The Strong and Harris copper-zinc-silver deposit is a sub-type of or related to a classic copper skarn (Einaudi and Burt, 1982; and Meinert et al, 2005). Skarn deposits range in size from a few million to 500 million tonnes and are globally significant, particularly in the southwestern US. They can be stand-alone copper skarns, which are generally small, or can be spatially and temporally closely associated with porphyry copper deposits, in which case they tend to be very large. The skarn at Strong and Harris and collectively in the Cochise mining district is presumably related to the Texas Canyon Quartz Monzonite, despite the intrusive itself hosting very little known economic mineralization. Mineralization in the quartz monzonite would require more specialized conditions involving the metal and volatile content of the magma, depth of emplacement, or other factors (Burt, 1977).

Copper skarns generally form in calcareous shales, dolomites and limestones peripheral or adjacent to the margins of diorite to granite intrusions that range from dikes and sills, to large stocks or phases of batholithic intrusions, and frequently are associated with mineralized intrusions. Copper mineralizing hydrothermal fluids are focused along structurally complex and fractured rocks and convert the calcareous shales and limestones to andradite-rich garnet assemblages near the intrusive body, and to pyroxene and wollastonite rich assemblages at areas more distal to the intrusive. Retrograde evolution of the hydrothermal fluids produces actinolite-tremolite-talc-quartz-epidote-chlorite assemblages that overprint earlier garnet and pyroxene. Strong and Harris occurs approximately two miles north of any known occurrences of the Texas Canyon Quartz Monzonite intrusion in the Cochise mining district, which is thought to be the source of mineralizing hydrothermal fluids. Therefore, Strong and Harris can be subcategorized as distal skarn related to a porphyry copper system. This assumption is supported by the high abundance of wollastonite alteration in the mineralized zones. The anatomy of a telescoped porphyry copper system model by Sillitoe (2010) can be used as a conceptual model to understand the spatial relationship of the Strong and Harris distal skarn and associated proximal skarns in the district.

Primary copper-zinc-silver mineralization at Strong and Harris is characterized by lenses of sulfide minerals emplaced more-or-less parallel to layering in favorable lithologic units, usually along bedding planes or in disseminated masses and blebs. Some mineralization is disseminated in certain lithologies. Less frequently, the mineralization is hosted in quartz +/- calcite +/- feldspar veins. The mineralization is typically accompanied by calc-silicate alteration of the carbonate host-rock (described as “tactite” in the logs). In some local areas or sub-units, the mineralization completely replaced the host rock with massive lenses or patches of sulfide minerals, some of which are now oxidized. The sulfide minerals include pyrite, pyrrhotite, chalcopyrite, chalcocite, and sphalerite. Minor tetrahedrite group minerals have also been reported in the historical drill logs.

Sub-units of the Earp Formation, particularly those immediately below its upper contact with the Colina Limestone, were the most favorable sites for deposition of the copper, zinc and silver minerals. However, mineralization is also present in the Colina Limestone above the Earp, as well as in the Horquilla Limestone below the Earp. Historical reports often referred to mineralization in the Horquilla as the “Peabody Sill”, as such mineralization and its host rock were termed at the historical Peabody Mine southwest of the Strong and Harris deposit. The contact between the Horquilla and this sill at the Peabody Mine was reportedly favorable, at the mine although the sill itself is thin and represents only a volumetrically minor portion of that deposit. The same relationship is observed on the western side of the Strong and Harris property where the diabase sill has been logged in several holes and is often mineralized. The thickness of the sill is typically less than 10 feet. Mineralization in tactites of the Horquilla Limestone, either stratigraphically above or below the sill, is equally if not more important than the sill itself at Strong and Harris. However, the sill is a favorable host where present.

The Strong and Harris deposit has been oxidized to varying degrees that generally decrease with depth. Three oxidation zones are currently recognized in the deposit: the oxide zone, the transition (or mixed) zone, and the sulfide zone. In the oxide zone, copper is dominantly hosted in chrysocolla with minor azurite, malachite, and tenorite. Zinc minerals noted in the oxide zone include rosasite, aurichalcite, and willemite. Sulfide zone mineralogy is dominated by chalcopyrite and sphalerite with associated pyrite and pyrrhotite. In the transition (mixed zone), the mineralogy consists of secondary sulfides (namely chalcocite) mixed with a combination of the above oxide and sulfide zone mineralogy.

The PEA presented in this report considers open-pit mining of the Strong and Harris project.

