Andrew W. Macfarlane

Geology and major and trace element chemistry of late Archean weathering profiles in the Fortescue Group, Western Australia: implications for atmospheric PO2

Abstract We have identified two thick weathering profiles between flows of the Mt. Roe Basalt unit of the Fortescue Group, in the Pilbara area of Western Australia. The age of weathering is constrained by UPb zircon dating to lie between 2765 and 2715 Ma. The profiles, herein referred to as the Mt. Roe #1 and #2 paleosols, were mapped in detail for 1500 m and 760 m along strike, respectively. Samples were collected along traverses across both profiles where they were well-developed and well-exposed. The profiles, which may be structurally offset parts of the same weathering horizon, have well-developed upper sericite zones (1–15 m thick); these grade downward into chlorite-rich zones which are chemically and physically heterogeneous. The chlorite zones grade downward into unweathered basalts. The Mt. Roe #1 profile is overlain by 2–6 m of bedded, probably lacustrine sediments; these in turn are covered by succeeding lava flows. Evidence of a surficial origin for the paleosols includes mud cracks, ripple marks, and rip-ups of the paleosol material in the post-paleosol sediments. Soft-sediment deformation and probable dewatering structures are present along the paleosol-sediment contact. The Mt. Roe #2 profile is overlain directly by the succeeding lava flow with little or no intervening sediments. Some brecciation and pillowing is observed along the base of the overlying basalt flow in both cases, probably due to emplacement of lava into a shallow lake or onto wet soil. In the sericite zones of both profiles nearly all of the Fe, Mn, Mg and Zn and a substantial fraction of the Ca, Pb, Na, Ni and Si have been leached. The chlorite zones have elevated Fe, Pb, Ni and Zn contents and appear to be repositories of elements lost from the sericite zone. Al, Ti, Zr and Th were immobile in the Mt. Roe # 1 paleosol, and are concentrated by factors of 2.5–2.6 over the unweathered basalt. V, U and Cr were also nearly immobile. Major element and trace element data indicate that weathering took place under conditions of low atmospheric oxygen. K is strongly enriched toward the upper contact of the sericite zone, and was introduced during post-weathering metasomatism together with Rb and Sr. Na and Ca are also enriched near the top of each profile, and small amounts of each may have been introduced during regional, low-grade metamorphism of the Hamersley Basin at 2168 ± 10 Ma.

REE chemistry and Sm-Nd systematics of late Archean weathering profiles in the Fortescue Group, Western Australia

Two weathering profiles, each consisting of an upper, sericite-rich zone and a lower, chlorite-rich zone, are preserved between flows of the Mt. Roe Basalt in the Fortescue Group, Hamersley Basin, Western Australia. REE concentrations in samples from these two profiles, which originally developed ca 2,760 Ma, show large variations depending on stratigraphic position. LREE abundances and (La/Yb)N are greatest at depths of 3-6 m below the paleosurface of the Mt. Roe #1 profile and are somewhat lower in samples above this level. The LREEs reach concentrations 6-9 times greater than in the underlying basalt, and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part. LREE concentrations in the #2 profile show a similar distribution but with a sharp increase in all REE concentrations within 50 cm of the paleosurface. The distinction between the REE profiles in the two paleosols may be related to the difference in the overlying material. The #1 paleosol is overlain by a few meters of sediments and then by basalt, whereas the #2 paleosol is directly overlain by basalt. The LREEs appear to have been mobilized both during chemical weathering of the parental basalt and during later lower-greenschist-facies metamorphism and metasomatism of the paleosols. Remobilization of the REEs during the regional metamorphism of the Fortescue Group is confirmed by a whole-rock Sm-Nd reference isochron of Mt. Roe #1 samples with an age of 2,151 +/- 360 Ma. Variable initial 143Nd/144Nd values of unweathered basalt samples which may represent the paleosol protolith prevents a confident determination of the magnitude of LREE mobility. Both the initial mobilization of the REEs during weathering and the metasomatic remobilization appear to have taken place under redox conditions where Ce was present dominantly as Ce3+, because Ce anomalies are not developed within the sericite zone samples regardless of concentration. Europium anomalies in the paleoweathering profile are somewhat variable and were probably modified by mobilization of Eu2+ at metamorphic conditions. In all samples, the HREEs appear to have been relatively immobile and correlate with Al, Ti, Cr, V, Zr, and Nb. Sm-Nd systematics and REE patterns of four unweathered basalt samples indicate derivation of the Mt. Roe Basalts from a heterogeneous and enriched source having epsilon Nd between -4.0 and -7.4. Initial 143Nd/144Nd values of these basalts are even lower than those reported by NELSON et al. (1992) for Fortescue Group basalts and indicate a substantial crustal component in the generation of Mt. Roe Basalts.

