Nonsulfide and sulfide-rich zinc mineralizations in the Vazante, Ambrósia and Fagundes deposits, Minas Gerais, Brazil: Mass balance and stable isotope characteristics of the hydrothermal alterations
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Keywords:
Dolomitization
Breccia
Ore genesis
Stockwork
The Madan Pb-Zn deposits are characterized by three morphogenetic types of ore bodies: veins, stockworks and replacement skarn-ore bodies. The subvertical veins and complex stockwork zones follow the tectonic boundaries of the NNW striking structures, cutting the various gneisses, amphibolites and marbles of the host metamorphic complex. Veins represent regularly-shaped, simple, steeply-dipping mineralized sections of the ore-bearing faults. Apophyses are common, generally joining the main vein in depth. Irregular in shape complex stockwork bodies are represented by sulphide veinlets and impregnations in deeper levels of the deposits and formed in zones with intensive hydrothermal alteration. Complex replacement metasomatic ore bodies are developed at the intersections of the ore-controlling faults with the marbles. Their morphological varieties include bed-like, columnar, mushroom-like or irregular, single or multilayered skarn- ore ledges, controlled by the lithological contacts of the skarns and marbles and screened by the other silicate rocks. The morphology of the replacement bodies is additionally complicated by post-depositional tectonic movements. The investigation summarizes the available data about the morphology of the ore bodies in the Madan district, taking into consideration the controlling factors. Understanding of the processes in the hydrothermal system and factors determining the deposition of rich ores can serve for the future successful exploitation of the deposits.
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The Rio Blanco-Los Bronces ore deposit is located at 33°l5'S in the Principal Cordillera of central
Chile. It lies midway between the Los Pelambres and El Teniente porphyry copper deposits, which
together define one of Chile's most economically significant metallogenic belts. Rio Batholith and
surrounding Tertiary volcanic and volcaniclastic rocks at approximately 5-4 Ma. The bulk of the
>50 million metric tonne copper resource is hosted by the Rio Blanco Magmatic Breccia and the
Sur-Sur Tourmaline Breccia.
The geometry of the Rio Blanco-Los Bronces system is asymmetrical, and appears to have been
controlled by three principal fracture orientations that trend N, NW and NE. Two N-trending
faults tenninate at the eastern and western boundaries of the Rio Blanco-Los Bronces deposit and
have acted as pull-apart structures that allowed hydrothermal fluids and felsic magmas to be
emplaced within the ore deposit. The N-trending faults are the largest and most deeply eroded
fault orientations in the district and at Sur-Sur are occupied by Tounnaline Breccia. The phyllic
altered Tourmaline Breccia is marginal to the potassic altered Rio Blanco Magmatic Breccia,
which constitutes the core zone between the two N-trending fault terminations. Although the NWand
NE-trending faults crosscut the Tounnaline Breccia, their fracture intensities are highest where
strongly mineralised zones occur along the strike length of the breccia body, indicating that these
fracture trends were active during brecciation and helped to localise fluid flow.
Paragenetic studies indicate nine main stages of veins, breccia and porphyry emplacement in the
Rio Blanco and Sur-Sur sectors of the ore deposit. The stages are: ( 1) magnetite-actinolite alteration;
(2) potassic stockwork veins; (3) Rio Blanco Magmatic Breccia and Sur-Sur Tourmaline
Breccia; ( 4) Feldspar Porphyry; ( 5) potassic stockwork veins; ( 6) Quartz Monzonite Porphyry and
Don Luis Porphyry; (7) molybdenite stockwork veins; (8) chalcopyrite stockwork veins; and (9) D
veins. Barren dacite and rhyolite intrusions cross-cut the ore deposit complex.
A change from potassic to phyllic alteration defines the contact between the Rio Blanco Magmatic
Breccia and Sur-Sur Tounnaline Breccia. The Rio Blanco Magmatic Breccia occupies a large
volume of rock within the Rio Blanco, La Union, Don Luis and Sur-Sur sectors, and extends over
a vertical interval of ~2 km. The Tourmaline Breccia lies transitionally outward from the Magmatic
Breccia mainly in the Sur-Sur sector. There is a 100m thick gradational contact between deeplevel
biotite breccia and shallow-level tourmaline breccia at Sur-Sur. Paragenetic studies ofbreccia
cement minerals in the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia reveal a
similar cement infill sequence involving initial biotite and/or tounnaline precipitation at clast
margins, followed by sulfate (anhydrite) and oxide (specularite) mineral phases, and in tum by
chalcopyrite and magnetite. Spatial zonation of breccia cement minerals occurs in the Rio Blanco and Sur-Sur breccias. Zonation
of biotite and tourmaline coincides with zonation of potassic and phyllic alteration, respectively.
