Summary:
Deposit Type
The Santa Cruz Project consists of a series of porphyry copper systems exhibiting typical features of porphyry copper deposits. Porphyry copper deposits form in areas of shallow magmatism within subduction-related tectonic environments (Sillitoe, 2010). The Santa Cruz Project has typical characteristics of a porphyry copper deposit defined by Berger et al. (2008):
• Copper-bearing sulfides are localized in a network of fracture-controlled stockwork veinlets and as disseminated grains in the adjacent altered rock matrix.
• Alteration and mineralization at 1 km to 4 km depth are genetically related to magma reservoirs emplaced into the shallow crust (6 km to over 8 km), predominantly intermediate to silicic in composition, in magmatic arcs above subduction zones.
• Intrusive rock complexes associated with porphyry Cu mineralization and alteration are predominantly in the form of upright-vertical cylindrical stocks and/or complexes of dykes.
• Zones of phyllic-argillic and marginal propylitic alteration overlap or surround a potassic alteration assemblage.
• Cu may also be introduced during overprinting phyllic-argillic alteration events.
Hypogene (or primary) mineralization occurs as disseminations and in stockworks of veins, in hydrothermally altered, shallow intrusive complexes and their adjacent country rocks (Berger, Ayuso, Wynn, & Seal, 2008).
The Santa Cruz Project has a history of oxidation and leaching that resulted in the formation of enriched chalcocite horizons, and later stages of oxidation and leaching, which modified the supergene Cu mineralization by oxidizing portions of it in place and mobilizing some of the chalcocite to a greater depth.
Property Mineralization
The Santa Cruz Project is comprised of five separate areas known as the Southwest Exploration Area, Santa Cruz deposit, East Ridge deposit, Texaco Ridge Exploration Area, and Texaco deposit which represent portions of one or more large porphyry copper systems. Each deposit contains porphyry- style hypogene sulfide mineralization and subsequent tertiary-supergene oxide copper and chalcocite enrichment. Intensity varies by deposit along with speciation, and characteristics depending on spatial and vertical positions and the timing and total amount of overlying post-mineral tertiary sediment deposition.
Mineralization at the Santa Cruz Project is generally divided into three main groups:
1. Primary hypogene sulfide mineralization: chalcopyrite, pyrite, and molybdenite hosted within quartz-sulfide stringers, veinlets, veins, vein breccias, and breccias as well as fine to coarse disseminations within vein envelopes associated with hydrothermal porphyry-style mineralization. Hypogene mineralization appears to be the most concentrated within the Southwest Exploration Area, Texaco Ridge Exploration Area, and Texaco deposit areas based on IE drillholes.
2. Secondary supergene sulfide mineralization: dominantly chalcocite which rims primary hypogene sulfides and completely replaces hypogene mineralization. Other sulfides that fall within this category include lesser bornite and covellite as well as djurleite and digenite which have been identified by historic XRD analyses. Supergene sulfide mineralization developed as sub-horizontal domains, known as “chalcocite blankets”, within the phreatic zone (below the paleo water table).
3. Supergene copper oxide mineralization: Supergene oxide mineralization is dominantly comprised of chrysocolla (copper silicate) with lesser dioptase, tenorite, cuprite, copper wad, and native copper, and as copper-bearing smectite group clays. This mineralization style resides immediately above supergene sulfide mineralization near the paleo water table. Superimposed in-situ within the copper oxide zone is atacamite (copper chloride) and copper sulfates (e.g., antlerite, chalcanthite). Atacamite accounts for much of the copper grades within the oxide zone and requires formation of a brine to precipitate. The timing and mechanism for brine formation and atacamite precipitation remains poorly understood.
Mineralization at the Santa Cruz Deposit.
Hypogene Mineralization
Lithologies hosting hypogene mineralization in and around the Santa Cruz deposit include Precambrian oracle granite, Laramide porphyry, and Precambrian diabase.
Primary hypogene sulfide mineralization consists of chalcopyrite, pyrite, molybdenite, and minor bornite hosted within quartz-sulfide stringers, veinlets, veins, vein breccias, and breccias as well as fine to coarse disseminations within vein envelopes associated with hydrothermal porphyry-style mineralization. Lateral and vertical continuity of highest hypogene grades locally varies within the deposit due to clustering of Laramide porphyry dike intrusions.
Supergene Mineralization
Prior to burial by Tertiary sediments, hypogene sulfide mineralization near the paleo ground surface was subjected to multiple cycles of oxidation and enrichment resulting in locally abundant atacamite, chrysocolla, and chalcocite mineralization that form a supergene zone with complex geometries up to 600 m thick in vertical drillholes. Supergene mineralization is generally subdivided into supergene sulfide and -oxide mineralization with minor quantities of exotic copper mineralization. Atacamite and associated copper sulfate mineralization occurs dominantly within the copper oxide zone, although the relative timing and mechanism for formation is less well understood. The exotic Cu mineralization is dominantly hosted in the overlying clastic and volcanic rocks at the Santa Cruz deposit. Supergene mineralization at the Santa Cruz deposit reflects a mature, long lived supergene system (nearly complete chalcocite replacement of hypogene sulfides) with a well-developed supergene stratigraphy consisting of distinctly zoned mineralization with chrysocolla overlying chrysocolla-atacamite, overlying atacamite, overlying chalcocite.
Mineralization at the Texaco Deposit
Hypogene Mineralization
Hypogene mineralization at the Texaco deposit has been intersected with over a dozen widely spaced drillholes, historical and modern. However, the hypogene system has not been systematically tested and remains open in several directions. Hypogene mineral assemblages consist of chalcopyrite, pyrite, and molybdenite hosted within sulfide and quartz-sulfide veins, veinlets, vein breccias, and breccias, as well as fine to coarse disseminations within vein envelopes (dominantly replacing mafic minerals biotite and hornblende).
Supergene Mineralization
The supergene mineralization at the Texaco deposit consists of a similar geochemical stratigraphy to that observed at the Santa Cruz deposit. Supergene mineralization contains a well-developed leached cap with abundant limonite consisting of hematite over goethite and minor jarosite. The limonite leached cap zone overlies a chalcocite enrichment blanket of variable thickness.
Mineralization at the East Ridge deposit
Hypogene Mineralization
Hypogene mineralization in the East Ridge deposit is correlative and displaced from the Santa Cruz deposit. Hypogene mineralization includes broad zones of low to moderate-density quartz-sulfide veins consisting of pyrite, chalcopyrite, molybdenite, and rare bornite mineralization. Lithologies hosting hypogene mineralization in and around the East Ridge deposit include Precambrian Oracle Granite, Laramide Porphyry, and Precambrian Diabase.
Supergene Mineralization
Supergene mineralization in the East Ridge deposit is also correlative and partially displaced from the Santa Cruz deposit. Supergene sulfides are present as thin, stacked intervals displaced from those in the Santa Cruz deposit by D2 faulting. Chrysocolla and atacamite mineralization is more broadly distributed, especially near the fault-controlled paleo-valley formed between the Santa Cruz deposit and the East Ridge deposit.