1J Barry Maynard

Basin Analysis and Ored Deposits: Manganese Ores of Mexico

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What is manganese (Mn) and what is it used for?

What mineral forms does it occur in?

Where does it occur in North America?

Why?

Outline: What Will We See?

Molango

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The only sizable Mn deposit in N America is Molango in Mexico

It is associated with strike-slip motion of the Yucatan peninsula

Mn appears abruptly at the base of the Kimmeridgian

Access to a spreading center in the Gulf of Mexico opening behind the translating Yucatan block is the likely culprit

What Will We Learn?

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What is Manganese?

Element 25, manganese was isolated by Johan Gottlieb Gahn, a Swedish chemist, in 1774 by heating the mineral pyrolusite (MnO2) in the presence of charcoal.

Nearly 90% of all of the Mn produced each year is used in the production of steel to make it easier to form and to increase its strength and resistance to impact.

Manganese is also used to give glass an amethyst color and is responsible for the color of amethyst gemstones.

First, it is not magnesium!

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How Does it Behave?Mn is nearly identical to Fe in its geochemistry.

Manganese Iron Mn/Fe

Formula wt 54.94 55.85

Oxidation states @ 1 atm, 398K 4+, 3+, 2+ 3+, 2+

Coordination number w O 6 6

Ionic radius, Å 0.83 0.78

Crustal abundance, ppm 950 56 000 0.017

Concentration in seawater, µg/L 0.072 0.25 0.288

in Black Sea surface water 0.56 0.29 1.93

in Black Sea deep water 333 4.14 80.4

Because of its much lower crustal abundance, it is submerged by Fe except in special environments

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How Does it Behave?Mn is nearly identical to Fe in its geochemistry.

Manganese Iron Mn/Fe

Formula wt 54.94 55.85

Oxidation states @ 1 atm, 398K 4+, 3+, 2+ 3+, 2+

Coordination number w O 6 6

Ionic radius, Å 0.83 0.78

Crustal abundance, ppm 950 56 000 0.017

Concentration in seawater, µg/L 0.072 0.25 0.288

in Black Sea surface water 0.56 0.29 1.93

in Black Sea deep water 333 4.14 80.4

Because of its much lower crustal abundance, it is submerged by Fe except in special environments

Dissolves

Unless S2- present

Sulfidic

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How Does it Behave?Mn is nearly identical to Fe in its geochemistry.

Manganese Iron Mn/Fe

Formula wt 54.94 55.85

Oxidation states @ 1 atm, 398K 4+, 3+, 2+ 3+, 2+

Coordination number w O 6 6

Ionic radius, Å 0.83 0.78

Crustal abundance, ppm 950 56 000 0.017

Concentration in seawater, µg/L 0.072 0.25 0.288

in Black Sea surface water 0.56 0.29 1.93

in Black Sea deep water 333 4.14 80.4

Because of its much lower crustal abundance, it is submerged by Fe except in special environments

Precipitates

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Where Does It Come From Today?

Most Mn is ultimately volcanic, but is hosted by sediments because it is carried so much farther than Fe

90 %10 %2+ 2+

Mn4+ oxidesFeS2

2+

O2

Fe3+ oxides

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Where Does It Come From Today?

Most Mn is ultimately volcanic, but is hosted by sediments because it is carried so much farther than Fe

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What are its Common Mineral Forms?

The dominant Mn mineral, percentage of land-based deposits

Rhodochrosite MnCO3 32.0

Braunite (Mn2O3)3(Mn,Fe)SiO3 24.3

Cryptomelane KMn8O16 8.7

Manganite MnOOH 7.8

Pyrolusite MnO2 4.9

Hausmannite Mn3O4 2.9

Psilomelane MnO2 3.9

amorphous oxides 1.9

Kutnahorite CaMn(CO3)2 1.9

Mn-calcite (Mn,Ca)CO3 1.9

Todorokite (Mn,Ca,Mg)Mn3O7_H2O 1.9

Others (oxides) 7.8

Mn2+

4*Mn2+ + 3*Mn4+

Mn nodules

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A Conflict Between Fact and Theory

Rhodochrosite and braunite, both of which are Mn2+ minerals, make up over half of the Mn phases in land-based ore deposits.

But we just said Mn is transported as Mn2+ and deposited as Mn4+

How then is the Mn in land-based deposits transported and precipitated?

Primary Mn Deposits

Molango – Jurassic, Mexico

Most Mn ore is black, dominated by low-oxidation state minerals

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Primary Fe Deposits

Hamersley IF – Paleoproterozoic, Australia

Most Fe ore is red, dominated by high-oxidation state minerals

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Sedimentary Basins and How They Relate to Ore Deposits

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1. Rift Basins

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Neoproterozoic Rapitan-type Fe, Mn

2. Collisional Basins

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Lake Superior-type Iron Formation (Proterozoic)

Cairncross & Beukes 2013

3. Strike-slip Basins

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Cairncross & Beukes 2013

Black-shale Hosted Mn Deposits – Restricted Basin Model

The Geology of Mexico – version 1

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The Geology of Mexico – version 2

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Mn

Tectonic Subsidence Analysis aka Backstripping

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Tectonic Subsidence Analysis – a rift example

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Tectonic Subsidence Analysis – a foreland example

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Tectonic Subsidence Analysis – a pull-apart example

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Tectonic Subsidence Analysis – a pull-apart example

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So What Happened?

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Mn

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Euxinic Basin Model of Ba Incorporation

SO4=

Mn++

Spreading Center

Emergence of spreading center + sea level rise provided source and pathway to bring Mn to the basin

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Mn Deposit Workers

Eric May

Mn Deposit Workers

Pat Okita

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Alphonso Martinez

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Rhodochrosite Braunite Cryptomelane

Manganite Pyrolusite Hausmannite

PsilomelaneKutnahorite Todorokite