Copper Sources, Metal Production, and Metals Trade in Late Postclassic Mesoamerica

Copper ore sources exploited during the Late Postclassic Period (1300 to 1521 CE) were located by means of lead isotope analyses of copper ores from 15 deposits in West Mexico, Oaxaca, and Veracruz and of 171 copper artifacts from nine Mesoamerican archaeological sites in West Mexico and in southern, central, and northeastern Mesoamerica. West Mexican ores provided copper metal for most artifacts from the west Mexican settlements of Atoyac and Urichu, as well as for some artifacts from Aztec towns, Huastec centers, a Maya site, and settlements in Oaxaca and Chiapas. West Mexico was not marginal to Mesoamerican events, but played the primary role in the production and distribution of copper metal and bronze artifacts, one of Mesoamericas key exotic goods.

La metalurgia del bronce en los Andes Sur Centrales: Tiwanaku y San Pedro de Atacama

This study is part of a larger investigation into the development of bronze metallurgy in the central and southcentral Andes during the Middle Horizon. It focuses on the kinds of bronze alloy produced and disseminated throughout the region that lies roughly between Tiwanaku and San Pedro de Atacama. Lead isotope analysis of bronze artifacts excavated at these two places and of a wide range of metallic ores sampled from deposits in Bolivia and Northern Chile indicates that the ore sources for most of the artifacts were located and exploited in the puna and high sierra of Bolivia. These results raise questions about the movement of ore, of bronze lingots, and of bronze objects within the south-central Andean interaction sphere during the Middle Horizon.

Distributions of Mercury and Phosphorous in Everglades Soils From Water Conservation Area 3A, Florida, U.S.A.

Soils in the southern half of Water Conservation Area3A are mostly peats with some organic-rich marls. Mercury contents of 64 surface samples over a500 km2 area average 28.7 ng cc-1 (209 ppb drysediment), which is typical of organic-rich soils. High Hg contents in Everglades fish are therefore notcaused by anomalously high soil Hg. Hg contents showno systematic lateral variation, consistent withdeposition from well-mixed atmospheric sources ratherthan nearby point sources or runoff from canals.Cores from 9 sites contain more Hg and P at or nearthe surface than at 20–30 cm depth. Hg and P contentsof individual cores correlate well and define separatebackground and anomalous populations. The subsurfacedistribution of P is determined largely by uptake bysawgrass and other plants. The correlation between Pand Hg suggests that, although atmospheric Hgdeposition has undoubtedly increased in recentdecades, postdepositional mobilization of Hg may beimportant in Everglades soils. This finding, togetherwith recent direct measurements of atmospheric Hgdeposition, indicates that previous estimates of Hgdeposition rates based on Everglades peat cores, whichassumed that Hg is immobile in peat after deposition,have yielded large overestimates.

