Johnson Camp, Gunnison Copper Project, Cochise Mining District, Cochise County, Arizona, USAi

Regional Level Types
Johnson Camp	Village (Abolished)
Gunnison Copper Project	Project (Active)
Cochise Mining District	Mining District
Cochise County	County
Arizona	State
USA	Country

A Cu-Ag-W mining area centered on a former mining camp and obliterated ghost town located 7 miles North of the Dragoon RR Station.

Paleozoic limestones NE of the granite-schist contact have been extensively metamorphosed to tactite, consisting largely of garnet and other silicates. This tactite zone has copper that was mined for more than 2 miles along the strike.

The rocks of the mining area are cut by a well defined set of faults and fractures striking N.5º to 30º E., and dipping 60º to 80ºE., called the "Northeasters;" a less well defined but important set striking .60º to 90º., and dipping 30º to 60ºS., the "Easters;" and a rare and relatively unimportant group striking .10º to 45ºW. and dipping over 65º SW. or NE, the "Northwesters." Subsidiary fractures striking essentially parallel to the Easters but dipping over 60ºS. are commonly associated with the Easters and also occur where no close relation to an Easter is evident.

With several minor exceptions, the faults are normal faults and the east or southeast sides are downthrown. The displacements range from almost nothing to a few tens of feet on most of the faults, but locally exceed 100 feet in the Mammoth and Copper Chief faults, 250 feet on the Republic fault and 1,000 feet on the Keystone fault. Many of the faults appear to have formed before the mineralization, for fractures belonging to each set have localized ore at one place or another in the district.

It is probable that the main movement on the large Keystone fault took place during the regional faulting of the area which preceded consolidation of the quartz monzonite for there is no indication that this fault cuts the quartz monzonite (Texas stock). It is possible that all faults were formed at this time.

Fault movements were renewed at intervals, and some movement took place after the ore was formed. At least two periods of movement, each followed by introduction of quartz, are indicated for some Northeasters by brecciated early quartz fillings cut by later unbrecciated fillings.

The rocks of the mining area are altered texturally and mineralogically in a way that is characteristic of igneous contact zones. Their appearance and mineral composition depend to a large extent on the original composition of the rocks from which they are derived.

Shale beds, like most of the lower member of the Abrigo formation and several beds in the Martin limestone have been altered in most parts of the area to compact fine-grained hornfels having mica, feldspars, diopside, tremolite, epidote and chlorite as the princpal minerals. Impure limestone, calcareous sandstone and calcareous shale, like the middle member of the Abrigo, are more or less altered to granular brown or greenish silicate rocks characterized by any or all of the calc-silicates, garnet, epidote, vesuvianite, and wollastonite, together with some diopside. Before becoming granular calc-silicate rocks the most shaly layers passed through a stage n which they were dark hornfels, preserved in some parts of the area because of arrested metamorphism. Sandy dolomite and dolomitic sandstone, like the upper member of the Abrigo and the middle part of the Martin, are altered to granular, nearly white, silicate rock weathering rusty brown and characterized by the calc-magnesia silicates diopside and tremolite. At a few places impure limestone has been converted to white silicate rock indistinguishable from that derived from dolomite, but no place is known where dolomite has been altered to garnet or any of the other magnesia-free minerals so characteristic of the metamorphosed limestones. Nearly pure dolomite like the lower part of the Escabrosa limestone and parts of the Martin limestone, is generally recrystallized to dolomitic marble altough in places it has been de-dolomitized into calcite-tremolite, calcite-forsterite or calcite-serpentine rocks. In general pure limestone like most of the Carbniferous limestones, has been recrystallized into calcite marble lacking silicate minerals.

The silicate rocks commonly contain considerable quantities of K feldspar, that makes up over 25% of many specimens and over 70% of some, quartz, and calcte. These minerals are, in part, products of late stage metasomatic replacement as shown b distribution and textural relations. The are also, in part, recrystallized minerals from the original sediment.

The orebodies occur at or near the intersection of mineralized fractures with favorable beds. Some orebodies are tabular deposits parallel to the beds, 3 to 15 feet thick and several hundred feet across. The largest deposits are chimneys which are more or less oval in cross-section and have the long axis and intermediate axis in the plane of the beds, and the short axis perpendicular to the beds. The chimney is known as a manto if its long axis lies at a large angle to the dip of the beds.

The primary ore consists of varying proportions of chalcopyrite, sphalerite, bornite, and pyrite, with a little molybdenite and scheelite in a gangue of calc-silicates, K feldspar, quartz and calcite.

The mineralization was probably a single but complex process brought about by ascending hot fluids. Two stages are recognized. At first there was a metamorphic stage during which most of the calc-silicates, and much of the K feldspar were formed. Fracturing and brecciation followed. Later there was a metasomatic stage during which ore minerals K feldspar, a new generation of calc-silicates, quartz and calcite were introduced. The metasomatic stage started at a high temperature, judging from the second generation of contact silicates. I continued to uch lowe temperatures, at which quartz and calcite formed in juxtaposition without reacting with one another to form silicates. In general the ore minerals are interstitial to the silicates, but locally the ore minerals fill fractures in the silicates and replace them.

Production was some $6,000,000 worth of Cu & Ag ore (period values).

Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/Johnson,_Arizona
Wikidata ID:	Q28222333
GeoNames ID:	5300068
Link to USGS MRDS:	10160924

Mindat Articles

Johnson Camp by Rolf Luetcke

Localities in this Region

Other Regions, Features and Areas containing this locality

References