PPM), Department. of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa, e-mail: glgy13{at}na.rau.ac.za or geo_russ{at}hotmail.com

D.L. Reid

Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, Cape Town, South Africa, e-mail: dlr{at}geology.uct.ac.za

The Broken Hill-type Ph-Zn-Cu-Ag massive sulphide mineralisation at Aggeneys in the Northern Cape Province, South Africa has been subjected to peak metamorphic temperatures ranging between 670 to 690°C and pressures between 3.4 and 4.5 kbar. Due to the Ag and Bi rich nature of the ore, these conditions, based on previous experimental studies, may have been sufficient to allow partial melting of the sulphides to occur. Such features as (1) concentrations of low melting point chalcophile elements (LMCE), which include the metals Pb, Zn, Ag, Ga, As, Se, Cd, In, Sb, Te, Hg, Tl and Bi, (2) sulphide inclusions containing a number of different LMCE-bearing minerals within high temperature metamorphic gangue minerals, (3) low interfacial angles between two LMCE-bearing sulphides, such as galena and sphalerite, and LMCE-bearing sulphides and resistate minerals, particularly silicates, (4) LMCE-bearing sulphides filling fractures within metamorphic silicate minerals, (5) coarse recrystallised pegmatite bodies of galena and chalcopyrite crosscutting the regional foliation in the ore body and (6) the presence of a transgressive Mn selvage around the orebodies have all been recognised at Aggeneys and may be indicators of sulphide partial melting. The abundance of pyrrhotite in the orebodies, particularly in the lower ore body (LOB), may, in part, be due to partial melting of pre-metamorphic pyrite during peak metamorphism. Some euhedral pyrite in the LOB may similarly have been derived from desulphidation of pyrrhotite with falling temperatures. The Aggeneys ores also shows other typical features of metamorphosed sulphide ores, such as recrystallisation, remobilisation, deformation and annealing, many of which have contributed to obscuring previous textural features. The extent of sulphide partial melting is likely to have been small, and its effects obscured by later deformation, remobilisation and recrystallisation. It may, however, have played a small role in locally upgrading the ore. This study examines the petrography of the sulphides and their ore textures. Analytical techniques to examine the composition of the sulphides and of multiphase sulphide inclusions are to be undertaken. More extensive studies using numerous techniques are also required in order to clarify this topic.

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