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ARIZONA

The Lost Mine of Salt River Canyon

GEOLOGY OF THE AREA


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The rugged Salt River Canyon cuts through the southeastern Sierra Ancha Range which lies in the so-called "disturbed belt" of complex geologic structure which separates the thick, nearly horizontal sedimentary rocks of the Colorado Plateau from the Basin and Range Province to the south and southwest. This "disturbed belt" comprises the central third of Arizona's three physiographic provinces and is designated the Central Highlands Province. This tectonically-active belt has been faulted, folded, and uplifted several times during its history. Uplift has been so dramatic that the thick mantle of younger Paleozoic and Mesozoic sediments that once covered this part of Arizona has been completely stripped away, revealing the ancient Precambrian basement rock beneath. The rugged country that stretches from Globe northeastward to the Salt River Canyon is mostly underlain by this ancient Precambrian basement rock. Igneous, sedimentary, and metamorphic rocks are all represented in this Precambrian terrane. Some younger Tertiary-age sedimentary and volcanic rocks occur in the area but this part of the Central Highlands Province is dominated by Precambrian crystalline and sedimentary rocks.

The oldest rocks in the Salt River Canyon area and indeed in the entire state are Precambrian schists. Nearly 2 billion years old, these ancient schists crop out in the cores of uplifted mountain ranges in the Central Highlands Province and Basin and Range Province. Much more abundant than these ancient metamorphic rocks are the younger Precambrian intrusives. These intrusive igneous rocks range in composition from mafic diabase to intermediate diorite and diorite porphyry to felsic granite. The oldest intrusive rocks in the Salt River Canyon country are granites and diorites. Some of the granites are nearly white in appearance; these "leucogranites" consist almost entirely of feldspar and quartz. A late-Precambrian mafic diabase magma has intruded the older igneous, metamorphic, and sedimentary rocks of the area, forming many sills, dikes, and plugs. This dark gray diabase is widely exposed throughout the area and constitutes the youngest Precambrian intrusive rock in the Salt River Canyon area. Precambrian sedimentary rocks are also abundantly exposed in this area. The Troy Quartzite and the Apache Group of limestones, shales, and quartzites overlie the older granites and are in turn overlain and intruded by younger diabase. This entire package of Precambrian rocks is cut by numerous faults, most of which trend north-south.

In this part of Arizona, the great mining districts of Clifton, Morenci, and Globe all share some common characteristics. In all three districts, ore minerals are hosted in slabs or wedges of highly faulted Paleozoic sedimentary rock. These mineralized blocks of Paleozoic rock are generally associated with large masses of Precambrian basement rock. Somewhat similar associations can be found throughout the Salt River Canyon area, except in this case the sedimentary rock is much older. Nevertheless, the limestones of the Precambrian Apache Group are favorable host rocks for the emplacement of silver-bearing veins.

PROSPECTING POTENTIAL

Most silver sulfide deposits are formed as cavity or vein fillings or as replacement bodies in nearby country rocks. They are usually associated with medium to low temperature hydrothermal assemblages. The most common gangue minerals in silver deposits are quartz, calcite, dolomite, and rhodochrosite.

Most metal-bearing fluids tend to react readily with the surrounding country rock. Limestones in particular have the ability to "fix" or precipitate most metals out of solution. Unfortunately, silver-bearing solutions generally do not react with limestone country rock unless halite or rock salt is also present in the sequence. The metal-bearing fluids simply migrate through the formation into the supergene zone beyond. There, the silver may be deposited as one of the silver sulfides.

The famous mining districts of Clifton, Morenci, and Globe are founded on that classic association between metallic ore deposits derived from magmatic fluids and sedimentary host rocks. Somewhat similar associations occur throughout the Salt River Canyon area. Prospectors may want to focus on the scattered blocks of Precambrian sedimentary rocks that crop out in the area.