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Papers by Lydia Lobato

Brazilian Journal of Geology, 2015

Ore Geology Reviews, Jun 30, 2015

The host rocks record effects of metamorphism under 3.5–4 kbar and 540–610 °C.The aqueous–carboni... more The host rocks record effects of metamorphism under 3.5–4 kbar and 540–610 °C.The aqueous–carbonic fluid suffered boiling and mixing with cooler fluids at 1–2 kb and 300–350 °C.The onset of the ore-forming process took place at ca. 2.1–2.2 Ga.Timing of Au formation was at 1.9–2 Ga and late hydrothermal influxes happened after 1.75 Ga.The ore-forming took place in two periods, with cooling rates of 1 °C/Ma and 2.5 °C/Ma.The Turmalina gold deposit comprises three epigenetic domains whose development is related to the propagation/reactivation events of the NW–SE Pitangui Shear Zone (PSZ). The lodes are hosted in a Late Archaean sedimentary sequence on top of a strongly deformed (mafic-dominated) metavolcanic pile metamorphosed under 3.5–4 kbar and 540–610 °C; the association forms the upper part of a lithostratigraphic succession (Pitangui Group) that overlies an older TTG gneissic basement. According to field evidence and petrographic observations, the ore-forming process is polyphasic, starting at the time when the PSZ crossed the metamorphic quartz + staurolite + biotite + almadine + hornblende isograde; the main evolving stages, however, mostly took place throughout the metamorphic retrogression path. Fluid inclusion microthermometry also shows that metamorphogenic aqueous–carbonic solutions (initially with ≈ 16–20 eq. wt.% NaCl and circulating at approximately 4 kbar and 550 °C) were subjected to repeated boiling and mixing with cooler aqueous solutions at approximately 1–2 kb and 300–350 °C. These boiling events, which were triggered by depressurization, were contemporaneous with gold (and later sulfide) deposition, preceding a late stage of hydrothermal activity under lower PT conditions (< 1 kbar and ≈ 130–230 °C). To constrain both the mineralization age and the source/pathways of the ore-forming fluids in the Turmalina deposit, a multi-system isotope (Pb–Pb, Rb–Sr and Sm–Nd) study was carried out using various whole-rock and mineral samples. The main results are as follows: (i) the onset of the ore-forming process took place at ca. 2.2–2.1 Ga; (ii) the critical timing for gold formation was confined to ca. 2–1.9 Ga; and (iii) the late hydrothermal influxes occurred after 1.75 Ga. Therefore, the ore-forming process can be envisaged as a result of successive physical–chemical processes that took place during two major, long-lasting (≈ 250 Ma) periods under initial cooling rates of approximately 1 °C/Ma and, after ca. 2–1.9 Ga, approximately 2.5 °C/Ma. Moreover, the rejuvenation episodes at ca. 2–1.9 Ga and ca. 1.75 Ga probably indicate reactivation events in the PSZ, which triggered new fluid inflows into the system and revitalized the ore-forming process. That was the case for fluids that circulated deeply through both the older basement rocks and the mafic volcanic pile, allowing either metal enrichment from multistage leaching processes of various reservoirs or possible U/Th decoupling during ore remobilization. The Turmalina ore-system lifetime is consequently confined to the Rhyacian period (Paleoproterozoic), which is compatible with the age constraints presented by other studies regarding the fold-thrust belt's development (ca. 2.125 Ga) and orogenic collapse (ca. 2.095 Ga).

