Cliff D. Taylor

Comparison of methods to determine degree of pyritization

Abstract Degree of pyritization (DOP) is a measure of the ratio pyrite iron/(pyrite iron + reactive iron) that can be related to the depositional environment of a sediment. Several methods of DOP determination have been used but not systematically evaluated. The determination/extraction of reactive (usually acid soluble) iron is critical to the DOP determination, and the method generally used is reaction of the sample for 1 to 2 min with hot 12 N HCl. We present results for timed experiments with 1 N, 6 N, and 12 N HCl on three different samples. We also show that a 24 h room temperature treatment with 1 N HCl is equivalent to the 24 h treatment with Na-dithionite. Experiments with several suites of samples show that all three of these methods leach comparable amounts of iron; therefore, the DOP values are similar. However, the 1 N HCl, 24 h procedure is preferable because laboratory handling is less and easier.

Link between ridge subduction and gold mineralization in southern Alaska

40 Ar/ 39 Ar geochronology reveals that turbidite-hosted gold deposits in the southern Alaska accretionary prism are the same age as nearby near-trench plutons. These early Tertiary plutons and gold lodes formed above a slab window during subduction of an oceanic spreading center. Ridge subduction is a previously unrecognized tectonic process for the generation of lode gold.

Environmental geochemistry of shale-hosted AgPbZn massive sulfide deposits in northwest Alaska: natural background concentrations of metals in water from mineralized areas

Abstract Red Dog, Lik and Drenchwater are shale-hosted stratiform Ag Pb Zn massive sulfide deposits in the northwestern Brooks Range. Natural background concentrations of metals in waters from the undisturbed (unmined) Drenchwater prospect and Lik deposit were compared to pre-mining baseline studies conducted at Red Dog. The primary factors affecting water chemistry are the extent of exposure of the deposits, the grade of mineralization, the presence of carbonate rocks in the section, and the proportion of Fe-sulfide in the ore. Surface water samples from the Drenchwater prospect, which has pyrite-dominant mineralization exposed in outcrop, have pH values as low as 2.8 and high dissolved concentrations of metals including as much as 95 mg l−1 Al, 270 mg l−1 Fe, 8 μl−1 Cd, 10 μl−1 Pb, and 2600 μl−1 Zn, with As up to 26 μgl−1. Surface waters from the Red Dog deposit prior to mining were also acidic and metal-rich, however, dissolved metal concentrations in Red Dog waters were many times greater. The higher metal concentrations in Red Dog waters reflect the high Zn grades and the abundant sphalerite, pyrite, and galena that were present in outcrop prior to mining. In contrast,l despite significant mineralization at the Lik deposit, carbonate rocks in the section buffer the system, resulting in less acidic, mostly near-neutral pH values with low concentrations of most metals except Zn.

The F’derik-Zouérate Iron District: Mesoarchean and Paleoproterozoic Iron Formation of the Tiris Complex, Islamic Republic of Mauritania

High-grade hematitic iron ores (or HIF, containing 60–65 wt% Fe) have been mined in Mauritania since 1952 from Superior-type iron deposits of the F’derik-Zouerate district. Depletion of the high-grade ores in recent years has resulted in new exploration projects focused on lower-grade magnetite ores occurring in Algoma-type banded iron formation (or BIF, containing ca. 35 wt% Fe). Mauritania is the seventeenth largest iron producer in the world and currently has about 1.1 Gt of crude iron ore reserves. The main host for Algoma-type iron ore in the district is magnetite quartzite layers (formerly BIF) within Mesoarchean granulite-facies rocks of the Tiris Complex. Superior-type iron ores are restricted to the allochthonous Paleoproterozoic sequences of the Kediat Ijil and Guelb El Mhaoudat, which overlie the Tiris Complex. Paleoproterozoic BIF are present in the Sfariat belt that hosts at least three known iron occurrences, and at Guelb Zednes, all of which are interpreted as fragments of Superior-type BIF that were imbricated with, or allochthonously deposited on, the Mesoarchean-Paleoproterozoic suture zone during the Birimian orogeny. Prominent linear, high-amplitude magnetic anomalies associated with BIF are characteristic of the region. Paleoproterozoic rocks in the Kediat Ijil, a klippe of metasedimentary rocks including ferruginous chert capped by a distinctive conglomerate unit, and in the El Mhaoudat Range, produce very prominent, broad, high-amplitude magnetic anomalies that can be extended at least 150 km along strike projected beneath the Taoudeni Basin to depths of >2000 m. New petrographic, geochemical, and geochronologic data presented here elucidate several features of the iron deposits, their tectonic history, and possible processes of enrichment of BIF protore to HIF. The Zouerate district iron ores are remarkably pure, consisting almost wholly of hematite and quartz. Contents of all major elements other than iron and silica are well below global averages for hematitic iron ores; minor and trace element contents are similar to those of other deposits worldwide. However, Zouerate HIF shows a general depletion of REE, a positive Eu anomaly, and a preferential enrichment of HREE compared to LREE. U–Pb ages of detrital zircons in the Tazadit and Seyala Conglomerate Formations of the Kediat Ijil are consistent with their derivation from the Tiris Complex. This interpretation suggests a passive margin depositional environment on the northeastern edge of the Mesoarchean Rgueibat Shield, unaffected by clastic input from early Birimian tectonic elements. In contrast, ages of detrital zircons from rocks faulted against the Mhaoudat Formation show depositional peaks consistent with input from Birimian and Neoproterozoic sources. These age data imply that tectonic emplacement of Guelb El Mhaoudat could have occurred as a result of Pan African orogenic events; such a Pan African record represents a previously unrecognized tectonic element in this part of the Rgueibat Shield. Timing of enrichment of protore BIF to HIF is poorly constrained and may have resulted from multiple metamorphic-hydrothermal events from 2.83 to 1.6 Ga. Geochemical trends are consistent with interaction by a relatively high-temperature, oxidizing, and possibly alkaline fluid. A supergene weathering profile exists in the Zouerate district and began forming during uplift related to opening of the Atlantic Ocean at about 160 Ma, based on a new apatite fission track age. However, effects of this supergene enrichment are relatively minor, being superimposed on one or several metamorphic-hydrothermal events responsible for the transformation of BIF to HIF.