Geology and alteration in the Golden Mile, Kalgoorlie

Abstract

The giant Kalgoorlie gold deposit (1,200 metric tons Au recovered) is surrounded by a wide wall-rock alteration halo (>1 X 3 km). This alteration primarily represents the selectively pervasive formation of carbonate, chlorite, and localized intense pyrite and muscovite alteration, which all formed synchronously with gold mineralization. The carbonate alteration halo is interpreted as epigenetic because of evidence that carbonate-chlorite assemblages overprint regional metamorphic fabrics. Furthermore, the geometry of the alteration zone boundaries is controlled by ductile shear zones formed during regional deformation and the geochemistry of all the alteration zones suggests that one fluid was responsible for all assemblages. There is no evidence of intense, premetamorphic carbonation in the Kalgoorlie area.Kalgoorlie is situated in the highly auriferous Eastern Goldfields Province of the Yilgarn Block, within a narrow corridor (rift zone) that contains several large gold deposits. Regional metamorphism has produced upper greenschist facies assemblages but has retained original rock textures. At Kalgoorlie, the most important gold host is the Golden Mile Dolerite (80% of gold produced), a differentiated tholeiitic sill intruded between a lower mafic-ultramafic pile, including the Paringa Basalt (the next most important gold host), and overlying felsic rocks and metasediments.The Kalgoorlie syncline is the dominant tectonic structure in the main gold-producing area of the Golden Mile. Adjacent to this infold of metasediments, the Golden Mile Dolerite and Paringa Basalt are the site of a number of ductile shear zones which formed synchronously with the syncline. These zones, and the many smaller shear zones between them, have provided fluid channelways throughout the Golden Mile and are the site of most economic gold mineralization.Three alteration zones have been mapped in the Golden Mile Dolerite on the basis of the mineralogic siting of Fe (i.e., the chlorite, carbonate, and pyrite zones). The additional actinolite zone, representing the unaltered regional metamorphic mafic assemblage, has been converted to chlorite zone assemblages throughout the mined areas of the Golden Mile. The carbonate zone (bleached dolerite) can be traced for 2 km through the main mineralized parts of the Golden Mile and to the full depth of Lake View mine. The boundary between the chlorite and carbonate alteration zones is locally sharp and controlled by ductile shear zones but is generally strata-bound and preferentially developed in the more Fe-rich parts of the dolerite. The pyrite zone, which includes all economically gold-mineralized areas, is the site of the strongest muscovite alteration and is most intense and continuous in ductile shear zones.The chlorite zone (chlorite + calcite + or - ankerite) forms from mild carbonation of actinolite zone assemblages; the carbonate zone (ankerite + siderite) is a more local expression of this carbonation. Even after extensive carbonate alteration, relative major element abundances are little changed and the main added components are limited to CO 2 , S, K, and Rb. The pyrite zone (i.e., the gold lodes) has ankerite, muscovite, quartz, albite, pyrite, and minor siderite-bearing assemblages; it represents significant addition of sulfur. The three alteration zones not only reflect differing degrees of alteration intensity but are also strongly influenced by host-rock Fe/Mg and this ratio largely determines the stability of chlorite (favored by low Fe/Mg) and thus the distribution of the carbonate zone (favored by high Fe/Mg). Progressive deformation within the shear zones reworked existing mineralization.Kalgoorlie fits a metamorphic replacement model of gold formation by auriferous metamorphic fluids interacting with Fe-rich host rocks. Its great size is a function of a very well-developed fluid channelway system, via ductile shear zones and later fault zones, and the favorable chemical nature of the host rocks, especially the Golden Mile Dolerite. The full width of carbonation (chlorite and carbonate zones) reflects overlapping alteration envelopes around the closely spaced fluid pathways. The processes responsible for the formation of most of the gold at Kalgoorlie were essentially similar to those at several other Archean gold deposits, but on a larger scale.