Mineralium Deposita, 2016

Brazilian Journal of Geology, 2000

Brazilian Journal of Geology, 1998

Ore Geology Reviews, 2016

RESUMO: O Lineamento Aurífero Córrego do Sítio corresponde a uma zona de cisalhamento dúctil com ... more RESUMO: O Lineamento Aurífero Córrego do Sítio corresponde a uma zona de cisalhamento dúctil com um trend NE-SW controlando quatorze depósitos de ouro, no qual se inclui o depósito Carvoaria Velha. A mineralização de ouro se hospeda em rochas arqueanas do greenstone belt Rio das Velhas, estas correspondem a filitos e xistos finos alterados a quartzo, carbonato e sericita. O estilo de mineralização principal ocorre em veios de quartzo-carbonato-sulfetos (±sulfossais). Os tipos de veios são classificados como: Veios V1quartzo-ankerita-sulfetos-berthierita-ouro -são paralelos à principal foliação Sn e estão localmente mineralizados em ouro. Veios V2quartzo-ankerita-pirita -são veios desenvolvidos no plano de clivagem de crenulação Sn+1, extensionais e raramente mineralizados. Veios V3 -quartzo-ankerita -são veios de preenchimento de fraturas Sn+3, geralmente não contêm sulfetos e sulfossais. Veios V4 -quartzocalcita -são veios restritos aos diques e sills metamáficos, são extensionais e sem orientação preferencial. Os minerais-minério são arsenopirita, pirita, berthierita e pirrotita. Microssonda eletrônica revela a presença de fases metálicas ricas em As, Sb e Co (boulangerita, tetraedrita, cobalita e ullmanita), que sobrecrescem aos minerais-minério. Dados de inclusões fluidas caracterizam fluido mineralizador com composição H2O-CO2±CH4-NaCl, com traços de N2, podendo conter outros cátions além de Na + , como Mg 2+ , Ca 2+ e Fe 2+ . Estes fluidos teriam pelo menos dois estágio de evolução: i) fluido inicial aprisionado em quarto fumê, com salinidade moderada (~8.5% em peso de NaCl eq.) e temperatura mínima de 280±17 o C; e ii) fluido tardio, representado por fluido aprisionado em quartzo recristalizado, menos salino (~4.6% em peso de NaCl eq.) e temperatura mínima de 315±33 o C. Os dados obtidos correspondem a valores típicos de fluido metamórfico característico de depósitos tipo orogenic gold.

... Atlas Vasconcelos Corrêa Neto Department of Geology, Universidade Federal do Rio de Janeiro, ... more ... Atlas Vasconcelos Corrêa Neto Department of Geology, Universidade Federal do Rio de Janeiro, Av. ... of the Metaultramafic, Mafic Meta-volcanic, Lower and Upper Metasedimentary units, which are the main host to gold, and the units Metagranodiorite, Alto Maranhão tonalite ...

Ore Geology Reviews, 2015

The Lamego orogenic gold deposit (440,742 oz gold measured reserves and 2.4 million t measured re... more The Lamego orogenic gold deposit (440,742 oz gold measured reserves and 2.4 million t measured resources, with an average grade of 5.71 g/t Au and a cut-off grade of 2.15 g/t Au; AngloGold Ashanti Córrego do Sítio Mineração S/A (AGA) personal communication, 2014) is located in the 5 km-long trend that includes the world-class Cuiabá deposit. It is hosted in the Neoarchean metavolcano-sedimentary rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil. Mineralization is associated mainly with metachert-banded iron formation (BIF) and carbonaceous phyllites in the reclined Lamego fold, in which the Cabeça de Pedra orebody represents the hinge zone. Mineralization is concentrated in silicification zones and their quartz veins, as well as in sulfide minerals, product of BIF sulfidation. Hydrothermal alteration varies according to host rock, with abundant sulfide-carbonate in BIF, and sericite-chlorite in carbonaceous phyllite. Quartz vein classification according to structural relationships and host rocks identified three vein systems. The V1 system, mainly composed of smoky quartz (Qtz I) and pyrite, is extensional, crosscuts the bedding plane S 0 of BIF, and is parallel to the fold axis. The V2 system, of the same composition, is represented by veins that are parallel to the S 1-2 foliation and S 0 . This system is also characterized by silicification zones in the BIF-carbonaceous phyllite contact that has its maximum expression in the hinge zone of folds. The V3 system has milky quartz (Qtz II) veins, which result from the recrystallization of smoky quartz, located mainly in shear zones and faults; these veins form structures en echelon and vein arrays. The most common ore minerals are pyrite, As-pyrite and arsenopyrite. Fluid inclusion-FI trapped in all quartz veins present composition in the H 2 O-CO 2 ± CH 4 -NaCl system. Fluid evolution can be interpreted in two stages: i) aqueous-carbonic fluid trapped in Qtz I, of low salinity (~2% equiv. wt.% NaCl), and ii) carbonic-aqueous fluid, of moderate salinity (average 9 eq. wt.% NaCl) hosted in Qtz II. Both stages are characterized by decrepitation temperatures in the range of 200 to N 300°C, and suggest a fluid of metamorphic origin. Applying an arsenopyrite geothermometer, the calculated formation temperature for the Cabeça de Pedra orebody is 300 to 375°C. The vertical intersection of the isochors allows a minimum pressure calculation of 2.6 kbar. The composition of individual FIs of this orebody, obtained by LA-ICP-MS analyses, compared with results of FIs for the Carvoaria Velha deposit, Córrego do Sítio lineament, highlights a standard composition typical of metamorphic fluids with Na N K N Ca N Mg, which increase or decrease in concentration as a function of salinity in both deposits. Trace elements vary according to fluid-rock reactions, and are directly related to the host rock composition. The comparison of data sets of the two deposits shows that the Cabeça de Pedra FIs have a higher enrichment in Zn, while Cu, As and Sb are richer in Carvoaria Velha, suggesting influence of the host rock geochemistry. The suggested mechanisms for gold precipitation at the Cabeça de Pedra orebody, Lamego gold deposit are: i) hydrolysis of the carbonaceous matter of phyllite and BIF, affecting fO 2 , destabilizing sulfur complexes and enhancing gold precipitation; ii) replacement of BIF iron carbonates by sulfides; and iii) continuous pressure changes that lead to silica precipitation and free gold. Other than playing the long-recognized role of the carbonaceous phyllites as a fluid barrier, the data highlight their importance as a source of metals.

