The DeLamar project is situated in the Owyhee Mountains near the east margin of the mid-Miocene Columbia River – Steens flood-basalt province and the west margin of the Snake River Plain. The Owyhee Mountains comprise a major mid-Miocene eruptive center, generally composed of mid Miocene basalt flows and younger, mid-Miocene rhyolite flows, domes, and tuffs, developed on an eroded surface of Late Cretaceous granitic rocks.
Earth Resources and NERCO geologists defined a local volcanic stratigraphic sequence in the DeLamar area. The mine area and mineralized zones are situated within an arcuate, nearly circular array of overlapping porphyritic and banded rhyolite flows and domes that overlie cogenetic, precursor pyroclastic deposits erupted as local tuff rings. The porphyritic and banded rhyolite flows and domes were interpreted to have been emplaced along a system of ring fractures developed above a shallow magma chamber that supplied the erupted rhyolites. This magma chamber was inferred to have been intruded within a northwest flexure of regional north-northwest trending Basin and Range faults.
Gold-silver mineralization has been recognized in two types of deposits: within 1) relatively continuous, quartz-filled fissure veins that were the focus of late 19th and early 20th century underground mining, and 2) broader, bulk-mineable zones of closely-spaced quartz veinlets and quartz-cemented hydrothermal breccia veinlets that are individually continuous for only a few feet laterally and vertically, and of mainly less than 1.3 centimeters in width. This second type of mineralization was mined in the open pits of the late 20th century DeLamar operation.
The fissure veins mainly strike north to northwest and are filled with quartz accompanied by variable amounts of adularia, sericite or clay, ± minor calcite. Much of the quartz is massive, but some has drusy or comb structure and a lamellar variety is locally abundant. Vein widths vary from a few centimeters to several meters, but persist laterally for as much as several hundreds of meters. Principal silver and gold minerals are naumannite, aguilarite, argentite, ruby silver, native gold and electrum, native silver, cerargyrite, and acanthite. Variable amounts of pyrite and marcasite, and minor chalcopyrite, sphalerite, and galena occur in some veins.
The bulk mineable type of mineralization has been delineated in four broad, lower-grade zones, two of which overlap and are centered on fissure veins. This type of mineralization has been described as zones of closely spaced veinlets and fracture fillings in porphyritic rhyolite. Most of the veinlets are less than 5 mm in width and have short lengths that are laterally and vertically discontinuous. Small veins can form pods or irregular zones up to 1- to 2-centimeters wide that persist for several centimeters before pinching down to more restricted widths. In highly silicified zones, porphyritic rhyolite is commonly permeated by anastomosing microveinlets typically less than 0.5-millimeters wide. Vein gangue minerals consist mainly of quartz, with minor amounts of adularia. Naumannite, acanthite and acanthite-aguilarite solid solution are the principal silver minerals, with lesser amounts of argentopyrite, Se-bearing pyrargyrite, Se-bearing polybasite, cerargyrite, Se-bearing stephanite, native silver, and native gold. Minor Se-bearing billingsleyite, pyrostilpnite, and Se-bearing pearceite have also been reported. Ore minerals are generally very fine grained.
In the flow banded rhyolite, scattered zones of mineralized breccia occur most frequently near the base of the unit. These breccias consist of close-packed angular fragments of flow-banded rhyolite in a chalcedonic matrix and crosscut flow layering. The gold and silver mineralization at the DeLamar project is best interpreted in the context of the volcanic-hosted, low-sulfidation type of epithermal model. Various vein textures, mineralization, and alteration features, and the low contents of base metals in the district, are typical of low-sulfidation epithermal deposits world-wide.