Sierrita Mts, Pima Co., Arizona, USA

Ref.: Wilson, E.D., et al (1950), Arizona zinc and lead deposits, part I, Arizona Bureau of Mines Bull. 156: 42-43.

MRDS database Dep. ID file #10161696, MAS ID #0040190558.

The structure of the Sierrita Mountains has been complicated by folds overturns, low-angle faults, and steeply dipping falts. Much of it remains obscure and unknown. Some of the structures localized emplacement of the igneous intrusions and deposition of the ores. Some post-ore faulting has taken place.

In Mineral Hill faulted Paleozoic beds dip steeply southwestward, with older beds above younger. This block may be interpreted as one limb of a southeastward-trending, overturned fold, or it may be a remnant of a thrust-fault slice.

Helmet Peak consists of Snyder Hill limestone, bounded on the NE by down-faulted red beds and on the SW by shaly and arkosic beds of uncertain structural relation. Galbraith reports that the imestone of the peak shows a closely compressed anticline plunging southeastward.

Flexures are evident in the hill west of the San Xavier Extension Mine. Some unrecognized large folds may be present, as suggested by the wide variation in dips of beds in the Twin Buttes area.

As interpreted by Mayuga, the sedimentary rocks in the Mineral Hill, San Xavier, and Olive Camp areas rest upon the intrusive granite. Fault planes and breccia exposed above the granite indicate the contact to be a low-angle fault zone of general eastward dip. his fault zone may have localized the granite intrusion, or movement may have occurred along it after the intrusion.

A the San Xavier Extension Mine, Naco Limestone has been faulted over Escabrosa, and the fault contact dips about 25º southeastward. In the Cretaceous area south of the San Xavier Mine, older beds have been thrustover younger beds. In the northern part of Mineral Hill, a block of Escabrosa and Martin limestones has been thrust over Permian and Naco beds. The thrust plane appears to dip NE about 40º.

Steeply dipping fissures are numerous in the district. On the basis of general strike, they may be grouped into NE, NW, and east systems. Some of them have effected displacements of several hundred feet. Faults and fractures of the NE and east systems appear to be most commonly associated with mineralization.

Silver-lead deposits in the San Xavier and other districts of these mountains were known to the Jesuit missionaries and early Spaniards.