1 Basin Analysis and Ored Deposits: Manganese Ores of Mexico

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 2

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? 3

What is Manganese? Element 25, manganese was isolated by Johan Gottlieb Gahn, a Swedish chemist, in 1774 by heating the mineral pyrolusite (MnO 2 ) 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! 4

How Does it Behave? Mn is nearly identical to Fe in its geochemistry. ManganeseIronMn/Fe Formula wt Oxidation 1 atm, 398K4+, 3+, 2+3+, 2+ Coordination number w O66 Ionic radius, Å Crustal abundance, ppm Concentration in seawater, µg/L in Black Sea surface water in Black Sea deep water Because of its much lower crustal abundance, it is submerged by Fe except in special environments 5

How Does it Behave? Mn is nearly identical to Fe in its geochemistry. ManganeseIronMn/Fe Formula wt Oxidation 1 atm, 398K4+, 3+, 2+3+, 2+ Coordination number w O66 Ionic radius, Å Crustal abundance, ppm Concentration in seawater, µg/L in Black Sea surface water in Black Sea deep water Because of its much lower crustal abundance, it is submerged by Fe except in special environments Dissolves 6 Unless S 2- present Sulfidic

How Does it Behave? Mn is nearly identical to Fe in its geochemistry. ManganeseIronMn/Fe Formula wt Oxidation 1 atm, 398K4+, 3+, 2+3+, 2+ Coordination number w O66 Ionic radius, Å Crustal abundance, ppm Concentration in seawater, µg/L in Black Sea surface water in Black Sea deep water Because of its much lower crustal abundance, it is submerged by Fe except in special environments Precipitates 7

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+ Mn 4+ oxides FeS 2 2+ O2O2 Fe 3+ oxides 8

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 9

What are its Common Mineral Forms? The dominant Mn mineral, percentage of land-based deposits Rhodochrosite MnCO Braunite (Mn 2 O 3 ) 3 (Mn,Fe)SiO Cryptomelane KMn 8 O Manganite MnOOH 7.8 Pyrolusite MnO Hausmannite Mn 3 O Psilomelane MnO amorphous oxides 1.9 Kutnahorite CaMn(CO 3 ) Mn-calcite (Mn,Ca)CO Todorokite (Mn,Ca,Mg)Mn 3 O 7 _H 2 O 1.9 Others (oxides) 7.8 Mn 2+ 4*Mn *Mn 4+ Mn nodules 10

A Conflict Between Fact and Theory Rhodochrosite and braunite, both of which are Mn 2+ minerals, make up over half of the Mn phases in land-based ore deposits. But we just said Mn is transported as Mn 2+ and deposited as Mn 4+ How then is the Mn in land-based deposits transported and precipitated? 11

Primary Mn Deposits Molango – Jurassic, Mexico Most Mn ore is black, dominated by low-oxidation state minerals 12

Primary Fe Deposits Hamersley IF – Paleoproterozoic, Australia Most Fe ore is red, dominated by high-oxidation state minerals 13

Sedimentary Basins and How They Relate to Ore Deposits 14

1. Rift Basins 15

16 Neoproterozoic Rapitan-type Fe, Mn

2. Collisional Basins 17

18 Lake Superior-type Iron Formation (Proterozoic) Cairncross & Beukes 2013

3. Strike-slip Basins 19

20 Cairncross & Beukes 2013 Black-shale Hosted Mn Deposits – Restricted Basin Model

The Geology of Mexico – version 1 21

The Geology of Mexico – version 2 22 Mn

Tectonic Subsidence Analysis aka Backstripping 23

Tectonic Subsidence Analysis – a rift example 24

Tectonic Subsidence Analysis – a foreland example 25

Tectonic Subsidence Analysis – a pull-apart example 26

Tectonic Subsidence Analysis – a pull-apart example 27

So What Happened? 28 Mn

Euxinic Basin Model of Ba Incorporation SO 4 = 29 Mn ++ Spreading Center

Emergence of spreading center + sea level rise provided source and pathway to bring Mn to the basin 30

Mn Deposit Workers Eric May 31

Mn Deposit Workers Pat Okita 32 Alphonso Martinez

RhodochrositeBrauniteCryptomelane Manganite PyrolusiteHausmannite Psilomelane 33