Sponsored by: ASARCO LLC
Genesis of the Chilito Porphyry Copper Deposit
by Donald Applebee, Corporate Exploration Manager, ASARCO LLC
Abstract: Chilito Porphyry Copper Deposit is located in the Dripping Springs mountains 4 miles N-NW of Asarco’s Hayden facility which includes both a concentrator and smelter. The site can be accessed by a dirt road along HWY 177 or from another dirt road via the back gate of the Hayden facility.
Early mining activities and prospecting began in the late 1880’s. Larger mining operations in the early 1900’s included the London Arizona Mine, Schneider Mine, Apex Mine, and 79 Mine. The London Arizona Mine produced 15,000 tons of copper ore (Tcu 4.5%) from the O’Carroll bed of the Martin formation. Smelter flux from the Troy quartzite was mined at the Schneider mine. Approximately $2,000 of gold was recovered from the Martin formation at the Apex Mine. Historical production from the 79 Mine has been estimated at 3,000 tons, averaging 24% Pb, and 1.75% Cu.
Chilito deposit is bounded by the Keystone fault to the west and the O’Carroll fault to the east. Both are normal high angle faults and the Chilito deposit is the horst block between the two faults. The Paleozoic section at Chilito has been eroded away leaving only the Precambrian section in place at the surface. Those Precambrian units exposed in outcrop include Troy quartzite, Mescal limestone, Apache group, and a Precambrian Diabase. Drill core has intersected the lower Apache group, Pioneer formation, Pinal Schist, and Ruin Granite. East of the O’Carroll fault, and west of the Keystone fault the Paleozoic section is exposed at surface within the grabens.
The Chilito Quartz Diorite was emplaced during the Laramide (66+/- ma.) and provided the hydrothermal fluids for mineralization. Favorable units within the Precambrian section became the host rocks for the pyrite, chalcopyrite, and minor molybdenum mineralization. Favorable hosts include the Mescal limestone, Diabase, sub-units in the Apache group, Pinal Schist, and Ruin Granite. Drilling over the last 50 plus years has demonstrated this mineralization does exist. Recent land acquisitions have opened up exploration potential in the Paleozoic section and that exploration work is ongoing.
Bio: Don Applebee began his geological career working for the U.S. Geological Survey’s Astrogeology Branch in Flagstaff, Arizona while pursuing a Master’s Degree in Geological Sciences from Northern Arizona University. After completing his degree in 1989 Don accepted a mine geologist position at the Oracle Ridge Mine in the Santa Catalina Mountains northeast of Tucson, AZ. His position entailed geologic mapping of development drifts in a copper skarn deposit, along with short range mine planning and mine site exploration. In 1996 he accepted a geologist position at Asarco’s Mission Mine, a copper porphyry skarn deposit. His duties at Mission included both underground and open pit mapping, along with mine site exploration programs. In 2003 he became the Senior Geologist at the Silver Bell Mine. In addition to mine geology and exploration, he assisted faculty from the University of Arizona in numerous field trips and short courses. In 2007 Don took a Senior Geologist Position with Freeport McMoRan, Morenci Operation to manage the deep sulfide exploration program. In 2009 he accepted a Senior Mine Geologist position at Kinross Gold’s Buckhorn Mine in Republic, WA, a gold skarn deposit where he was responsible for geologic modeling and short range mining. In 2011 Don returned to Tucson to work for Asarco’s Exploration Department, eventually becoming Corporate Exploration Manager and oversees both mine site exploration along with advance stage exploration projects. His interests include porphyry copper deposits and structural modeling of these deposits.
