This study was initiated to investigate the occurrence of cinnabar in the oxidation zones of some mercury sulfide deposits. Seven mines or mine districts were visited and samples were taken from both the oxidation and primary zones of these deposits in order to compare their mineralogies and assess the chemical changes that took place during oxidation. As a result of the study of these specimens, several limiting factors are suggested for the chemical environments and processes active in both the deposition and oxidation of mercury sulfide deposits.
Mt. Jackson Mines Guerneviile, California
The Mt. Jackson mine is a cinnabar deposit at the intersection of several faults with a serpentinite “sill” in Franciscan sandstones and shales. Footwall silica-carbonate rock consists of magnesite, quartz, and chalcedony, with accessory chromite, kaolinite, pyrite and millerite, and is cut by magnesite, dolomite, and quartz veins. Cinnabar occurs in these veins and as disseminations in the silica-carbonate rock. Bravoite and vaesite occur in some quartz-cinnabar veins.
The deposit is typical of many mercury deposits in serpentinite. It appears that serpentinite containing Ni, Fe, Mn and Al, when exposed to a solution of the composition of geologically related thermal springs can reasonably produce the silica-carbonate rock, veins and ores. These springs average in ppm: HCO3-, 3500; Cl- 2800; Na+ 2650; H2S + H2SO4, 260; Hg++ <0.02; and ΣSiO2 + B + Fe + Mg + Ca + K + NH4, 775. A reaction can be written for this process:
Two principal ore zones consisting of mineralized silica-carbonate rock (the west and east ore zones) occur along the west trending serpentinite – sediment fault, contact at its intersection with the northwest trending cross-fault zone, one on either side of the cross-fault zone. Between the zero and the 500′ levels, this cross-fault zone is essentially vertical and cuts the west trending boundary fault with a strike. Below the 500′ level, the cross-fault zone trend shifts until it trends N60°W and dips 75°NE on the 1086′ level.
Cinnabar occurs principally in the footwall silica-carbonate rock, which consists of magnesite, quartz, chalcedony, with accessory chromite, kaolinite, pyrite, and millerite, and is cut by magnesite, dolomite, and quartz veins. The silica-carbonate rock is altered serpentinite.
The veins are composed principally of gangue minerals magnesite, dolomite, quarts, and chalcedony. The veins may be composed of one or more of these minerals, and they cross-cut each other in complex patterns. In general, the veins are related to the complex fault patterns of the cross-fault and serpentinite – sediment, contact fault zones. The veins are concentrated in the vicinity of those fault zones, but they are also partially influenced by the original shear directions in the serpentinite.
Cinnabar is the only ore mineral at the Mt. Jackson Mine and it occurs as disseminations in the silica-carbonate rock and to a lesser extent as a vein mineral. It is generally fine grained, but it may occur as coarse grained aggregates, especially in the veins. Cinnabar is pulverulent in some veins. Cinnabar is bright red when it is fine grained, but as its grain size increases, it becomes dull purplish black. Qualitative electron probe analysis indicates the presence of a few percent zinc, probably in solid solution with HgS. Disseminated cinnabar in the silica-carbonate rock is always observed associated with quartz, and as a vein mineral usually occurs in the quartz and chalcedony veins.
The Oxidation Zone and the Oxidation of Cinnabar
The mineralogy of the Mt. Jackson Mine oxidation zone consists principally of quartz, chalcedony, and goethite, with minor kaolinite, cinnabar, aragonite, and psilomelane. The oxidized material as a whole maintains the coarse texture of the silica-carbonate rock and veins. But the magnesite and dolomite have been leached out leaving only the quartz – chalcedony framework of the silica-carbonate rock and quartz and chalcedony veins. Some of these veins contain aragonite, which may have been precipitated after the Mg was leached from the late vein dolomite. The quartz and chalcedony are generally stained brown by a goethite film. In addition, some seams of black psilomelane with goethite occur in the quartz – chalcedony framework, suggesting that Mn originally occurred in the silica-carbonate rock, presumably as a magnesite solid solution component. Some zones of the oxidized silica-carbonate rock are highly porous with much open space in which the quartz – chalcedony framework is broken and discontinuous resulting from the leaching of a dominantly magnesite zone.
The cinnabar in the oxidation zone is of special interest. It commonly occurs in quartz-enclosed unoxidized and unleached primary rock and vein remnants. But in the oxidized portions of the oxidation zone, the occurrence of cinnabar is highly variable. Veinlets of well crystallized cinnabar deposited on quartz occur in thoroughly oxidized silica-carbonate rock. They may represent complex primary veins of cinnabar deposited on quartz and coated by magnesite, which was subsequently leached in the oxidation zone leaving an open faced,cinnabar-lined quartz vein. Those veinlets also contain goethite masses presumably derived from pyrite. Well crystallized cinnabar also occurs in the leached and oxidized silica-carbonate rock as individual grains, probably as remnant disseminated cinnabar. In these well crystallized cinnabar occurrences, the grains may or may not exhibit rounded crystal form indicative of some leaching. Some cinnabar in the lower oxidation zone may be remnant supergene cinnabar discussed below. Well crystallized cinnabar is bright red to dull purple depending on its grain, size.
Native mercury, which is commonly thought to be supergene, occurs as primary inclusions in unoxidized quartz and chalcedony veins at small mines (i.e. Knepper and Aurora Mines) in serpentinite in the New Idria District. It has been reported in the literature but it was not observed at the other mines visited. Hence it is probably not present as a significant oxidation product in the oxidation zone or percolated down into the primary zone.
All of the above cinnabar occurrences are considered likely to be supergene cinnabar. Analytical tests for the presence or absence of primary zinc solid solution in these cinnabars is being conducted in hope of verifying their supergene nature. It seems likely that some supergene enrichment of mercury as socondary cinnabar deposited on primary, cinnabar occurs in these, mines. It would explain the observation that the richest ore in many small mines occurs in the general vicinity of the lower boundary of the oxidation zone.