Abstract

The Finlandia Pb-Zn-Ag-Au vein cuts andesitic Tertiary volcanic rocks and is unusual in that the Ag-Au mineralization is earlier and of a higher temperature than the Pb-Zn mineralization. Hypogene mineralization can be divided into 7 paragenetic stages: (I) quartz; (II) quartz + muscovite + tetrahedrite + silver sulfosalts + electrum + sphalerite + galena + pyrite + siderite; (III) quartz; (IV) sphalerite + galena; (V) quartz; (VI) sphalerite + galena; (VII) yellow-green sphalerite + galena + barite. Stage II muscovite yields a K-Ar age of 10.3 + or - 0.5 m.y., which corresponds to the late Tertiary volcanism of the central Andes.Uncorrected fluid inclusion filling temperatures are 270 degrees + or - 20 degrees C for stages I, II, and III, 260 degrees + or - 20 degrees C for IV, 240 degrees + or - 20 degrees C for V, 180 degrees + or - 20 degrees C for VI, and 200 degrees to 140 degrees C for VII. Sulfur isotope temperatures from sphalerite-galena pairs agree with these temperatures, implying that the deposit was quite shallow when formed and that little or no pressure correction is necessary for the fluid inclusion data.The distribution and nature of the stage II mineralization is strongly controlled by elevation. The stage II Ag-Au bonanza ore shoots are restricted to a 130-meter vertical interval in the vein, where the ore is fine grained and contains at least 13 minerals. The fine-grained nature of the Ag-Au-rich stage II ore, its strong control by elevation, and the observation that primary fluid inclusions exhibit markedly variable liquid to vapor volume ratios all indicate that boiling was the controlling factor in the ore formation for this stage. Boiling of the hydrothermal fluid may also have been a factor in the deposition and localization of ore in a number of classic Ag-Au bonanza ore deposits and evidence of boiling may be a potential tool in exploration, especially for blind bonanza ore shoots.Chemical and isotopic data from fluid inclusions suggest the existence of two distinct hydrothermal fluids throughout the course of mineralization. One was a meteoric water with delta D (sub H 2 O) [asymp] - 100 per mil, the other a sedimentary formation (connate?) water, with uniformly high salinity (6 to 13 equiv. wt % NaCl) and delta D (sub H 2 O) between -75 and -48 per mil. The principal stages of sphalerite and galena deposition (IV and VI) are associated with the predominant presence of formation water, but meteoric water predominated during quartz and Ag-Au mineralization (stages I, II, III, and V).Both types of fluids evolved to higher values of delta 18 O (sub H 2 O) with time, from approximately - 8.5 per mil during stage I to +2 per mil during stage VI and then a final drop to approximately - 1 per mil in stage VII. Increasing delta 18 O (sub H 2 O) of both fluids and K/Na (from <0.06 to 0.214) ratios in the fluids of meteoric origin indicate that both fluids were reacting with increasing amounts of fresh rock at increasing temperatures while the temperature at the site of deposition was decreasing.Fluid inclusion and mineralogical data indicate that stage II fluids had the following chemical composition (calculated for 270 degrees C) : log f (sub S 2 ) = - 10.2 + or - 0.6, pH = 5.1 + or - 0.4, f (sub O 2 ) = - 34.6 + or - 1.6, log f (sub CO 2 ) = 1.3 + or - 0.2, and log m (sub Sigma S) = - 2.5 + or - 1.0.

First Page Preview

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