Summary:
The Globe-Miami mining district in which the Van Dyke project occurs is known mainly for its large porphyry copper deposits, including the Miami-Inspiration, Miami East, Pinto Valley, Copper Cities and Castle Dome mines, and copper-bearing veins of the Old Dominion mine.
Porphyry copper deposits consist of disseminated copper minerals and copper minerals in veins, stockworks and breccias that are relatively evenly distributed throughout large volumes of rock. Porphyry copper deposits are typically high tonnage (greater than 100 million tons) and low to medium grade (0.3–2.0% Cu). They are the world’s most important source of copper, accounting for more than 60% of the annual world copper production and about 65% of known copper resources.
The geometry and dimensions of porphyry copper deposits are diverse, in part because of post-ore intrusions, varied types of host rocks that influence deposit morphology, relative amounts of hypogene and supergene ore each of which has different configurations, and erosion and post-ore deformation including faulting and tilting. Porphyry copper deposits commonly are centered on small cylindrical porphyry stocks or swarms of dikes.
The vertical extent of hypogene mineralization in porphyry copper deposits is generally less than or equal to 1 to 1.5km. The predominant hypogene copper sulphide minerals are chalcopyrite, which occurs in nearly all deposits, and bornite, which occurs in about 75% of deposits. Molybdenite, the only molybdenum mineral of significance, occurs in about 70% of deposits. Gold and silver, as by-products, occur in about 30% of deposits.
The Van Dyke copper deposit has a drill-defined, north-easterly strike length of 1500m, a width of 1300m, and a thickness between 40m to over 230m.
Mineralization includes both hypogene (primary sulphide) and supergene (secondary oxidization/enrichment -oxide-silicate+/-sulphide) types, but the latter far outweighs the former in terms of abundance, grade, and therefore economic potential. Secondary copper mineralization comprises most of the Van Dyke deposit. Mineralization, consisting primarily of malachite, chrysocolla, azurite, cuprite and tenorite occurs over a 1,500m horizontal distance principally in tectonically fractured to brecciated panels of Pinal Schist. The secondary minerals in the vicinity of the Van Dyke shaft occur primarily as bands and crustifications, textures that suggest formation was by filling of open spaces, whereas in other parts of the deposit, the secondary copper minerals occur as staining on cleavage planes, in fractures and as in-situ replacement in quartz veins. There are no relict sulphide grains in the upper part of the deposit. Beneath the secondary copper mineralization there exists a weakly developed Supergene zone; containing primarily chalcocite with sparse malachite, azurite and chrysocolla and is transitional down-section locally into weakly-developed zones of hypogene mineralization, primarily located in the central and western parts of the project area.
The Van Dyke deposit is located immediately southwest of the Miami Caved deposit and east of the Miami East deposit. It is separated from the Miami Caved deposit and from the Miami East deposit by the Van Dyke fault. The Van Dyke deposit is interpreted to be the eastern extension of the porphyry copper deposit mined by the Miami-Inspiration operation and the southern extension of the Miami caved porphyry copper deposit. The deposit is covered by from 186m to 627m of alluvium and post-mineral Gila Conglomerate.
The Van Dyke deposit is hosted primarily in the Pinal Schist and to a lesser extent in porphyritic dykes of Schultz granite. Secondary copper mineralization comprises much of the Van Dyke deposit. The Oxide zone consists primarily of malachite, chrysocolla, azurite and cuprite. These copper minerals occur in fractures, in quartz veins and along cleavage planes but primarily in fractured to brecciated areas of Pinal Schist. Beneath the oxide copper mineralization there exists a weakly developed Supergene zone containing mainly chalcocite with sparse malachite, azurite and chrysocolla; it istransitional down-section into local, weakly-developed zones of hypogene chalcopyrite-pyrite-molybdenite mineralization particularly in the center and western parts of the project area. Hypogene copper-molybdenum mineralization is subordinate to the secondary copper mineralization that comprises much of the Van Dyke copper deposit.