ALBERT

Description: The Cerro la Mina Au (Cu-Mo) porphyry-high sulfidation prospect is located in Chiapas State, southeastern Mexico, outside of the major metallogenic provinces of Mexico. The prospect is hosted by Pleistocene alkaline volcanic rocks of the Chiapanecan volcanic arc that formed in a complex triple-junction tectonic setting. Cerro la Mina’s stratigraphy comprises pyroclastic flows that were intruded by monzodiorites and diorites at 1.04 ± 0.04 Ma (U-Pb, zircon), and that were overlain by debris flows and synvolcanic trachyandesite domes. The volcanic stratigraphy of Cerro la Mina is dominated by pyroclastic flows and rare basalts that are cut by the Cerro la Mina breccia pipe, a matrix-rich granular, vertically oriented, downward-tapering, polymict lithic rock unit that is host to all of the significant alteration and mineralization. A NW-trending sinistral wrench fault, which was active throughout the history of Cerro la Mina, is responsible for dismembering the prospect after mineralization. The magmatic hydrothermal system was composed of early porphyry-style potassic veins (quartz + K-feldspar ± biotite) and stage 1 pyrite that are preserved in clasts within the breccia pipe, suggesting that brecciation disrupted an embryonic porphyry system. Late potassic alteration occurred after the formation of the breccia pipe, as its matrix is strongly K-feldspar altered. Hydrothermal fluids then produced phyllic alteration composed of quartz, muscovite, illite, illite-smectite, and chlorite that is associated with stage 2 pyrite ± chalcopyrite ± molybdenite ± quartz veins. An unusual zoned pattern of advanced argillic-argillic alteration overprinted potassic and phyllic alteration. This zoning included a low-temperature (〈110°C) halloysite + kaolinite that extends from 800 to 250 m below present-day surface and is deeper than higher temperature (〉120°C) quartz + dickite ± kaolinite ± pyrophyllite ± alunite that occurs from 250 m to the present-day surface. The advanced argillic-argillic altered rocks host the most significant Au-Cu mineralization, which is associated with stage 3 marcasite, sphalerite, galena, and barite, and stage 4 arsenian pyrite ± enargite ± covellite. The magmatic hydrothermal system at Cerro la Mina began sometime between monzodiorite emplacement (1.04 ± 0.04 Ma; zircon U-Pb) and the precipitation of porphyry stage 2 molybdenite at 780 ± 10 ka (Re-Os). 40 Ar/ 39 Ar dating of biotite (689 ± 13 ka) records the age at which the hydrothermal system cooled below the biotite closure temperature of ~300°C and provides a maximum estimate for the onset of advanced argillic-argillic alteration. Sulfur isotope results of sulfides (–2.5 to +4.9; mean +0.7; n = 20) and a sulfate (barite; +10.5; n = 1) suggest a magmatic source of sulfur for all four stages of mineralization. The lack of residual quartz, rare alunite, and anomalous halloysite-kaolinite alteration may be explained by the high acid-buffering capacity of alkaline volcanic host rocks, high CO 2 contents of the alkaline magma, and/or potentially by a highly reduced magmatic hydrothermal fluid. At the regional metallogenic scale, the Cerro la Mina prospect along with the nearby Santa Fé mine and Campamento deposit represent parts of a porphyry copper system—specifically, a porphyry/high-sulfidation, proximal skarn and intermediate sulfidation deposit, respectively. The characteristics of Cerro la Mina (i.e., anomalous halloysite-kaolinite alteration) broaden the window for additional discoveries to be made in the porphyry-epithermal environment.

Description: In tropical climates, postdrilling oxidation of sulfide-rich core can severely degrade drill core, producing low-temperature iron oxyhydroxides, sulfates, and clays. Variable growth of these secondary minerals in exposed drill core, combined with the hydration and degradation of primary hydrothermal minerals, may lead to the production of spurious results in near-infrared (NIR) spectroscopic studies. However, the NIR technique can remain an effective tool in assessing hydrothermal alteration, even in extremely degraded core. We have assessed the usefulness of the NIR technique on degraded core at the Ladolam gold deposit, Papua New Guinea. Here, we seek to determine whether the primary alteration mineralogy had been significantly transformed by postdrilling oxidation over several years of weathering. In doing so, the study tested whether NIR analysis can be an effective tool in the discrimination of primary hydrothermal minerals in degraded core. Our study was made possible using semiquantitative X-ray diffraction (QXRD) analyses of a drill hole in 2004, where samples were collected at 50-m intervals. We subsequently repeated NIR and QXRD analyses on the same drill core in 2012. After nine years of storage, the drill core had degraded considerably, with the growth of jarosite and other sulfates. Despite this, XRD results from 2004 and 2012 show no major differences in the primary alteration mineralogy. Closely spaced NIR analyses were conducted at 1-m intervals to increase the chance of obtaining a spectrum of the primary mineralogy and to exclude secondary oxidation minerals. The drill core, where possible, was broken immediately prior to analysis to obtain a fresh surface. On average, over a 10-m interval, approximately 25% of the NIR spectra did not contain secondary minerals and relict primary alteration minerals could be detected. The remaining spectra were affected by the occurrence of secondary jarosite, gypsum, and/or residual water, but in most cases, the primary alteration mineralogy could be determined. We conclude that NIR analyses remain an effective tool in the construction of geological deposit models when logging degraded historic core, even for sulfide-rich core that has degraded in tropical environments.