Ore Geology Reviews, 2015

ABSTRACT

Journal of South American Earth Sciences, 2014

ABSTRACT The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important ura... more ABSTRACT The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important uranium province in the country and presently produces this metal in an open-pit mine operated by Indústrias Nucleares do Brasil. Uranium-rich zones are associated with plagioclase (dominantly albite ± oligoclase) -rich rocks, albitites and metasomatized granitic-gneisses, distributed along NNW/SSE striking shear zones. We have used the ID-TIMS U–Pb method to date zircon and titanite grains from the São Timóteo granitoid, and albite-rich rocks from the Lagoa Real district in order to assess the age of granite emplacement, deformation/metamorphism and uranium mineralization. The isotopic data support the following sequence of events (i) 1746 ± 5 Ma – emplacement of the São Timóteo granitoid (U–Pb zircon age) in an extensional setting, coeval with the beginning of the sedimentation of the Espinhaço Supergroup; (ii) 956 ± 59 Ma hydrothermal alteration of the São Timóteo granitoid and emplacement of the uranium mineralization (U–Pb titanite age on an albite-rich sample); (iii) 480 Ma metamorphism, remobilization and Pb loss (U–Pb titanite age for the gneiss sample), during the nucleation of shear zones related to the collision between the São Francisco-Congo and Amazonia paleoplates. The 956 ± 59-Ma mineralization age is apparently associated with the evolution of the Macaúbas-Santo Onofre rift. This age bracket may bear an important exploration implication, and should be included in the diverse age scenario of uranium deposits worldwide.

Brazilian Journal of Geology, 2015

Ore Geology Reviews, Jun 30, 2015

The host rocks record effects of metamorphism under 3.5–4 kbar and 540–610 °C.The aqueous–carboni... more The host rocks record effects of metamorphism under 3.5–4 kbar and 540–610 °C.The aqueous–carbonic fluid suffered boiling and mixing with cooler fluids at 1–2 kb and 300–350 °C.The onset of the ore-forming process took place at ca. 2.1–2.2 Ga.Timing of Au formation was at 1.9–2 Ga and late hydrothermal influxes happened after 1.75 Ga.The ore-forming took place in two periods, with cooling rates of 1 °C/Ma and 2.5 °C/Ma.The Turmalina gold deposit comprises three epigenetic domains whose development is related to the propagation/reactivation events of the NW–SE Pitangui Shear Zone (PSZ). The lodes are hosted in a Late Archaean sedimentary sequence on top of a strongly deformed (mafic-dominated) metavolcanic pile metamorphosed under 3.5–4 kbar and 540–610 °C; the association forms the upper part of a lithostratigraphic succession (Pitangui Group) that overlies an older TTG gneissic basement. According to field evidence and petrographic observations, the ore-forming process is polyphasic, starting at the time when the PSZ crossed the metamorphic quartz + staurolite + biotite + almadine + hornblende isograde; the main evolving stages, however, mostly took place throughout the metamorphic retrogression path. Fluid inclusion microthermometry also shows that metamorphogenic aqueous–carbonic solutions (initially with ≈ 16–20 eq. wt.% NaCl and circulating at approximately 4 kbar and 550 °C) were subjected to repeated boiling and mixing with cooler aqueous solutions at approximately 1–2 kb and 300–350 °C. These boiling events, which were triggered by depressurization, were contemporaneous with gold (and later sulfide) deposition, preceding a late stage of hydrothermal activity under lower PT conditions (< 1 kbar and ≈ 130–230 °C). To constrain both the mineralization age and the source/pathways of the ore-forming fluids in the Turmalina deposit, a multi-system isotope (Pb–Pb, Rb–Sr and Sm–Nd) study was carried out using various whole-rock and mineral samples. The main results are as follows: (i) the onset of the ore-forming process took place at ca. 2.2–2.1 Ga; (ii) the critical timing for gold formation was confined to ca. 2–1.9 Ga; and (iii) the late hydrothermal influxes occurred after 1.75 Ga. Therefore, the ore-forming process can be envisaged as a result of successive physical–chemical processes that took place during two major, long-lasting (≈ 250 Ma) periods under initial cooling rates of approximately 1 °C/Ma and, after ca. 2–1.9 Ga, approximately 2.5 °C/Ma. Moreover, the rejuvenation episodes at ca. 2–1.9 Ga and ca. 1.75 Ga probably indicate reactivation events in the PSZ, which triggered new fluid inflows into the system and revitalized the ore-forming process. That was the case for fluids that circulated deeply through both the older basement rocks and the mafic volcanic pile, allowing either metal enrichment from multistage leaching processes of various reservoirs or possible U/Th decoupling during ore remobilization. The Turmalina ore-system lifetime is consequently confined to the Rhyacian period (Paleoproterozoic), which is compatible with the age constraints presented by other studies regarding the fold-thrust belt's development (ca. 2.125 Ga) and orogenic collapse (ca. 2.095 Ga).

