New Mexico Geological Society Annual Spring Meeting — Abstracts

Hydrothermal systems can vary from oxidizing to very reducing. Oxidizing systems deposit Mn-Fe-W-Sn-Pb-Zn-Ag oxides. Reduced systems deposit precious and base metals with Au mineralization associated with the most reduced mineralizing waters.

Study of the Luis Lopez Mn deposits indicates that oxidizing epithermal systems are short-lived and have a number of geothermal characteristics that differ from reduced systems.

Epithermal systems may start as oxidizing and evolve in time to reducing systems. Characteristics of these two types of geothermal systems are:

Similarities
Oxidizing type (Luis Lopez Mn deposits)Reducing type (Precious-base metal deposits)
Temperature of mineralization: 200-350 same
Salinity of ore-forming fluids: 0.0- 0.5 eq. wt. % NaCI 0.0-10 eq. wt. %NaCl
Boiling condition during mineralization same Shallow depth of mineralization same Association with intrusives same CO₂ content of ore-forming fluids: 1-2% same

Differences Oxidizing type (Luis Lopez Mn deposits) Reducing type (Precious-base metal deposits)
No H₂S in ore forming fluids 0.01 -1.0% H₂S in ore-forming fluids No organic materials in ore-forming fluids 0.01 -1.0% organic materials in ore-forming fluids Associated and/or hosted with volcanic rocks Associated and/or hosted with volcanic and sedimentary rocks No alteration associated with the veins (Pervasive alteration, hematization) Low-pH alteration type
Ores are oxides of Mu-Fe-W-Sn-Pb-Zn-A.g Ores are sulfides of Fe-Pb-Zn-Ag plus Au High δ¹⁸O fluids Low δ¹⁸?O fluids Low to medium water/rock ratio High water/rock ratio Fast moving ore-forming fluids Slower moving ore-forming fluids Fluids not in contact with sedimentary rocks Fluids in most cases in contact with sedimentary rocks