Economic and Environmental Geology Notes (Geos 340)

Introduction, syllabus. Field trip planning

Processes.

Conceptual

Chemical separation (oxidation/reduction, dissolution/precipitation, examples Fe oxidation, write reaction, multiple examples dissolution/pption)

Physical separation (gravity-density settling, magnetic, sorting by stream/wind)

Biological activity (changes in solubility/precipitation either direct or indirect, production of fossil fuels, examples)

notes: properties important both for ore/resource production and for environment. Also define pollutant distribution and migration.

Earth Processes (radioactive, internal heat driven largely)

magmatic processes (energy sources, phase equilibria, partitioning, fractional crystallization, settling, Cr layered intrusions combination of fractional crystallization and settling-magma injection, magma mixing, double diffusive mixing, crystal fractionation, convection, density currents, nucleation densities, crystallization in thermal gradients, Ni compatible, found in mafic-ultramafic - F, Be, Li, Ree incompatible found in pegmatites - explain relation to degree of fractionation from mantle composition)

note on double diffusive convection: The best known double-diffusive instabilities are ``salt-fingers'' (Stern, 1960). These arise when hot salty water lies over cold fresh water of a higher density and consist of long fingers of rising and sinking water. A blob of hot salty water which finds itself surrounded by cold fresh water rapidly loses its heat while retaining its salt due to the very different rates of diffusion of heat and salt. The blob becomes cold and salty and hence denser than the surrounding fluid. This tends to make the blob sink further, drawing down more hot salty water from above giving rise to sinking fingers of fluid. There are many other different forms of double-diffusive convection.

notes for Layered basic intrusions and ophiolites

Layered basic intrusions occur throughout the geologic record on the continents. They are generally large, funnel-shaped bodies of crytalline igneous rock that as an overall composition close to that of a mid-ocean ridge basalt (though this was not known at the time these rocks were first studied). More importantly, most of them apparently crystallized in situ from a single batch of very primitive magma. During the 1950's, this allowed the evolving field of experimental petrology the opportunity to test the phase relationships they were determining for basaltic systems.

Another and very important aspect of layered basic intrusions is their sulfide mineralization, which accounts for some of the richest Cu and PGE ore deposits in the world. The layering in an LBI can take many forms, the most important are:

· Mineral or modal layering

· Grain size layering

· Cryptic layering

The earliest theories for the origin of layering in igneous rocks envisioned the gravitational settling of crystals to the floor of a magma chamber, as pictured at the right. However, there are a variety of problems with this model. The most obvious is that the density relationships often do not allow it -- plagioclase, which is lighter than the magma is often found in alternating bands with olivine, which is denser than the magma.

· Modern theories for the origin of layering take other processes into account such as varying nucleation rates for the different mineral phases, crystallizazion along thermal gradients and double diffusive convection.

Despite the fact that actual crystal settling probably has little to do with the formation of igneous layered rocks, these rocks are generally still refered to as cumulates.

Metamorphic process (contact, oxide and sulfide ores, Au, Ag, hydrothermal from intrusion, more water rich sialic more mineralization often, and limestone more reactive than shale or ss, emeralds, ruby, sapphire in these environments, very rare, exogenetic region vs endogenetic, zonation [with distinct mineral assemblages and associated ore types]. regional, dewatering=hydrothermal, remobilization, effect of metamorphism on physical properties (building stone) as well as chemical, garnets refractories like Al-rich minerals, and abrasives like corundum, metamorphic grade)

from Kula notes.

zonation due to rock type, tectonic setting.

Hydrothermal process (driven by magmatic or metamorphic, generally T dependent, ppt/solution depends on T, pH, Eh, P, Cl, S, Na, K, CO2; discuss solubility, effect of H+, effect of Cl making soluble species in water, leChatlier principle. Get gradients and zones due to systematic variations in T, etc. Epithermal([Au, Ag, Sb], mesothermal[Cu, Pb, Zn], hypothermal[Sn, W], note commonly observed associations. Also wall rock alteration zones, metasomatism: potassic zone (K-spar), phyllic zone (micas), argillic zone (clay minerals, no Ca-rich minerals, and no Kspar), propyllitic zone (least distinction from unaltered, has carbonates, no biotite)

Notes on reactions (concepts also true for water pollutants and sediment)

PbCl2 = Pb+ + 2Cl-

or PbCl2 = PbCl- + Cl-

S precipitates, acidity affects, warmer = more soluble.

transport often as Cl, pption as S, hydrothermal zoning often is controlled by Cl, not S, solubilities.

Black smokers, dissolved metals and sulfur ppt where encounters sea water, changing T, pH, etc. Some ore deposits presently being mined on continents thought to have originated in such an environment. Talk more about when talk about Cu ores.

