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Geology topics

S. E. Church

Publications and source records attributed to S. E. Church.

At least 73 records · Page 4Linked to original sources
USGS1987

Mineralogical basis for the interpretation of multi-element (ICP-AES), oxalic acid, and aqua regia partial digestions of stream sediments for reconnaissance exploration geochemistry

We have applied partial digestion procedures, primarily oxalic acid and aqua regia leaches, to several regional geochemical reconnaissance studies carried out using Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) analytical methods. We have chosen to use these two acids because the oxalic acid primarily attacks those compounds formed during secondary geochemical processes, whereas aqua regia will digest the primary sulfide phases as well as secondary phases. Application of the partial digestion technique has proven superior to total digestion because the concentration of metals in hydromorphic compounds and the sulfides is enhanced relative to the metals bound in the unattacked silicate phases. The aqua regia digestion attacks and leaches metals from the mafic chain silicates and the phyllosilicates (coordination number of VI or more), yielding a characteristic geochemical signature, but does not leach appreciable metal from many other silicates. In order to interpret the results from these leach studies, we have initiated an investigation of a large suite of hand-picked mineral separates. The study includes analyses of about two hundred minerals representing the common rock-forming minerals as well as end-member compositions of various silicates, oxides, sulfides, carbonates, sulfates, and some vanadates, molybdates, tungstates, and phosphates. The objective of this study is to evaluate the effect of leaching by acids of particular lattice sites in specific mineral structures.

Journal of Geochemical Exploration↗
USGS1986

Lead-isotopic data from sulfide minerals from the Cascade Range, Oregon and Washington

Lead-isotopic studies of mineral deposits associated with Tertiary plutons found in the Cascade Range of Oregon and Washington demonstrate a rather uniform isotopic composition in various sulfide minerals ( 206 Pb 204 Pb = 18.84 to 19.05 "> 206 Pb 204 Pb= 18.84 to 19.05 ; 207 Pb 204 Pb = 15.57 to 15.62 "> 207 Pb 204 Pb= 15.57 to 15.62 ; 208 Pb 204 Pb = 38.49 to 38.74 "> 208 Pb 204 Pb= 38.49 to 38.74 ), show less variation than data from the volcanic rocks of the Cascade Range and fall within the mixing array defined by the MORB regression line and continental sediments. An evaluation of the role of crustal assimilation by hydrothermal convection during emplacement was made on five sulfide deposits associated with a single composite batholith, the Cloudy Pass pluton. The Pb-isotopic data and mass balance calculations suggest that only minor amounts of the lead were derived from the overlying Precambrian (?) Swakane Biotite Gneiss during emplacement. The bulk of the metal that occurs in sulfide deposits in the Cascade mineral belt appears to have been derived from subducted continental detritus. The variation of the Pb-isotopic signature of Sulfides from specific districts or deposits suggests that there is a correlation with age and structure of the crust. 206 Pb 204 Pb "> 206 Pb 204 Pb is greater than 18.92 in northern Washington and southern Oregon where deposits have intruded Mesozoic or older crust. However, the ore deposits between the northern Oregon border and central Oregon, south of Eugene, have intruded younger crust composed largely of mafic and andesitic volcanic rocks and 206 Pb 204 Pb "> 206 Pb 204 Pb lies between 18.84 and 18.92. This region, previously called the Columbia embayment, appears to be underlain by Tertiary volcanic rocks. Lead-isotopic data may be used to define the boundaries between discontinuous blocks of Mesozoic crust and Tertiary volcanic cover.

Geochimica et Cosmochimica Acta↗
USGS1985

Genetic interpretation of lead-isotopic data from the Columbia River basalt group, Oregon, Washington, and Idaho

Lead-isotopic signatures of basalts from the Columbia River Basalt Group of central Oregon, Washington, and western Idaho, and of the high-alumina olivine basalts from southeastern Oregon plot mostly in the field of Pb-isotopic data from volcanic rocks of the Cascade Range. In contrast to the mid-ocean ridge basalts (MORB), the Pb-isotopic data for several of the formations form small, tight clusters, and the Nd- and Sr-isotopic data show discrete variation between these basalt groups. Pseudoisochrons are not observed. A single model will account for the observed isotopic and trace-element data from samples from most of the Columbia River Basalt Group. This model calls for partial melting of the convecting oceanic-type mantle and mixing with, or contamination by, fluids derived from continental-derived sediments. It requires that crustal components are not only subducted along the trench, but that they are also transported in the low-velocity zone of the mantle at least 400 km behind the active volcanic arc into the back-arc environment represented by the Columbia Plateaus province. Lead isotopes in the Saddle Mountains Basalt, the youngest formation, suggest that these magmas were formed by partial melting of a 2.6-b.y.-old subcontinental lithosphere that extends as far west as south-central Washington near Ice Harbor Dam. The source material for these flows is characterized by high Th/U, Th/Pb, Rb/Sr, and Nd/Sm ratios and light REE enrichment. An additional component, presumably granitic crust, is necessary to explain the Pb-isotopic data of the Umatilla and perhaps the Pomona Members of the Saddle Mountains Basalt. This contamination model is supported by published δ 18 O data and petrological interpretation. Published studies of basalts from the Deccan Traps, the Tasmanian dolerites, the Snake River Group, and the volcanics of the Yellowstone Plateau province also suggest that melting of old subcontinental lithosphere beneath the continental crust may be an important process in basalt genesis on a global scale.

Idaho, Oregon, Washington↗