Presented at the Geological Society of Nevada symposium on Geology and Ore deposits of the American Cordillera, in Reno, NV April 10- 13, 1995

Dolostone-Hosted Gold-Rich Jasperoids and their Relationship to Copper Replacement Mantos, LS &A and Queen Creek Mines, Superior District, Arizona

Guillermo A. Pareja
Department of Geology
Stanford University
Stanford CA 94305-2115

The Lake Superior & Arizona (LS&A) and Queen Creek mines are located near Superior , 50 miles east of Phoenix, Arizona. Their combined production from 1885 to 1959 is over 90,000 metric tons of oxidized ore with average grades of 14.8 ppm Au, 27.6 ppm Ag, and 0.9% Cu, plus around 50,000 metric tons of manganese ore of unknown grade.

Gold in these mines is found within quartz-iron oxide bodies (jasperoids) that replace a 7 m thick dolostone bed located at the base of a 600 m thick Paleozoic limestone sequence . These strata overlie a Proterozoic basement, and dip 30 to 40 degrees to the east. The host rock is a fine- to medium-grained finely laminated dark gray dolostone, regionally known as the "fetid dolomite" of the Devonian Martin Formation. About 500 m down dip from the deepest level at the LS & A mine that same bed has been replaced by specular hematite + pyrite + chalcopyrite ± magnetite concordant bodies ("mantos"). These mantos have been mined at the Magma (Superior) mine , and yielded several million tons of ore with grades of over 4% Cu and 0.9 ppm Au.

The gold-bearing bodies at the LS&A and Queen Creek mines are parallel to bedding (concordant), crop out discontinuously for over 2 km along the north-south strike of the beds, and are frequently centered on and elongated along steep east-west fractures and faults. Those structures are considered to be the "feeders" of the ore bodies, although there is no significant alteration and/or mineralization along them, outside of the favorable horizon. The jasperoid bodies are generally 0.3 to 1 m thick, 10 to 15 m wide, and 40 to 60 m long, although the mined ore bodies were more than 2 m thick, 20 to 50 m wide, and up to 200 m long. At the surface, they are composed of massive earthy hematite, hematite-stained very fine- grained quartz, and minor limonite. Most of the iron oxides are the product of weathering of pyrite and copper sulfides, although minor specular hematite is locally found. Oxidation is complete down to the deepest level of the LS&A mine (more than 400 m below the surface), and minor pyrite and chalcocite were found only on the lowest mine level. The lack of major amounts of copper carbonates suggests that the original copper content of these bodies was relatively low. Most ore bodies have sharp contacts with unaltered host rock, although hydrothermal dedolomitization (replacement of sedimentary dolomite by hydrothermal calcite) commonly occurs as an alteration halo that extends laterally for up to ten meters. Small bodies and pods of talc-quartz-calcite and talc-tremolite-calcite replacing dolostone are locally found in the area, although their relationship with a given style of mineralization is not clear.

All of the manganese production came from veins composed of fine-grained manganese oxides + Fe-Mn-carbonates in a matrix of hematite-stained very fine-grained quartz. These veins are also located along east-west structures, and range from 0.3 to 3 m thick and up to 700 m along strike. They formed mostly by wall rock replacement with only minor open space filling. There are also some small concordant manganese bodies (mantos) replacing a few coarse- grained biosparitic (crinoidal) limestone beds of the upper Martin Formation. Manganese mineralization spatially overlaps gold mineralization, but extends several kilometers beyond the limits of the latter.

No major intrusive bodies have been found in the area, although dikes, sills, and small irregular bodies of porphyry of intermediate composition are common in the center of the district. Although none of these intrusions have been dated, similar igneous rocks of the nearby Globe-Miami and Ray porphyry districts are about 70 to 60 m.y. old. Pre-mineralization "pebble dikes" occur within some of the major "feeder" structures. They may suggest the presence of a stock (possibly a porphyry body) at depth.

The similarity in the structural and stratigraphic controls on the location of the replacement ore bodies at the LS&A, Queen Creek, and Magma mines, and their close spatial location suggest that all those ore bodies are genetically related. The high-gold, low-copper character of the LS&A-Queen Creek mine bodies, and their small size relative to the Magma mine mantos, might reflect a zoning pattern from copper-rich, gold-poor bodies in the center of the district (Magma mine, Cu/Au ~ 50,000) to gold-rich, copper-poor bodies at the fringes of the system (LS&A and Queen Creek mines, Cu/Au ~ 600). The presence of this metal zoning pattern and of peripheral manganese mineralization is similar to that observed in some porphyry- and/or intrusion-related carbonate-hosted distal gold deposits (e.g., Yauricocha (Perú), Bau (Malaysia), Cove (Nevada)).

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