1383 The Canadian Mineralogist Vol. 42, pp. 1383-1403 (2004) TEMPERATURE GRADIENTS RECORDED BY FLUID INCLUSIONS AND HYDROTHERMAL ALTERATION AT THE MOUNT CHARLOTTE GOLD DEPOSIT, KALGOORLIE, AUSTRALIA TERRENCE P. MERNAGH § Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia CHRISTOPH A. HEINRICH Swiss Federal Institute of Technology, ETH Zürich, Isotope Geology and Mineral Resources, ETH Zentrum, NO CH-8092 Zürich, Switzerland EDWARD J. MIKUCKI 2285 Coolgardie Street, Mundaring, WA 6073, Australia ABSTRACT The Mount Charlotte gold deposit, Kalgoorlie, Australia, comprises a series of steeply plunging orebodies in which a network of conjugate quartz veins are surrounded by sulfide-rich alteration haloes that host most of the gold. There are three mineralogically distinct types of alteration halo, which are systematically distributed on the orebody scale, with pyrite-rich haloes at the top and in the periphery of the deposit, and pyrrhotite-rich alteration at depth and in the center of the fluid-upflow zones. Fluid inclusions in the main-stage quartz from veins within each of the three alteration assemblages have been analyzed to determine possible thermal and chemical trends on the scale of the deposit. Fluid-inclusion assemblages associated with the shallow and more distal pyrite–muscovite alteration contain three-phase H 2 O–CO 2 inclusions and coexisting liquid-rich, aqueous inclusions with less than 15 vol.% vapor. The lowest-temperature Fluid Inclusion Assemblages (FIAs) from this alteration assemblage homogenize between 210 and 245°C. FIAs in quartz veins in pyrite–pyrrhotite alteration contain two-phase H 2 O–CO 2 –CH 4 inclusions and liquid-rich, aqueous inclusions. The lowest-temperature FIAs in this assemblage homogenize between 240 and 275°C. Veins in pyrrhotite–albite alteration contain two-phase H 2 O–CO 2 –CH 4 inclusions and liquid-rich, aqueous inclusions, with the lowest- temperature FIAs homogenizing from 260 to 310°C. A late, secondary population of liquid-rich H 2 O–CH 4 inclusions was also observed in veins associated with pyrite–muscovite and pyrrhotite–albite alteration. These inclusions homogenize between 239 and 286°C. Although the temperature ranges overlap, the fluid-inclusion data and alteration data consistently indicate a signifi- cant gradient in temperature between the base and the top of the deposit (up to 100°C / km) during vein stockwork formation and alteration at Mount Charlotte. In contrast, the chemical composition of the fluids in terms of H 2 O : CO 2 : CH 4 : NaCl, as deter- mined by microthermometry and Raman-microprobe analyses, do not vary in a systematic manner on the deposit scale, although the CH 4 : CO 2 ratio varied across a large range. These observations indicate that gold deposition at Mount Charlotte occurred by desulfidation of bisulfide complexes due to reaction with Fe-rich wallrocks to form pyrrhotite and pyrite, as previously suggested. These reactions are strongly temperature-dependent. The relatively high grade of gold in the Mount Charlotte orebody can be explained by the unusually steep temperature-gradient along the fluid-flow path through the vein network. Keywords: gold deposit, Mount Charlotte, hydrothermal alteration, gold mineralization, fluid inclusions, Kalgoorlie, Australia. SOMMAIRE Le gisement aurifère de Mount Charlotte, à Kalgoorlie, en Australie, est fait d’une série de zones minéralisées à fort pendage, dans lesquelles des veines de quartz conjuguées sont entourées d’une auréole à sulfures qui contient la plupart de l’or. Nous distinguons trois types d’altération, dont la distribution est systématique à l’échelle du gisement, avec une auréole riche en pyrite vers le haut et près de la périphérie du gisement, et une auréole à pyrrhotite en profondeur et au centre des zones de montée de la phase fluide. Nous avons analysé les inclusions fluides piégées dans le quartz du stade principal de minéralisation des veines de § E-mail address: terry.mernagh@ga.gov.au