Chalcopyrite is spatially associated with stage 3 Magmatic Breccia and biotite alteration in
the Rio Blanco to Don Luis sectors, and with stage 3 Tounnaline Breccia and quartz-sericite
alteration in the Sur-Sur sector. Outward from the potassic and phyllic altered zones; a propylitic
assemblage occurs that is defined by chlorite alteration and pyrite-specularite breccia cement minerals.
New 40 Arf39 Ar geochronology of hydrothermal biotite from the base of the Sur-Sur Tourmaline
Breccia and whole rock sericite from the top of the Sur-Sur Tounnaline Breccia yielded ages of
4.78 ± 0.04 Ma and 5.42 ± 0.09 Ma, respectively. Both 87Sr/86Sr and aNd analyses for tounnaline
and anhydrite from the Rio Blanco Magmatic Breccia and Sur-Sur Tounnaline Breccia range
between 0.7040 and 0.7044, and +1.70 and +2.53, respectively. 206Pbf2°4Pb values for anhydrite
cement in the Rio Blanco Magmatic Breccia and the Sur-Sur Tourmaline Breccia ranged between
17.558 and 18.479, 207Pbf204pb values ranged between 15.534 and 15.623, and 208Pb/204Pb values
ranged between 37.341 and 38.412. The early-fonned anhydrite cement has Pb isotopic compositions
that are less radiogenic than the sulfide ores and host rocks, and also has elevated initial Sr
ratios compared to the host rocks. Pb and Sr in anhydrite are interpreted to have been sourced
from rocks and/or waters external to the main magmatic-hydrothermal system.
Oxygen/deuterium isotopes for tourmaline breccia cement minerals have 'magmatic' values, however
recalculated values of propylitic-altered samples from previous workers indicate a meteoric
water component of up to 25%. Zonation of sulfur isotope compositions occurs in the mineralised
breccias, particularly at Sur-Sur. The Rio Blanco sector is characterised by sulfides with 834S
values between -3.94 and +3.34 and sulfates between +10.07 and + 17.86 values. The Sur-Sur
sector is characterised by sulfides with 834S values between -4.12 and +2.65 and sulfates between
+11.15 and +13.39. These values are consistent with a magmatic-hydrothermal sulfur source. At
Sur-Sur, the most negative ()34S compositions (834S < -3 per mil) are spatially associated with the
highest copper grades and specularite cement.
The Rio Blanco Magmatic Breccia and Sur-Sur Tourmaline Breccia contain co-existing low salinity
liquid-rich and vapour-rich fluid inclusions and localised zones containing co-existing vapour-rich
and hypersaline fluid inclusions. Homogenisation temperatures from >600 to 131 oc have been
measured, but most are between 450° and 300°C. Complete salinity arrays from 0-69 wt.% NaCl
equivalent were observed, and eutectic temperatures are commonly below -35°C. Minimum pressure
estimates from fluid inclusions are between 48 and 368 bars. An average lithostatic formation
depth of 200 m and a hydrostatic formation depth of 2300 m below the palaeo-surface have been
calculated, indicating that up to approximately 1 km of erosion has occurred since breccia formation. The mineralised breccias in the Rio Blanco and Sur-Sur sectors are magmatic-hydrothermal in
origin. They formed when magmatic-hydrothermal fluids (brine and gas) exsolved from deepseated
magma and potentially mixed with an external water. Hydrostatic pressures catastrophically
exceeded lithostatic load plus the tensile strength of the confining rocks leading to brecciation.
At Sur-Sur, fault rupture along the Rio Blanco Fault may have been a trigger for magmatic-hydrothennal
brecciation. Phase separation of magmatically-derived aqueous fluid occurred at the onset
ofbrecciation, with a low density gas phase (carrying H20, S02, HCl and B20 3) separating physically
from the dense copper-bearing brine. The gas phase fluxed through the breccia column first,
where it condensed into exotic groundwaters of uncertain derivation, resulting in the deposition of
oxide-stage cements (anhydrite, specularite, tourmaline) from a hybrid low salinity water. Subsequent
upwelling of the magmatic-hydrothermal brine resulted in main stage sulfide deposition, possibly
in part due to fluid mixing with the hybrid water.