Road and Ramp Design
Road designs have been completed for the PEA to allow primary access for people, equipment, and consumables to the site. This includes haul roads between the designed pits, dumps, and proposed leach facility. Within the pit designs, ramps have been established for haul truck and equipment access. The in-pit ramps will only require a single berm. Ramps outside of the pit will require two safety berms. Onelane traffic ramps are also utilized near the bottom of pits where the strip ratio is minimal, and the traffic requirements are low.

The ramps and haul roads assume the use of CAT-777 haul trucks with an operating width of 20 feet. For two-way access the goal of the road design is to allow a running width of near 3.5 times the width of the trucks. MSHA regulations specify that safety berms be maintained at least ½ of the diameter of the tires of the haul trucks that will travel on roads. The ½ height of the CAT-777 haul trucks tires is 4.5 feet. An extra 10% was added to berm height design to ensure that all berms are sufficient in height.

Safety berms assume a slope of 1.5 horizontal to 1.0 vertical. Considering that ramps in the pit only need one berm, the road width of 85 feet was determined for two-lane traffic, which allows for 3.5 times the operating width of the haul trucks. Single-lane traffic roads are estimated to require 55 feet which allows 2.0 times the operating width of the CAT-777 haul trucks.

Roads outside of the pit will require two berms and widths are estimated to be 100 feet, allowing 3.5 times the width of the CAT-777 haul trucks.

Road designs are intended to have a maximum of 10% gradient, though some may exceed this for short distances around inside turns. Where switchbacks are utilized, the centerline gradient is reduced to about 8%. This keeps the inside gradient approximately 12%. Switchback designs have not added the detail for super elevation through the curves, but is it assumed that this will be done when they are constructed.

Pit Design
Pit designs were completed for Strong and Harris using Surpac™ software (version 2020.1). Each of the designs utilize 20-foot benches with a catch bench of 19 feet wide installed every other bench, or 40 feet apart. The bench face angle used is 70°. The resulting inner-ramp slope is 50°.

Strong and Harris pit designs were completed using four pit phases. Phase one is mined as a starter pit in the southern portion of the main pit. Phase two is mined around phase one to complete the southern portion of the main pit, followed by phase three which is designed as the northern portion of the main pit. Phase four is the small pit located to the south of the main pit designs.

Dump Design
A single waste dump is designed to the east of the ultimate pit along with in-pit back fill located in the phase two pit design. These designs are shown in the site layout in Figure 18.1. The in-pit backfill is minimized and is used to maintain the ramp for the northern phase 3 design. The east dump was designed to contain up to 163,212,000 cubic yards of material, which is within 1% or 2,000 cubic yards less than the total capacity needed considering a 1.4 swell factor. The remainder of the waste material can be placed into the phase 1 and 2 pit designs. Depending on efficiencies, it may be beneficial to costs to add more material to the in-pit backfill. This will be confirmed in additional studies.

Production Schedule
Production scheduling was completed using MineSched software (version 2020.1). The production was primarily driven by constraining the throughput of oxide and mixed leach material to 7.2 million tons per year, and mixed and sulfide material to the flotation plant to 1.8 million tons per year. Additional constraints to production limits included up to 2 benches per month could be mined in any given pit phase to manage the monthly sink rate. Mining assumes the use of contractors and their equipment to sustain the productivity required to feed the leach pad and flotation plant.

The resulting production schedule requires approximately 12 months of preproduction to strip waste above the deposit with some leach material being mined in month minus one. Leach production is ramped up through the first four months of production to full capacity in month five.

Flotation production is assumed to start at the beginning of production year two. Some flotation material will be stockpiled from the lower portions of phase 1 mining. It is assumed that some ramp up and commissioning of the plant will happen in year one, but no production is attributed to year one.

A grizzly may be used with the primary crusher to remove oversized material for further breakage. The secondary crusher discharge will be fed to the grinding mill, along with sufficient water to produce the slurry density wanted for flotation.

At this point in the development of the Strong and Harris project, no recent testing has been done on the processing of material from the three mineralogical zones. For purposes of this assessment, two conceptual processing routes have been developed using information on successful processing of material that is chemically and geologically similar to that found at Strong and Harris, as well as the historical testwork. For the sulfide material, flotation will be used to recover and then separate the copper and zinc. Acid heap leaching, followed by SX-EW will be used to treat the oxide material. The transition material may be treated by either flotation or heap leach.

Recovery of Copper and Zinc by Flotation.
The generic PFD developed for the Strong and Harris sulfide and selected transition material Generally speaking, the flow of material is from top to bottom. Both ROM and crushed rock stockpiles are included to smooth the flow of material to grinding. A grizzly may b ........