The Mercedes Gold-Silver District, Sonora, Mexico: Geology, Geochemistry and Structure of a Sierra Madre Low-sulfidation Epithermal System

The Mercedes district contains a series of Oligocene-aged low-sulfidation quartz-carbonate veins with economic Au and Ag mineralization. The veins are hosted in dextral-normal faults contained within andesitic flows, flow breccias and lahar deposits within the Sierra Madre Occidental volcanic sequence. These are localized within two structural basins adjacent to a northwest-trending anticline that exposes pre-mineral tuffs and sedimentary units. The vein mineralogy is characterized by a unique ferruginous green quartz and Mn-oxide bearing calcite assemblage along with both primary and supergene Au-Ag mineralization. Three vein systems within the district were studied: the Mercedes, Klondike and Lupita/Diluvio systems. The Mercedes vein contains the highest grades within the district, is highly-brecciated and displays weak and anomalous geochemical zonation patterns attributed to a high degree of hydrothermal and tectonic brecciation and permeability controlled supergene remobilization. The vein possesses a 4-phase paragenesis. Phase I was the major mineralizing phase, introducing native Au as well as unidentified silver minerals (likely both electrum and silver sulfosalts) along with green quartz and pyrite. Phase II began during a hydrothermal/tectonic brecciation event and appears as a dark Mn-oxide bearing calcite and rhodochrosite cement between phase I breccias. Phase III was a later quartz and calcite veinlet event with local amounts of dark carbonate. Phase IV was the final event, and is a post-mineral oxidation event creating hematite after pyrite, limonite, zeolites and cerargyrite. Statistical evaluation of the geochemical data reveals that Au and Ag grades are poorly correlated with one another, due to post-mineral reworking of the silver mineralogy. Au is most strongly correlated with Pb, Cu, Hg, Zn and Se in that order. Ag is somewhat correlated with Se and Hg, with only weak (<0.5) correlations with other elements. Ca is positively correlated with only C and Mn, supporting the fact that the dark carbonate phase is a Mn-oxide/calcite intergrowth. This is interpretation is further supported by XRF analysis of the carbonates and the presence of boxworks of pyrolusite and other Mn-oxides in former carbonate sites leached by acidic groundwaters. The Klondike vein displays a similar mineralogy and paragenesis to the Mercedes vein, but retains a higher degree of structural order that results in a less-permeable system displaying a stronger and more apparent classical geochemical zonation. Geochemical correlations display a similar pattern to Mercedes with Pb, Cu and Hg being the most correlative elemental indicators of Au grade. Ag correlations with these elements are somewhat higher than in Mercedes. The Lupita/Diluvio system is hosted within a listric structure and consists of the fault-hosted Lupita vein and the overlying Diluvio stockwork, hosted within a gravitationally-displaced block of a quartz-lithic tuff that had been structurally prepared for mineralization during its displacement along the Lupita structure. The Lupita/Diluvio system displays a similar mineralogy and paragenesis to the Mercedes and Klondike vein systems. From these data, a number of inferences can be made. First is that the high degree of brecciation and transport within the Mercedes vein has destroyed the classical epithermal geochemical zonation that is somewhat present in the Klondike system and better defined in the Lupita/Diluvio system. This "washing-out" was accomplished through both physical transport and overprinting during multiple hydrothermal pulses. Second, supergene remobilization related to meteoric fluids has altered the grades and distribution of precious metals as well as further modified the geochemical zonation. The presence of cerargyrite and mineralogical evidence suggests that these processes may be related to the poor Ag recoveries (~30%) and erratic Au/Ag ratios within the vein systems. Third, it is apparent that the structural weaknesses that localized the mineralization on the property reflect the regional structural fabric of both dextral and extensional tectonics.