STRATABOUND BASE METAL DEPOSITS

Mississippi Valley-type, SEDEX, Supergene/Hypogene Zn-Pb-Cu Nonsulfide Ores

Focus

Field-studies, sampling
Thin and polished section petrography
Cathodoluminescence
XRD, SEM, WDS
Major and minor element geochemistry
Stable and radiogenic isotopes
Fluid inclusion thermometry and Cryo-Raman spectroscopy
Crush-leach analyses

Field-studies, sampling
An essential prerequisite for a metallogenetic study is a good field mapping and a careful sampling both in the field or from cores and cuttings. If possible, all the sampling stages should be undertaken by the responsible scientist. In the field, the areal distribution of the different ore types and of macroscopically observable alteration phases has to be established. Also the areal distribution of the ore types within the stratigraphic successions can be outlined. It is essential to follow the possible association of the mineralized intervals with major/minor structural lineaments.

Thin/polished section petrography
Standard petrographic studies in transmitted and reflected light are a first step in outlining the ore- and gangue minerals association as well as their paragenesis. Different investigation methods will be used for (a) primary (Zn-Pb- Cu)sulfide ores and (b) secondary/supergene (Zn-Pb-Cu-V)nonsulfide concentrations.
Staining with different solutions, e.g. the Zinc Zap (a solution of 3% potassium ferricyanide – K3Fe(CN)6 – and 0.5% diethylaniline dissolved in 3% oxalic acid) further helps in the analysis of the distribution of specific mineral phases.

Cathodoluminescence
Cathodoluminescence (CL) microscopy is a powerful tool for better outlining different diagenetic phases; it gives also the first hints to trace element contents of these phases. Cold and/or hot cathodoluminescence can be applied to gangue- and alteration carbonates as well as to other ore minerals. The use of a cathodoluminescence microscope is also an easy way to distinguish between different nonsulfide minerals, even if occurring in complex intergrowths.

XRD, SEM, WDS

XRD analysis is crucial for the determination of the mineralogical phases present in the area under exploration. Both qualitative-semiquantitative, and quantitative methods will be applied. The quantitative phase analysis (QPA) can be performed using the Rietveld method and the results will be complemented with assay data of the mineralized samples, measured in different chemical laboratories.

SEM (Scanning Electron Microscopy) is of special importance for the distribution in space of the occurring mineralogical phases. Element mapping and qualitative energy-dispersive (EDS) spectra can be obtained with the INCA microanalysis system (Oxford). SEM can also be very useful to observe the different minerals (e.g. clays) in three dimensions.
To get the precise mineralogical composition of selected mineral phases, a wavelength dispersion spectrometry (full WDS) on a Cameca SX50 (or SX100) electron microprobe can be used.

Major and minor element geochemistry
Geochemistry is an essential tool for most applied studies, especially when very small and area-wise strictly defined samples can be taken and analyzed. Sr, Na, Fe and Mn are important tracers for the gangue minerals. Several metals are good tracers for the source and genesis of many ore mineral associations, both sulfides and hypogene and supergene nonsulfides.

Stable and radiogenic isotopes

Stable isotopes (C, O, S) are a standard tool in ore deposits and fluid flow studies. They allow, for instance, deciphering if a mineral (carbonate/silicate/sulfate) possibly has seawater values or has been strongly modified. It is also important to compare the values of less diagenetically altered host rocks with those of altered ones. Negative or positive excursions, especially of carbon isotopes, can often be used for isotope stratigraphy. Stable oxygen and carbon isotope data of
carbonate minerals associated to diagenetic, hydrothermal, and magmatic sulfide ore deposits have for many years yielded valuable information on the temperatures of mineralization, origins and evolution of ore-forming fluids, mechanisms of ore deposition, and patterns of wall-rock alteration.
Stable isotope (C, O) studies might be also very important to distinguish supergene from hypogene base metal carbonate deposits, to evaluate the effects of oxidative heating and the role of microbes during sulfide oxidation. Additionally, the isotope data may help to determine the type of fluid involved in sulfide oxidation, to trace changes of isotope compositions of meteoric waters and thus to gain indirect information on the duration of sulfide oxidation, and to trace metal enrichments versus dilution processes. Additionally, even paleoclimatic information might be gained from such analyses.
The principle use of sulfur isotopes has been to understand the formation of sulfide ore deposits, which may originate in either sedimentary or igneous environments. Variations in the ∂34S values are caused by two kinds of processes: reduction of sulfate to sulfide by anaerobic bacteria which results in an increase in the 34S of the residual sulfate, and various kinds of exchange reactions which result in 34S being concentrated in the compound with the highest oxidation state of S.

The sulfur associated with sedimentary processes generally reflects the composition of biogenic sulfide produced by bacterial reduction of marine sulfate, and has negative ∂34S values. On the other hand, the S associated with igneous rocks derived from the mantle is isotopically similar to that of meteorites and has ∂34S values close to 0‰.

When sulfide minerals are precipitated, isotopic equilibration among solids and liquid may cause small differences in the ∂34S values of co-genetic minerals. The differences between minerals can be used to estimate the temperature of equilibration. The ∂13C and ∂34S of co-existing carbonates and sulfides can be used to determine the pH and oxygen fugacity of the ore-bearing fluid during ore formation.

Lead and strontium isotopes have been extensively used in the study of Mississippi Valley-type and other carbonate- hosted Zn-Pb deposits (1) to define possible metal sources, (2) to trace the chemical and isotopic evolution of the mineralizing fluids, (3) to constrain the extent of hydrothermal fluid flow and fluid mixing, and (4) to characterize the size of the mineralizing fluid systems. Sr isotopes are also important tracers for the amount of recrystallization and the rock-water interactions in a closed or open diagenetic system, but also for contact of more radiogenic fluids derived from specific lithologies.

Fluid inclusion thermometry and Cryo-Raman spectroscopy
Fluid inclusion studies provide information on the relative salinity of the fluids, from which the different mineral phases precipitated, as well as on their temperature, if pressure corrections are applied. However, the salinity calculated only from microthermometry may lead to an underestimation of true salinities, because not only Na ions may be present and complex mixtures are difficult to calculate. Fluid inclusions microthermometry is extremely important for the study of hydrothermal ore deposits, whereas it is generally of lesser utility in the research for low-temperature supergene ores.

Cryo-Raman spectroscopy, i.e. a combination of Raman spectroscopy and low-temperature microthermometry reveals in much greater accuracy the salinity and the major types of dissolved cations and anions in the fluid inclusions. This improved analytical method of fluid inclusions analysis is a valuable contribution for the interpretation of fluid history and fluid flow.