Josemaria is a copper-gold (Cu-Au) porphyry system.
The copper-gold mineralization at Josemaria is mostly hosted by a Miocene porphyry system which forms an elongated body with minimum dimensions of 800 to 900 m north-south, 600 to 700 m east-west and 600 to 700 m vertically. The porphyry has been dated at approximately 24.7 Ma. The deposit is centered on a composite porphyry stock, which intrudes rhyolitic volcanic and tonalitic plutonic rocks of Permo-Triassic age. The earlier, more intensely quartz- ± magnetite-veined porphyry phases and nearby wall rocks display a telescoped sequence of alteration-mineralization zones, from shallow advanced argillic (mainly quartz-pyrophyllite) and underlying sericitic to deeper chlorite-sericite and remnant potassic; all of which are mineralized. The outermost parts of the copper-gold-zone are overlapped by a pronounced molybdenum-bearing zone.
The deposit was partially exposed by erosion, and then covered by post-mineral sedimentary and volcanic rocks. A north-trending, post-mineral fault zone has localized an economically important zone of high-grade copper mineralization hosted in supergene chalcocite enrichment in the northern part of the deposit.
The Josemaria deposit remains open to the south, beneath a thickening cover of post-mineral volcanic rocks, and also at depth.
A detailed description of the Josemaria deposit geology was published in the May 2019 edition of Economic Geology and can be accessed on the GeoScienceWorld website.