Abstract

The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold mineralization is hosted by a thin (0.5-2.0 km) but regionally extensive sequence of basalt and andesite, interbedded graywacke and argillite, and andesitic-lamprophyric intrusions. This sequence is bounded on the east by andesitic conglomerate and graywacke and on the west by interbedded graywacke and argillite. In the Syama mine area, this ore-bearing sequence is structurally layered, overturned, and east vergent. Massive competent basalt and conglomerate bound the hanging wall and footwall, respectively, of the mineralized interval.Lateritic weathering developed an oxide orebody that overlies sulfide mineralization. The oxide orebody extends to 35 to 40 m below the surface; sulfide mineralization extends to a depth of at least 500 m. Mineable oxide reserves are 3.0 million tons with an average grade of 3.20 g/ton. Geologic sulfide reserves from the top of sulfides to 320 m below the surface are 21 million tons with an average grade of 4.02 g/ton.The oxide deposit was expressed on the surface as a series of 3- to 15-m-deep ancient mine workings, which extended 800 m along the strike of the deposit, and sparse outcrops of silicified rock. Oxide ore occurs as two deposits: a surficial deposit consisting of ancient tailings and dump material which are 0.1 to 5 m thick and extend up to 200 m away from the ancient workings; and an in situ "lode" or saprolite deposit which lies immediately below the ancient workings and extends downward into sulfide mineralization.The sulfide deposit consists of lenticular bodies of intense ankerite-quartz veinlet stockworks, zones of sheeted ankerite-quartz veinlets, and breccia bodies. Pyrite is the principal gold-bearing mineral and occurs disseminated in veinlet halos and breccia clasts. Green chloritic basalt is the preferred but nonexclusive host of the mineralization; it is commonly bleached by ankerite, albite, and sericite alteration in the pyritic gold-bearing zones. Silica, Na 2 O, Au, CO 2 , S, and C have been added to mineralized zones hosted by basalt; Fe 2 O 3 , Al 2 O 3 , and trace metals have been removed. Andesitic-lamprophyric intrusions and graywacke-argillite layers also host disseminated pyrite mineralization. Faulted beds of graphitic graywacke-argillite and zones of silicification bound most individual orebodies.The volcanic rocks at Syama were probably originally deposited in a narrow rift or graben in an intra-arc setting. Gold mineralization overlapped with compressional deformation and intrusive activity. The mineralized horizons at Syama lie within a sequence of rocks which has been either stacked or attenuated by layer-parallel reverse faults. These faults coincide with abrupt regional- and mine-scale contrasts in rock competency. Intrusive rocks form a volumetrically significant part of the mineralized sequence and may have aided hydrothermal alteration as a source of heat. Both the graywacke-argillite layers and the altered basalt and intrusions may have been a source for some of the elements and fluids responsible for the mineralization and alteration.The mineralization at Syama is similar to other middle Proterozoic gold deposits in the Birimian Shield of west Africa and to Archean gold deposits hosted by mafic volcanic rocks in terms of tectonic setting, wall-rock alteration, form of mineralization, and associated structures.

First Page Preview

First page PDF preview
You do not currently have access to this article.