University of Tasmania
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Geology and genesis of the Cerro la Mina porphyry - high sulfidation epithermal prospect, Mexico

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posted on 2023-05-26, 01:45 authored by Jansen, NH
The Cerro la Mina prospect is located in Chiapas State, southern Mexico. In 2006, exploration drilling at Cerro la Mina intersected a near surface oxide gold zone and significant Au (Cu-Mo) sulfide mineralisation with porphyry alteration and thick advanced argillic alteration with associated epithermal mineralisation. A total of 64 drill holes totalling approximately 22,000 m were drilled at Cerro la Mina before exploration ceased. Cerro la Mina is located in a tectonic setting characterized by a complex triple junction between the North American, Caribbean, and Cocos plates. The state of Chiapas has a pre-Mesozoic metamorphic, sedimentary, and igneous basement, overlain by Mesozoic detrital-calcareous units and Cenozoic interbedded sandstone and siltstone. The Chiapanecan volcanic arc began around 3 Ma, when volcanism on the Pacific coast parallel to the trench moved inland and approximately 30º oblique to the Middle American trench using the northwest orientated strike-slip fault zone as magma conduits. The strike-slip fault zone is a continuation of the Motagua-Polochic sinistral fault zone which separates the North American and Caribbean plates. The Selva Negra volcanic rocks, part of the north-westernmost Chiapanecan volcanic arc, comprise widespread monzodiorite to diorite intrusions with a crystallization age of `~`1.0 Ma (zircon U-Pb), trachyandesite volcanic rocks and rare basalt flows of shoshonitic composition. The shoshonitic and LILE-rich composition of the Selva Negra volcanic rocks is the result of low degree partial melts which have ascended along the inland faults of the strike-slip zone 300 to 330 km from the trench. The radiogenic isotope data suggest that the ascending magmas interacted with the thick crust (`~`50 km) through MASH processes. The combination of low degree partial melts of a heterogeneous mantle enriched in LILE and LREE and crustal contamination resulted in a shoshonitic and enriched LILE geochemistry for the Selva Negra volcanic rocks. The geology of the Cerro La Mina prospect, hosted in the Selva Negra volcanic rocks, comprise four major units. The volcanic stratigraphy consists of ignimbrites (Unit 1) that are intruded by monzodiorites to diorites (Unit 2). Unit 1 is overlain by debris flows or lahars and trachyandesite syn-volcanic domes and associated autoclastic rocks (Unit 3). U-Pb ages for volcanic rocks in the Selva Negra region indicate volcanism occurred from `~`1.2 to at least `~`0.75 Ma. The matrix-rich, granule breccia (Unit 4) cross cuts the volcanic rocks (Units 1 to 3) and is inferred to be a breccia pipe formed by the explosive release of hydrothermal fluids from an intrusion at depth causing brecciation of the overlying rocks. A major northwest fault off-sets the stratigraphy and the breccia pipe and is probably part of the regional strike-slip fault zone of the region. Early porphyry-style alteration includes potassic (quartz + potassium feldspar ± biotite) veins and wall rock alteration (Stage A), and later sericitic alteration (Stage B). The potassic veins are infilled with late fractured pyrite (Stage 1) that has been fractured to a jigsaw-fit texture, consistent with its pre-brecciation timing. Potassic alteration also occurred after breccia pipe formation, altering the matrix of the breccia pipe (Unit 4). A late sericitic alteration (Stage B) comprises quartz, muscovite, illite, illite/smectite, chlorite, calcite, gypsum (after anhydrite), and tourmaline. The sericitic alteration (Stage B) is zoned, with a quartz + muscovite core centred on the breccia pipe and transition to lower temperature clay assemblages distally. Sericitic alteration (Stage B) is associated with Stage 2 mineralisation comprising inclusion-rich pyrite ± chalcopyrite ± molybdenite veins and disseminations that are most abundant in the quartz-muscovite core. Stage 2 molybdenite yielded an age of 0.780 ± 0.010 Ma (Re-Os). The Cu and Mo ore grade shells have a vertical orientation coincident with the shape of the breccia pipe. Sulfur isotope data suggest that Stage 1 and 2 pyrite precipitated from magmatic fluids and are enriched in Co, Ni and Se, have oscillatory zoning and local arsenic-rich zones. The high temperature quartz-muscovite alteration is associated with the highest Co, Ni and Se concentrations which decrease distally, coincident with decreasing temperature clay assemblages. The hydrothermal system cooled below 300ºC at 0.689 ± 0.013 Ma (hydrothermal biotite Ar-Ar age) with the onset of Stage 3 and 4 mineralisation associated with advanced argillic kandite alteration (Stage C). The kandite alteration (Stage C) is hosted in the breccia pipe and is telescoped onto the early porphyry system. The kandite alteration (Stage C) grades between the quartz + dickite ± pyrophyllite ± alunite ± kaolinite (Stage CQD) at the surface and halloysite + kaolinite zones (Stage CHK) extending 800 metres below the surface. The advanced argillic alteration is not typical of a lithocap with no vuggy quartz and very little alunite, consistent with fluids interacting with the basement evaporite and limestone. The kandite-altered rocks host the most significant Au-Cu mineralisation. Mineralisation in Stage `C_(QD)` consists of arsenian-pyrite + enargite + galena + sphalerite + barite (Stage 3 and 4). In the Stage `C_(HK)` zone mineralisation consists of marcasite + arsenian-pyrite ± sphalerite ± galena ± barite (Stage 3 and 4). The sulfides occur in breccia cement, veins, and disseminations. Stages 3 and 4 mineralisation have similar geochemistry, being enriched in As, Cu, Au, Zn, Mn, Sb, Pb, Ag, Tl, Te and Bi, typical of high sulfidation epithermal systems. Sulfur isotope data of epithermal mineralisation (Stages 3 and 4) suggests a mix of magmatic waters and meteoric waters that have interacted with volcanic rocks and/or evaporite units in the basement. The kandite alteration (Stage C) contains abundant halloysite. Traditionally the low temperature halloysite clay is thought to be a supergene mineral. However, at Cerro la Mina the halloysite occurs in veins and wall rock alteration associated with gold-bearing sulfides below the oxide zone. In addition, the halloysite is developed to at least 800 m below the present day surface in the fluid pathway of the breccia pipe, deeper than the hypogene clay minerals dickite, alunite, and pyrophyllite. A TerraSpec study showed that the relative abundance of halloysite to kaolinite changes in association with the thickness of rimming pyrite (Stage 4). A SEM study revealed the transition from halloysite to kaolinite. All the above evidence indicates that the halloysite is hypogene in origin. The Cerro la Mina prospect has potassic and advanced argillic alteration and heavy sulfur isotopic values consistent with a reduced calc-alkaline porphyry system. This is unusual as the geochemistry of the volcanic rocks and intrusions are alkalic, therefore calc-alkaline magmatism, not yet recognized in the district, may be related to the alteration and mineralisation. Abundant hypogene halloysite is unusual although the identification of halloysite is complicated by dehydration and may be underestimated in high sulfidation deposits. In summary, the Cerro la Mina prospect consists of a breccia pipe with weakly developed porphyry Cu-Mo mineralisation and associated potassic alteration overprinted by hypogene advanced argillic alteration and epithermal Au-Cu mineralisation. The geological and geochemical features of the prospect are not consistent with traditional models with its calc-alkaline alteration and mineralisation with alkalic rocks and the abundant low temperature hypogene halloysite. The results of the thesis add to the understanding of advanced argillic alteration telescoped onto porphyry systems. The increased understanding of Cerro la Mina will also give explorers new insights for discoveries in an under explored region of Mexico.


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