Milpillas Mine, Milpillas, Santa Cruz Municipality, Sonora, Mexicoi

Regional Level Types
Milpillas Mine	Mine (Active)
Milpillas	Village
Santa Cruz Municipality	Municipality
Sonora	State
Mexico	Country

Mine Information: Production started in 2006 with a planned 11 year mine life. By late 2011 most of the 7 principal copper oxide bodies were mined out, and company sources indicated that, unless more carbonate-rich ores are found, production will be almost purely chalcocite ores by mid to late 2012. Faults and water courses with secondary copper oxide species extend locally downwards into the supergene sulfide zone, but most post-2012 oxide species have come from upper levels where miners had the opportunity to collect after the areas were abandoned and before back-filling. (2015): Exploration has been successful in finding additional sulfide ores at depth and lateral to the known deposit… they now estimate a total of 12 years [to 2027] of mine life at the 2011 production level (388 kilotons at 26.4 kilotons of copper per year). Only small amounts of additional oxide materials have been found.

Specimen collecting by miners and tours by dealers are actively discouraged.

Geology: The district lies within the NW‐SE trending metallogenic Laramidian (late Cretaceous to Eocene) copper belt of Southwest North America, which extends from Sonora over Arizona to New Mexico. The Milpillas deposit, located in an extensional zone called the Cuitaca Graben, is a partially oxidized porphyry copper deposit with a series of alternating copper carbonate-oxide and chalcocite enrichment blankets on top of low grade primary chalcopyrite-bornite mineralization. Mineralization is covered by 250 meters of gravels and extends to 720 meters depth. Most of the copper carbonate oxide ores occur in the top 200 meters of the deposit. Host rocks are volcaniclastic from the Jurassic Henrietta formation and the Laramide Mesa formation, in which monzonitic to quartz monzonitic stocks intrude. The sericitically altered stocks and the intruding volcaniclastic rocks host the main copper mineralization.

The supergene profile in Milpillas is made up of 4 principal zones, within which certain overlap exists. They are from top to the bottom as follows:
1. The leached cap is characterized by oxidized rocks: goethite, hematite and jarosite are abundant, copper values are very low (between 0.01 and 0.05 %) with a common thickness of 100 ‐ 300m. Montmorillonite, sericite and kaolinite dominate as secondary alteration minerals of the volcanic host rock.
2. The oxide zone above the water table is situated at the lowermost part of the leached zone: sub‐horizontal mineralized blankets, made up mostly by copper oxides, copper carbonates and sulfates formed mostly by oxidation of pre-existing chalcocite. Minor amounts precipitated from oversaturated groundwater. The mix of different copper species is a result of at least three supergene enrichment cycles in Milpillas due to uplift or changes in the water level.
3. Supergene enrichment zone: situated at the base of the weathering profile, with reducing conditions underneath the paleo water level. In this zone, the copper solubility is decreased, which results in the replacement of iron in the primary hypogene minerals: chalcocite dominates together with covellite when copper enrichment is less pronounced. Native copper can be found at the borderline of reduced and oxidized conditions. 2015: Pyrite crystals to 15 cm with very thin coatings of chalcocite (not bornite as previously described; XRD, SEM-EDS and optically verified; https://www.mindat.org/forum.php?read,11,380551,432315#msg-432315) have emerged from this zone.
4. Hypogene primary mineralization: the surface between this zone and the supergene enriched one is irregularly shaped. In a transition zone, different “copper oxide” minerals as well as sulfides can be abundant. Underneath the transition zone the mineralization is primary as it is described above.

Geochronology

Mineralization age: Paleocene : 63.5 Ma to 62.6 Ma

Important note: This table is based only on rock and mineral ages recorded on mindat.org for this locality and is not necessarily a complete representation of the geochronology, but does give an indication of possible mineralization events relevant to this locality. As more age information is added this table may expand in the future. A break in the table simply indicates a lack of data entered here, not necessarily a break in the geologic sequence. Grey background entries are from different, related, localities.

Geologic Time				Rocks, Minerals and Events
Phanerozoic
	Cenozoic
		Paleogene
			Paleocene	ⓘ Molybdenite (youngest age) 62.6 Ma ⓘ Molybdenite (oldest age) 63.5 Ma

Other Regions, Features and Areas containing this locality

References