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This report presents results from laboratory studies involving the net acid production (NAP), acid neutralizing capacity (ANC), and magnetic mineralogy of thirtyfour samples collected in the Upper Animas River watershed near Silverton, Colo., during the summer of 2003. Sampling focused mainly on the volumetrically important, Tertiary-age volcanic and plutonic rocks that are host to base and precious metal mineralization in the study area. Rocks in the study have all been subjected to a regional propylitic alteration event that modified the primary mineralogy of the host rock, while introducing minerals with an acid neutralizing capacity (ANC) including calcite, chlorite and epidote. Locally, hydrothermal alteration has consumed any ANC and introduced minerals, mainly pyrite, that has a high net acid production (NAP). Laboratory studies included hydrogen pyroxide (H 2 O 2) acid digestion and subsequent sodium hydroxide (NaOH) titration to determine NAP, and sulfuric acid H 2 SO 4 acid titration experiments to determine ANC on selected samples that generally had low NAP. In addition to these environmental rock property determinations, mineralogical, chemical, and petrographic characteristics of each sample were determined through multiple methods including semi-quantitative Xray diffractometry (Rietveld method), optical mineralogy, wavelength dispersive X-ray fluorescence, total carbon-carbonate, and 40-element inductively coupled plasma analyses. Magnetic susceptibilities, converted to estimates of volume-percent magnetite were also calculated. Although magnetite is a minor mineral constituent, it is easily measured, and can be positively correlated to measurable percentages of important acidneutralizing minerals, such as chlorite and calcite and inversely correlated to NAP indicator minerals including pyrite and clay minerals. Ranks were assigned to the samples based on ANC quantity in kilograms/ton (kg/ton) calcium carbonate equivalent, and ratios of ANC to NAP. Results show the Pyroxene Andesite Member of the Silverton Volcanics has highest ANC with little to no NAP in either the propylitic or weakly sericitically-altered samples. Samples of the propylitically altered Pyroxene Andesite Member also contains the highest mean magnetite abundance (over 8 volume percent) and therefore, may permit its regional mapping using the airborne magnetic and electromagnetic survey data. Samples from the Burns Member of the Silverton Volcanics, in general have a low ANC, high to moderate NAP, and in general, contain little to no magnetite. Samples containing pyrite (≤ 1 weight percent) have NAP that ranges from non-detectable to 39 kg/ton CaCO 3. Samples with no detectable calcite often contain abundant chlorite species (clinochlore and chamosite). Acid titration was performed on a chlorite mineral separate comprised mainly of the minerals clinochlore and chamosite, collected from a Burns Member lava with a high ANC (second highest ANC of all samples studied) and that lacks calcite. Acid titration results indicate that chlorit...

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Pyrite and marcasite (FeSi) precipitation was studied experimentally from 0 to 200° C under conditions that may have existed during the formation of carbonate-hosted Pb-Zn (Mississippi Valley Type) deposits. Dissolved ferrous iron (from FeSO4 or FeCla), elemental sulfur (S°), and hydrogen sulfide (H^S) were the reactants, with sulfuric acid (H2SO4) or hydrochloric acid (HCI) added to adjust the initial pH to 3.2 or less. A separate series of experiments using added CaCOs was run to examine the acid-neutralizing effect of CaCOs on FeS2 formation. Reaction times were from 1 to 110 hours with most reactions conducted for 24 hours. On the time scale of these experiments, iron disulfides did not form or formed in small amounts at temperatures of 150° C or less, whereas larger yields of disulfides were obtained at 200° C. In many instances, very pure disulfide phases were produced (>.95 weight percent FeS2 as marcasite or pyrite). Marcasite was usually the dominant disulfide mineral produced when both t^S and S° were initially present in acid-sulfate solutions (50-95 weight percent of FeS2(totai)» where FeS2(totai)=pyrite + marcasite). Nearly pure marcasite (90 to >95 weight percent of FeS2(totai)) formed in acid-sulfate solutions when only S° (not H^S) was added. Over the range of 0.005-0.20 M H2SO4, increasing acidity of the solution promoted marcasite formation over pyrite. Beyond a total H2SO4 concentration of 0.20 M, however, the marcasite fraction of FeS2(totai) decreased. At an H2SO4 concentration of 0.24 M, only a trace of FeS2 formed (pyrite) and marcasite was not detected. A similar trend was found in acid-chloride solutions over the HCI concentration range of 0.005-0.03 M, where the marcasite fraction was 57-92 weight percent of FeS2(totai)« However, FeS2(totai) and weight percent marcasite maxima were reached at 0.01 M HCI, and then decreased as the HCI molarity increased to 0.03 M. Pyrite and marcasite were produced from ferrous chloride or ferrous sulfate solutions in the presence of CaCOs (without added acid or H2S), but pyrite was usually the dominant disulfide (greater than 50 weight percent of FeS2(totai))« Minerals formed in these experiments in addition to disulfides include iron monosulfide, anhydrite, gypsum, siderite, magnetite, and hematite. Marcasite comprised 50 weight percent or more of FeS2(totai) in only 2 of the 15 CaCOs experiments. In the other 13 experiments, marcasite did not exceed 33 weight percent in 7 experiments and was not detected in the other 6 experiments. Thus, the presence of CaCOs apparently decreases the relative amount of marcasite produced. The results are consistent with the fact that marcasite usually forms at pH values less than 5 and that pyrite tends to be the dominant disulfide formed at higher pH values.