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Abstract: Fluidization processes based on experiments are reviewed to gain some useful insights and comparisons with those that occur in hydrothermal systems. Field and petrographic work, and microscope observation were carried out on samples from the Qiyugou Au‐bearing breccia pipes from the East Qinling region, Henan Province. Evidence from macro‐ and micro‐textures suggests that the style of breccias in the Qiyugou area can be grouped into three types: (1) jigsaw fit‐stockwork texture, in which the interval between clasts is marked by fractures or filled with calcite or quartz veins; (2) larger breccias that are supported by smaller breccias, rock flour and alteration materials; in this type clasts moved over short distances, creating open spaces; (3) fluidized texture, where the clasts of different lithologies have rounded shapes. These observations are compared with those resulting from experiments on fluidization processes. The results of this comparison suggest that fluidization is an important geological process in the formation of the Qiyugou Au‐bearing breccia pipes and gold mineralization. In addition, fluidization processes such as expansion, bubbling, slugging, channeling and spouting must have contributed to the formation of the pipes and were conducive to the development of gold mineralization. In the Qiyugou breccia pipes, gold mineralization occurs as disseminations, in stockwork veins, and open space infills. The ore zones form subparallel sheets that are nearly perpendicular to the walls of the pipes.
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ABSTRACT Gosowong province has economic mineralisation which is classified as low sulphidation epithermal veining system containing high-grade of gold and silver. It is characterizes by various veins hosted in Holocene Quaternary volcanic rocks, mineralisation generally occurs in host rock andesite. The Kencana epithermal vein system in Gosowong Au-Ag district hosted by two main sub-parallel North-West trending major fault structures, named Kencana One (K1) and Kencana Two (K2) with strike lenght extend up to 600m. Both structures joined by link structures, appears to be Kencana Link (K-Link) as the thickest link structure. The Kencana One (K1) is the first underground mine in Gosowong district. The nature of underground conditions at Kencana has a high variability of gold grades. Thus due to extremely high grades ore in Kencana, it is need to be precise and thoroughly handling to obtain best results. Primary mineralisation occurs in multistage veins, breccias and stockwork veins as infill structures. This study confirming that there is a strong correlation between quartz vein infill structure mode versus gold grade which classified into (1) 1.01-3.0g/t Au, dominated with wall-rock breccia and stockwork, (2) 3.01-6.0g/t Au, dominated with wall-rock breccia follows by stockwork, (3) 6.01-20.0g/t Au, dominated with wall-rock breccia follows by quartz lode and stockwork, (4) 20.01-80.0g/t Au, dominated by wall-rock breccia follows by vein breccia and quartz lode, (5) >80g/t Au, dominated by wall-rock breccia. In terms of production quartz vein infill mode together and quartz vein texture with gold grade has been used as the main reference for geologist to make a direction in the process of ore drive determination in underground mining operations. Keywords: Kencana, low sulphidation epithermal, quartz vein infill mode SARI Gosowong memiliki mineralisasi ekonomis yang merupakan epitermal sulfida rendah yang mengandung emas dan perak dengan kadar tinggi. Cebakan dicirikan dengan kehadiran urat yang bervariasi dengan batuan vulkanik kuarter sebagai batuan induk berumur Holosen, mineralisasi umumnya terjadi di dalam batuan induk andesit. Kencana berada di dalam area Cebakan Au-Ag Gosowong. Kencana terdiri dari dua struktur besar yang berdampingan, dinamakan Kencana Satu (K1) dan Kencana Dua (K2) dengan arah jurus sepanjang 600m. Keduanya dihubungkan dengan struktur yang dinamakan Kencana Link (K-Link) yang merupakan struktur bukaan yang paling tebal. K1 adalah tambang bawah tanah pertama di Gosowong. Kondisi bawah tanah di Kencana menunjukkan variasi tubuh bijih emas yang mengandung kadar emas yang bervariasi. Dengan adanya kadar emas yang tinggi, maka penangannya harus tepat dan seksama untuk mendapatkan hasil terbaik. Secara umum mineralisasi terjadi secara bertahap dalam urat kuarsa, breksiasi, dan urat stockwork. Penelitian ini menegaskan bahwa terdapat korelasi yang kuat antara tipe struktur urat kuarsa dengan kadar emas yang diklasifikasikan menjadi (1) 1 . 01-3.0 g/t Au, didominasi oleh breksiasi batuan dinding dan stockwork, (2) 3.01-6.0 g/t Au, didominasi oleh breksiasi batuan dinding diikuti oleh stockwork, (3) 6.01-20.0 g/t Au, didominasi oleh breksiasi batuan dinding diikuti oleh quartz lode dan stockwork (4) 20.01-80.0 g/t Au, didominasi oleh breksiasi batuan dinding diikuti oleh breksiasi urat kuarsa dan quartz lode (5) > 80g/t Au, didominasi oleh breksiasi batuan dinding. Di dalam proses produksi, tipe struktur urat kuarsa dan tekstur urat kuarsa bersama dengan kadar emas menjadi petunjuk untuk geolog dalam menentukan arah penambangan emas di dalam tambang bawah tanah. Kata kunci: epitermal sulfida rendah, Kencana, tipe struktur urat kuarsa
