FIELD AND PETROGRAPHIC OBSERVATIONS AT THE MAJUBA HILL CU-(SN) DEPOSIT, PERSHING COUNTY, NEVADA

The Majuba Hill Intrusive Complex represents an opportunity to study a rhyolite-hosted Cu-(Sn) deposit. Majuba Hill consists of hypabyssal, fine-grained, quartz rhyolite porphyries emplaced during the late Oligocene (ca. 25 Ma) along a NE-SW trend into Triassic slates. Previous works (Stevens, 1971; MacKenzie and Bookstrom, 1976) studied the local geology and described primary ore mineralization. However, little research has been conducted on the genetic relationships between ore mineralization and intrusion-related processes. The aim of this CREG project is to determine the detailed paragenetic sequence of magmatic and hydrothermal events and the absolute timing of Cu and Sn mineralization.

MacKenzie and Bookstrom (1976) identified five temporally distinct intrusive assemblages. Field relationships and petrographic descriptions of units in these assemblages reveal the intricate emplacement history of the Majuba Hill complex. Nearly all rhyolite outcrops are altered to white, aphanitic groundmass textures. Porphyritic rhyolites with 1-5 mm-size euhedral to subhedral quartz phenocrysts are common. Tourmaline pseudomorphs (after K-feldspar) are also common in porphyritic rhyolite samples. Aphyric rhyolites are present but are less abundant.

Tourmaline-rich magmatic-hydrothermal breccias affect the upper part of the complex. Textural observations suggest successive episodes of brecciation occurring before and after intrusion of certain rhyolite units.

Copper sulfide veining constitutes the dominant form of ore mineralization at Majuba Hill. Cassiterite, the primary Sn ore, is also present in minor amounts. Disseminated pyrite and chalcopyrite were observed with tourmaline in drill cores. The juxtaposition of Sn and Cu mineralization indicates the possibility of magmatic-hydrothermal overprinting with Cu-bearing fluids occurring after Sn-bearing fluids. Additionally, while Majuba Hill is classified as a Cu-Sn deposit, exploration projects have identified significant Ag mineralization.

References:

Stevens, D.L., 1971, The geology and ore deposits of the Antelope (Majuba Hill) mining district, Pershing County, Nevada: M.S. thesis, University of Nevada, Reno.

MacKenzie, W.B., and Bookstrom, A.A., 1976, Geology of the Majuba Hill Area, Pershing County, Nevada: NBMG Bulletin 86.