A mineralogical and geochemical study of element mobility in sulfide mine tailings of Fe oxide Cu-Au deposits from the Punta del Cobre belt, northern Chile.

Two flotation tailings sites (Ojancos and P. Cerda) from the Fe oxide Cu–Au Punta del Cobre belt, south of Copiapo´ , Atacama desert, northern Chile, are geochemically (largely using sequential extractions) and mineralogically compared. Main ore minerals are pyrite, magnetite and/or hematite and chalcopyrite. Gangue is dominantly calcite with minor quartz. The host silicate assemblage is largely controlled by hydrothermal alteration and consists of variable amounts of the following minerals: K-feldsparFCa-amphiboleFbiotiteFsericiteFchloriteFtourmalineFepidoteFquartz. In this study, both the Ojancos and the P. Cerda tailings were deposited in valley dam impoundments and when they were filled, new tailings were deposited upstream. As a result, high quantities of seepage migrated downstream into the older tailings impoundment. At Ojancos, the recent upstream tailings have excess of acid potential (7.1 wt.% calcite and 3.5 wt.% pyrite), whereas the older downstream tailings are characterized by alternations of several meter-thick intervals with high neutralizing potential (about 40 wt.% calcite and 2 wt.% pyrite) and intervals with high acid potential (about 3 wt.% calcite and 4 wt.% pyrite). Acid mine drainage (AMD) with the precipitation of schwertmannite (pH 3.15) and chalcoalumite (pH 4.9) flows out at the interface between the uphill and downstream tailings. Strong downstream element transport is taking place and contributes to the formation of the cementation zone (mainly gypsum, ferrihydrite and goethite, and locally jarosite) in the older downstream impoundment. The cementation zone (pH = 4) shows strong enrichment of heavy metals (e.g., up to 6800 ppm Cu, 680 ppm Zn, 1100 ppm As), mainly adsorbed and as secondary sulfides (e.g., covellite). In contrast, at the P. Cerda, tailings impoundment carbonates are homogeneously distributed and the overall neutralization potential exceeds the acid potential (average of about 10 wt.% calcite and up to 2.5 wt.% pyrite). The up to 5-m thick oxidation zones (paste pH = 6.9–8.3) at P. Cerda are characterized by interlayering of coarser dark gray unoxidized layers with fine-grained, Fe(III) hydroxide-rich, ochre to red-brown colored horizons. The hyperarid climate dries out first the coarse, sulfide-rich horizons of the tailings and limits so the oxidation, which is restricted to the finegrained, due to their higher moisture retention capacity. However, results indicate that during operation an important element transfer from the younger upstream tailings to the older downstream tailings impoundment took place, possibly by sorptive transport at ferric polymers or colloids in the form of neutral mine drainage (NMD). This would explain the metal enrichments in the cementation zone, which are mainly associated to the exchangeable fraction and not as secondary sulfides. This results, in both cases (in Ojancos mainly as AMD and in P. Cerda mainly as NMD), in Fe(III) input as ferric cation, as ferric polymer, or CO3 complexes to the downstream impoundment. This constitutes a very effective acid potential transfer to the older downstream material because oxidation via input of external Fe(III) produces 16 mol of protons per mol FeS2, i.e., eight times more than via oxidation with oxygen. In addition, the created acidity favors dissolution of the abundant Fe oxides magnetite and hematite of this ore deposit type providing so additional Fe(III) for sulfide oxidation

Apparent stable-isotope heterogeneities in gangue carbonates of the Mississippi Valley-type Zn-Pb deposits of San-Vicente, Central Peru

The aim of the present communication is to emphasize that some variations of the measured delta(13)C and delta(18)O values are apparent, and due to analytical interferences caused by the presence of sulfur and organosulfur compounds in the analyzed carbonates. This is particularly relevant for isotopic studies on carbonate-hosted mineral deposits, where the nearly ubiquitous association of the host carbonates with organic matter and sulfides can certainly affect the metallogenetic interpretations. In this work two methods were used to overcome the disturbing effects of sulfides and organic matter: (1) sample pretreatment following the method proposed by Charef and Sheppard (1984), combining the oxidation of organic matter with sodium hypochlorite and trapping of the sulfur species with silver phosphate; and (2) laser-based microprobe extraction. Apparent isotopic variations in sparry dolomite from a single hand sample of zebra ore from the MVT Zn-Pb deposit, San Vicente, central Peru, are as large as 6 parts per thousand delta(13)C and 4 parts per thousand delta(18)O. These variations are reduced to several tenths of a per mil when the samples are pretreated. A careful examination of the effects of treatment with NaOCl and/or Ag3PO4 in relation to the concentration of sulfide inclusions indicates that the main disturbing effects for delta(13)C values are the presence of sulfur species and organic matter, whereas the delta(18)O values are mainly affected by the presence of sulfides. Fine- and medium-grained replacement carbonates from MVT and other sediment-hosted base metal deposits are potentially the most affected during isotope analysis, due to the common presence of organic matter and sulfides. Using in situ laser microprobe techniques, it is possible to determine isotopic variations at a sub-millimeter scale. Our results show that laser extraction analysis allows a more precise sampling of the carbonate minerals, and minimizes contamination of the sample with sulfides and to some extent with intergrown organic matter. However, there is an isotopic shift associated with the laser extraction technique, of the order of 0.5-1 parts per thousand for delta(13)C and delta(18)O values