terrestial Flashcards
(37 cards)
Basinal fluids
This describes all of the fluids that are indigenous to basins. This includes oceans and meteoric fluids.
Connate and diagenetic fluids
These are dericed from within the basin often within pore spaces which is important for leaching rocks
What controls redox conditions in terrestial deposits?
It is principally a function of organic content. It is important because metal sulfides are only stable in reducing conditions. It is from bacterial sulfate reduction which produces H2S that interacts with metals to form metal sulfides.
Where do lead, copper, and zinc come from in basinal systems?
Pb - sandstones derived from sandstones (Pb-related SS has Pb»Zn»Cu)
Cu - Basalts or redbeds (Cu>Pb+Zn for kuperschiefer/red bed Cu)
Zn: It is found within calcite and calcite-cemented rocks (MVT and SEDEX where Zn»Pb»Cu)
What are the methods for causing fluid flow?
- Topography-driven flow - basically the water infiltrates at a higher elevation and then filters to a lower elevation where it exits at an impermeable layer
- Compaction - rapid sedimentation above the sediments decreases pore space and forces fluids out
- Convection - resulting from dissolved species or heat
- Tectonic compaction
For the creation of an ore deposit it is key that there is a point of concentration of these basinal fluids
Mississippi Valley Type Pb-Zn: grade, size, fluids, commodities
Pb=Zn< 10%, low Ag, Mn, minor Cu, Co, and Ni. large (100’s Mt) occuring as clusters within a district influenced by the platform-facies carbonates often with gypsum lenses. Ore forms ~10-100 myr after sediments are deposited. Fluids are greater than 10% NaCl with temperatures between 60-160 C
MVT ore deposit geometry, mineralogy, and examples
These are lens-like ore bodies that are semi-continuous along the strike of the favorable strata. They have breccias at the intersection of the SS and the dolostone/LS, generally higher elevations have less sphalerite. In SE Missouri they occur with the onlap of typical reef sequence onto a basin high (domal volcanics). Examples are the Pb-dominant Old Lead/Viburium trend (SE Missouri) and Zn-dominant TN mines.
SEDEX: commodities, grades, processes, and examples
These are Pb-Zn-Ag +- Cu deposits formed during diagenisis accounting for ~1/3 of Zn production. They are formed from the exhault of low temperature fluids (< 250C) in seawater crease corridors of ore. These require anoxic seawater to be anoxic. Ore forms upon the water entering the cold water and precipitating fine grains of ore that show soft sediment deformation.
SEDEX geometry
Generally fluids are within a horst and graben structure and settle onto the hanging wall. This creates a zoned orebody which is very long (~1 km) but thin (< 40 m). Generally there is greater Pb, BaSO4 at the top, greater Mn, Zn at base. Near the footwall there are breccias.
Kuperschiefer: commodities, grade, examples, geological settings.
There are Cu, Co deposits (#2 for Cu and #1 for Co) with minor Ni, Ag, Zn, Pb, and U that form at the margins of intracontinental basins where there are layers of SS and basalt. They form 10’s of myr after sedimentation from warm (100-150 C) slightly saline water where ore hits the oxidation-reduction boundary. Principal examples are Kuperschiefer, Spar Lake/Belt-Purcell (ID), Paradox Basin (CO plateau) and Central African Cu-belt (Kitanga ~5% Co)
Kuperschiefer Zoning
They show the classical reduced to oxidized sequence of minerals: pyrite, sp+gn, cpy, bornite, chalcocite, copper, malachite
Uranium deposits: processes, geometry, examples, grade, tonnage, types
These are syn-diagenetic deposits that form from the migration of low temperature fluids within permeable sandstones that are at a reduced front. Grade is ~.1-.5% U3O8 There are unconformity-related, breccia-pipe, tabular, and roll-front deposits
Basin deposit fluids: ligand development and implications for geography
The primary ligand is Cl but it deposits with S, the exception is U. These solutions primarily form from evaporation or dissolution of salts. This means that most of these deposits form @ ~27N and 10S because these are areas of high heat and low rain
How is ore typically precipitated in terrestial hydro systems?
Typically the metals are transported with H2S which then is reduced by carbon or other materials. Alternatively, the fluid interacts with a store of S in black shales.
What is the age of most MVT deposits?
Most MVT are Phanerozoic (compared to Proterozoic + Phanerozoic SEDEX). This is because in the Phanerozoic is when shelled organisms arise (Post-Cambrian explosion)
Geopetal and zebra rock textures
Geopetal textures are when there are fine MS minerals on top of clasts of carbonate breccia. Zebra rock describes fine grained dark LS with course dolostone bands which is is important for creating porosity.
What is the evidence for mixing and rapid changes in environment in MVT?
There are open space crackle breccias and massive sphalerite which show there was mixing and rapid ore precipitation
Differences between MVT and SEDEX
SEDEX occur prior to the Phanerozoic (similar to VMS but after Archean), are found within deeper carboates, not reef platform facies, and have Ag.
How does the barite in sedex form?
Barite is incredibly insoluble so it requires the mixture of barium and sulfate rich fluids. Ba is usually from K-minerals/spars. It was once used for drilling mud because it is dense.
SEDEX examples
Selwyn Basin, BC and Belt-Purcell Basin, BC, ID, MT these have MVT in shallow reef facies and SEDEX at depth. They have variants in the metal compositions with Co, Au, Cu in some deposits related to mafics from rifting. Broken Hill is considered a metamorphosed SEDEX with notable Mn
Oxide dominated Zn deposits
These are rare deposits of oxide (usually hydrated carbonates like smithsonite) that are hypogene. The principal example is Franklin NJ which had over 20 Mt of ore at ~20% Zn. It is poorly explained but suggested that instead of mixing with sulfide rich fluids in reef facies it mixes with SiO2 rich fluids. These are highly sought after because the oxide Zn is easier to process.
Discordant Cu
These are deposits that are found in discordant rocks and basically describes the Keenawah Pennisulla deposits of massive native Cu. These arose from a ~1.1 Ga rift that introduced basalts and gabbros. The deposit is hosted within basalt vesicles. In Mt. Isa there is some epigenetic cross cutting Cu ~100 mya after Zn+Pb were deposited.
Syngenetic U-V deposits
These include paleoplacers (Rand), black shales, and phosperites.
Unconformity type U deposits
These are the dominant source of U at a grade < 10% (form mixing) U and ~10 Mt. They form at the interface of SS on basement rocks, classically it is where a reducing layer intersects the SS to create a large haloe of clay alteration, hematite, and even chlorite at the center. The type locality is the Cigar Lake deposit.