Diagenesis: low-T modification, recrystallization, cementation. sometimes mineral changes such as dolomite.

Cellulose (plant) = loss of S, P, H, concentration of C, = ranks of coal (lignite, bituminous, anthracite)

low cellulose marine plankton (high in lipids, some proteins and carbohydrates)= kerogen = petroleum, as slowly heated and compressed.

methane: both biogenic (CH4 produced as bacteria break down organics, swamp gas, flying saucers), and thermogenic (CH4 produced as more complex hydrocarbons break down with heating)

Earth Processes (solar, external heat driven largely)

weathering: climate, pH, Eh, T (effect of H+ on feldspar stability)

2KAlSi3O8 + 2H+ + 9H2O = H4Al2Si2O9 (kaolinite) + 4H4SiO4 (silicic acid) + 2K+

(direction of reaction when decrease pH, increase H+)

Leaching, chelating agents, 2ndary concentration processes (redox, Cu, U), Clays common product, also bauxite

from Easterbrook. Note that this is also important for pollutants, solubility determines mobility. Comment on bauxite (Al rich) versus laterite (typical red tropical soil, Fe rich)

sedimentation: sorting, river, wind, energy/density, sometimes Eh! (U in pC), placer deposits.

Crystallization from surface water, usually evaporation, can also be mixing zone where activities of components change. NaCl(mined for salt), CaSO4(H2O), KCl (mined for fertilizer). Flux of sea water to produce concentrations (such as onto shallow continental shelf or restricted basin), phosphorous from teeth bones on continental shelfs, Often pption is biologically enhanced (e.g. manganese nodules probably related to microbe activity).

Complex reactions where polluted stream encounters other water.

Plate Tectonics:

environments: subduction, (andesitic, Cu); divergence (basaltic, alkali basaltic, REE, Ni, Cr), Continental (Granitic, Au, REE, Be).

also affects landscape, erosion, swamps, rivers etc.

from Shackleton, 1986

also include diamonds, kimberlites, at continental divergent boundaries, and layered intrusives at oceanic and continental divergent boundaries, as well as granites and REE, other incompatibles in continent interior.

Petroleum and related products:

Language and economics

oil not randomly distributed

is in sedimentary rocks

largest reservoirs are in younger rocks (less likely as rock gets older)

usually less than 10000 feet

about 60% of oil is in 2% of fields

pools, fields, provinces.

lead time: time between when you decide to do something and when it is in operation (about 5-7 years for natural gas) (note: conservation effectively produces oil without lead time)

sandstone is 30% porosity, but if grains touching the permeability is too low to extract

Proven reserves: how much can get out at present day costs and technology

Ultimate reserves: how much is there and ultimately recoverable

Probably reserves: How much can get out by special extraction methods

Possible reserves: How much could get out by some imagined but possible incremental jump in science and technology

Cost of energy (e.g. per kWhour) has been decreasing since 1920’s. Now much cheaper than 1920’s or 1980’s for example.

wasting assets: non-renewable resources which, once they are gone, they’re gone. As amount decreases, cost will theoretically increase to prevent complete consumption (draw cost versus demand diagram, at some point, oil will no longer be economically viable as the world primary energy source, also used for chemicals, plastics, herbicides, pesticides, lubricants)

Prediction of discovery rates: in general, the rate of discovery declines as the number of feet drilled increases (can’t assume discovery rates will remain the same as today). Extraction efficiencies are also difficult to predict, depending on technological advance.

Fossil fuels presently about 80.3% of US energy use.

advantages: low cost, ease of transport (for oil in particular), existing infrastructure,

Disadvantages, political, wasting asset, sulfur and other pollutants

There is no energy source that is free of environmental cost (e.g. manufacture of solar cells often has more environmental cost than use of oil).

Carbon Dioxide is introduced into the atmosphere by many means, including burning of fossil fuels. This introduction of excess carbon dioxide into our atmosphere has the potential of effecting our climate by "greenhouse" warming. However, there are also many processes that remove carbon dioxide from the atmosphere. Like other problems in chemical differentiation (discussed in a previous section of this course), the element carbon is not created or destroyed in these chemical processes, but rather it is partitioned into different phases. The chart below illustrates in a schematic way some of the processes involved, and the reservoirs that carbon is stored in on Earth. The percentages of carbon in each reservoir is shown (upper number in parentheses), as well as the percentage of human-released carbon that is thought to
be in each reservoir (lower number in brackets).