Lead isotope studies of the Guerrero composite terrane, west-central Mexico: implications for ore genesis

New thermal ionization mass spectrometry and multi-collector inductively coupled plasma mass spectrometry Pb isotope analyses of three Cenozoic ores from the La Verde porphyry copper deposit located in the Zihuatanejo–Huetamo subterrane of the Guerrero composite terrane are presented and the metal sources are evaluated. Lead isotope ratios of 3 Cenozoic ores from the El Malacate and La Esmeralda porphyry copper deposits located in the Zihuatanejo–Huetamo subterrane and of 14 ores from the Zimapan and La Negra skarn deposits from the adjoining Sierra Madre terrane are also presented to look for systematic differences in the lead isotope trends and ore metal sources among the proposed exotic tectonostratigraphic terranes of southern Mexico. Comparison among the isotopic signatures of ores from the Sierra Madre terrane and distinct subterranes of the Guerrero terrane supports the idea that there is no direct correlation between the distinct suspect terranes of Mexico and the isotopic signatures of the associated Cenozoic ores. Rather, these Pb isotope patterns are interpreted to reflect increasing crustal contribution to mantle-derived magmas as the arc advanced eastward onto a progressively thicker continental crust. The lead isotope trend observed in Cenozoic ores is not recognized in the ores from Mesozoic volcanogenic massive sulfide and sedimentary exhalative deposits. The Mesozoic ores formed prior to the amalgamation of the Guerrero composite terrane to the continental margin, which took place during the Late Cretaceous, in intraoceanic island arc and intracontinental marginal basin settings, while the Tertiary deposits formed after this event in a continental arc setting. Lead isotope ratios of the Mesozoic and Cenozoic ores appear to reflect these differences in tectonic setting of ore formation. Most Pb isotope values of ores from the La Verde deposit (206Pb/204Pb = 18.674–18.719) are less radiogenic than those of the host igneous rocks, but plot within the field defined by the Huetamo Sequence, suggesting that these ores may also contain metals from the sedimentary rocks. The Pb isotope ratios of ore samples from the Zimapan deposit (206Pb/204Pb = 18.771–18.848) are substantially higher than the whole-rock Pb isotope compositions of the basement rocks. The similarity of ore Pb to igneous rock Pb in the Zimapan district (206Pb/204Pb = 18.800–18.968) may indicate that the proximal source of ore metals in the hydrothermal system was the igneous activity.

PROCESAMIENTO DE METALES DURANTE EL HORIZONTE MEDIO EN EL ALTIPLANO SURANDINO (ESCARAMAYU, PULACAYO, POTOSÍ, BOLIVIA)

En este trabajo se presentan y se analizan los resultados alcanzados en el sector metalurgico del sitio Pulac 050 (Escaramayu, Pulacayo, Bolivia). Los estudios desarrollados en el sector han puesto en evidencia un complejo de cuatro hornos metalurgicos, los cuales se encuentran espacialmente asociados con un conjunto de recintos fechados en el Horizonte Medio. Restos de varios materiales incluyendo escorias, metales y minerales fueron encontrados junto a los hornos. Los analisis de isotopo de plomo de estos materiales sugieren fuertemente que la mena fundida en el sitio fue obtenida del deposito polimetalico de plomo, plata, cobre y zinc, de Pulacayo. Tres de los hornos parecen haber sido utilizados para fundir menas metalicas; mientras que el cuarto horno se uso para refinar, quizas, el metal impuro contenido dentro de crisoles de ceramica.

Isotope and Fluid Inclusion Studies of Geological and Hydrothermal Processes, Northern Peru

Mineralization in the Hualgayoc district of northern Peru occurs in altered Miocene felsic intrusions and in mid-Cretaceous platform sedimentary rocks of the Goyllarisquizga, Inca, and Chulec formations. The ores occur both as stratiform and stratabound pyritiferous base-metal deposits (mantos), and as steeply dipping, sedimentary and intrusive rock-hosted base-metal veins. Igneous rocks in the district are affected by propylytic, sericitic-argillic, sericitic, potassic, and acid-sulfate alteration. K-Ar and Rb-Sr dating and geological evidence indicate multiple stages of intrusive activity and hydrothermal alteration, including close spatial emplacement of two or more separate Miocene magmatic-hydrothermal systems. K-Ar dates on sericite, hydrothermal biotite, and alunite indicate that the most important hydrothermal episodes in the district took place ≈13.24 and 12.4 Ma. Other K-Ar dates on altered rocks in the district may reflect various amounts of resetting by the emplacement of the 9.05 ± 0.2 Ma Hua...