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Multiple large mineralized breccia pipes (Cu grades up to >10%; individual pipes with >10 × 106 metric tons of Cu) are prominent, if not dominant, features in the three giant Andean Cu deposits of Los Pelambres, Los Bronces-Rio Blanco, and El Teniente of central Chile. At Los Bronces-Rio Blanco, over 90% of the >50x 106 metric tons of hypogene Cu occurs within the matrix of breccias and/or clasts and wall rock altered in association with the formation of these breccias, while at the other two deposits a lesser but still significant amount of Cu ore also is directly related to breccias. At both Los Pelambres and Los Bronces-Rio Blanco, high-grade (>0.5%) Cu occurs in zones of potassic alteration characterized by stockwork biotite veining and intense biotitization associated spatially, temporally, and genetically with biotite breccias. At Los Bronces-Rio Blanco, high-grade ore also occurs in younger tourmaline breccia pipes, emplaced both within and around the older central biotite breccia complex and potassic alteration zone after a period of uplift and erosion. Potassic alteration, sericitization, silicification, and mineralization of clasts in these tourmaline breccias occurred during their formation. At El Teniente, a significant amount of high-grade Cu ore also occurs in different tourmaline-rich breccias, including the marginal portion of the Braden breccia pipe and a related zone of quartz-sericite alteration that surrounds this pipe. Small, shallow, weakly mineralized or barren silicic porphyry intrusions occur in each of these three deposits, but their main role has been to redistribute rather than emplace mineralization. The mineralized breccia pipes in each deposit were emplaced into early and middle Miocene volcanic and plutonic rocks during the late Miocene and Pliocene by the expansion of boiling aqueous fluids. Fluid-inclusion and stable-isotope data indicate that the high-temperature, saline, metalrich fluids that produced the brecciation, precipitated the Cu ore in the matrix of the breccias, and generated the associated alteration and mineralization in clasts and wall rock were magmatic in origin. These magmatic fluids were not derived from the early and middle Miocene host plutons, which already were solidified at the time of breccia emplacement. Sr- and Nd-isotopic compositions of breccia-matrix minerals indicate that breccia-forming fluids were exsolved from magmas that were isotopically transitional between older volcanic and plutonic host rocks and younger silicic porphyry stocks, dikes, and extrusives. The fact that the roots of the breccias have not yet been encountered implies that these magmas cooled at depths >3 km to form plutons not yet exposed at the surface. The generation of the multiple mineralized breccias at each deposit occurred over a relatively short (but still significant) time period of 1 to 3 million years, during the final stages of existence of the long-lived (7gt;15 m.y.) Miocene magmatic belt in central Chile. The decline of magmatic activity in this belt was tectonically triggered, as subduction angle decreased in association with the subduction of the Juan Fernandez Ridge. This caused a decrease in the sub-arc magma supply and subsequently eastward migration of the magmatic arc, as well as crustal thickening, uplift, and erosion, which led to the superposition of younger and shallower alteration and mineralization events on older and deeper events in each deposit. The giant Cu deposits of central Chile cannot be explained by a static model in which their size is a function of the mass of a single pluton or the longevity of a single hydrothermal convection system. These deposits are giant because they were produced by multistage processes involving the formation, over a period of 1 to 3 million years, of multiple superimposed mineralized breccias and associated alteration zones resulting from the exsolution of metalrich magmatic fluids from independent magma batches cooling at depths >3 km. Neither an unusually large magma supply nor Andean magmas of unusually high Cu content is required to produce the sequence of multiple mineralization
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