1: burning of carbon or carbon-containing molecules: C + O2 = CO2.
2: respiration, the oxidation of carbon by living things to generate energy
3: weathering of rocks that contain carbon to release CO2
4: heating of limestone to produce Calcium for use in cement. This releases CO2.
A: burial of living things in sediment and conversion of living carbon to coal or oil.
B: Photosynthesis takes carbon from the atmosphere and fixes it in sugars, cellulose, etc
C: Animals eat plants
D: CO2 partitions between water and air.
E: CO2 in water mixes between the upper and lower parts of the ocean.
F: living things and inorganic processes extract CO2 from water as sediment.
G: sediments are lithified, becoming rock.
H: CO2 partitions between the air and ice (as snow falls, for example)
I: Ice containing CO2 melts
J: rivers flow into oceans.

footnote: (a), the carbon in terrestrial plants includes carbon present in animals.

Oil chemistry:

natural gas (CH4) , wet and dry (wet contains other gases such as ethane (C2H6), propane (Ch3H8), butane (C4H10), (draw pictures of C and H chains), CO, CO2, H2S, He, N2, H2)

petroleum, hundreds of hydrocarbon compounds.

three major types of petroleum:

Paraffin rich (light crude oils): CnH2n+2, straight or branched chains fom methane to paraffin wax: products include natural gas, LPG, gasoline, kerosene

Naptha rich (are rare crudes): CnH2n, including carbon ring: products include heavy fuel oils and asphalt residue

Aromatic hydrocarbons (heavy crude oils): CnH2n-6, at least one benzene ring: products include benzene and toluene.

from Shackleton

Sweet (low S) vs Sour (higher S): Talk about environmental impact, S+H = H2S, acid rain.

Temp effects: biogenic (methane) and thermogenic (cracking, organic to kerogen and petroleum and then conversion of kerogen to thermogenic wet gas), diagenesis (<50C, a few hundred meters depth), catagenesis (50-150C, up to about 3500-5000m, to 1.5kb), metagenesis (

kerogen=insoluble organic matter

Sequence:

lipids, proteins, carbohydrates[which contain C,H, P, S, N, O compounds] to CH4 and residue, then to kerogin and petroleum, then to petroleum and wet gas, then to dry gas. Product is also affected by the type of kerogen (depends on type of organic source, plant or animal, terrestrial or marine). cracking=decreasing number of C in chains.

from craig et al 2001

Degradation also plays a role in petroleum production:

from Shackleton

oil accumulation

source rock (vitrinite reflectivity indicator of maturity of organic sediment, comparable to coal rank, it is the reflectance of vitrinite particles in source rock.)

reservoir rock (porous and permeable) generally >30% porosity to get adequate permeability (more than close-pack sandstone). More permeable = sandstone, conglomerate, limestone. Petroleum resides in pore space.

traps: Structural (show anticlinal, fault, salt dome-explaining how upturn caused by plastic diapir), stratigraphic (show lens, transg-regression sanstone ‘V’ tilted, impermeable rocks above an angular unconformity). Cementation trap (explain differential cementation), hydrologic trap

can’t have too much fracturing of the impermeable rock.

from Tarbuck and Lutgens.

Bore Hole Logging:

More clays, more gamma, lower porosity and higher density, higher velocity

From Shackleton.

Coal accumulation:

Organic production vs decay rate (T, H2O, pH, affect both, bacterial activity affect decay, consider balance of production vs decay)

consider subsidence, transgression rates to make room for deposition

cyclothems, alternating near sea level changes

most coal in carboniferous, or late Cretaceous to early Cenozoic (hell creek and fort union groups)

cellulose in Devonian, thus no coal before that

Coal rank: increasing heating, pressure, loss of O, P, N, S, H, increasing C

craig et al.

Shackelton

craig et al

Coal Types:

humic: from organic debris that produces peat (trees), begins as peat, most abundant, may be autochthonous

sapropelic: fine grained from algal accumulations in O2-poor pools (similar to kerogen), may be allochthonous, examples: boghead and cannel coals.

Coal particles, Macerals (like coal minerals)

vitrinite: jellified plant residue, translucent to golden, some retaining some cell structure

Exinite: smaller, resistant plant debris such as spore cases, algal material,

Fusinite: charcoal like opaque carbonized cell structure

micrinite: opaque residue

sclerotinite: fungal residue.

Anthraxylon: translucent material from woody parts of trees

Attritus: opaque material and translucent material that is not anthraxylon

in situ (autochthonous): site evidence pg 21

accumulated in place of growth

evidence:

plant debris not sorted

underlying rock has roots penetrating

little to no clastic material

existing modern thick peat accumulations in place

stratigraphy suggests accumulation at or near sea level (cyclothems)

Drift (allochthonous): site evidence pg 21. especially ash connection with clastics

material transported to location

evidence

underlying sediment doesn’t fit model of where plant debris might accumulate

sedimentary structures:humus and plant material rafts, sedimentary structures, cross bedding, soft sediment deformation etc