Mineralium Deposita, 2016

Brazilian Journal of Geology, 2000

Brazilian Journal of Geology, 1998

Ore Geology Reviews, 2016

RESUMO: O Lineamento Aurífero Córrego do Sítio corresponde a uma zona de cisalhamento dúctil com ... more RESUMO: O Lineamento Aurífero Córrego do Sítio corresponde a uma zona de cisalhamento dúctil com um trend NE-SW controlando quatorze depósitos de ouro, no qual se inclui o depósito Carvoaria Velha. A mineralização de ouro se hospeda em rochas arqueanas do greenstone belt Rio das Velhas, estas correspondem a filitos e xistos finos alterados a quartzo, carbonato e sericita. O estilo de mineralização principal ocorre em veios de quartzo-carbonato-sulfetos (±sulfossais). Os tipos de veios são classificados como: Veios V1quartzo-ankerita-sulfetos-berthierita-ouro -são paralelos à principal foliação Sn e estão localmente mineralizados em ouro. Veios V2quartzo-ankerita-pirita -são veios desenvolvidos no plano de clivagem de crenulação Sn+1, extensionais e raramente mineralizados. Veios V3 -quartzo-ankerita -são veios de preenchimento de fraturas Sn+3, geralmente não contêm sulfetos e sulfossais. Veios V4 -quartzocalcita -são veios restritos aos diques e sills metamáficos, são extensionais e sem orientação preferencial. Os minerais-minério são arsenopirita, pirita, berthierita e pirrotita. Microssonda eletrônica revela a presença de fases metálicas ricas em As, Sb e Co (boulangerita, tetraedrita, cobalita e ullmanita), que sobrecrescem aos minerais-minério. Dados de inclusões fluidas caracterizam fluido mineralizador com composição H2O-CO2±CH4-NaCl, com traços de N2, podendo conter outros cátions além de Na + , como Mg 2+ , Ca 2+ e Fe 2+ . Estes fluidos teriam pelo menos dois estágio de evolução: i) fluido inicial aprisionado em quarto fumê, com salinidade moderada (~8.5% em peso de NaCl eq.) e temperatura mínima de 280±17 o C; e ii) fluido tardio, representado por fluido aprisionado em quartzo recristalizado, menos salino (~4.6% em peso de NaCl eq.) e temperatura mínima de 315±33 o C. Os dados obtidos correspondem a valores típicos de fluido metamórfico característico de depósitos tipo orogenic gold.

... Atlas Vasconcelos Corrêa Neto Department of Geology, Universidade Federal do Rio de Janeiro, ... more ... Atlas Vasconcelos Corrêa Neto Department of Geology, Universidade Federal do Rio de Janeiro, Av. ... of the Metaultramafic, Mafic Meta-volcanic, Lower and Upper Metasedimentary units, which are the main host to gold, and the units Metagranodiorite, Alto Maranhão tonalite ...

Ore Geology Reviews, 2015

The Lamego orogenic gold deposit (440,742 oz gold measured reserves and 2.4 million t measured re... more The Lamego orogenic gold deposit (440,742 oz gold measured reserves and 2.4 million t measured resources, with an average grade of 5.71 g/t Au and a cut-off grade of 2.15 g/t Au; AngloGold Ashanti Córrego do Sítio Mineração S/A (AGA) personal communication, 2014) is located in the 5 km-long trend that includes the world-class Cuiabá deposit. It is hosted in the Neoarchean metavolcano-sedimentary rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero, Brazil. Mineralization is associated mainly with metachert-banded iron formation (BIF) and carbonaceous phyllites in the reclined Lamego fold, in which the Cabeça de Pedra orebody represents the hinge zone. Mineralization is concentrated in silicification zones and their quartz veins, as well as in sulfide minerals, product of BIF sulfidation. Hydrothermal alteration varies according to host rock, with abundant sulfide-carbonate in BIF, and sericite-chlorite in carbonaceous phyllite. Quartz vein classification according to structural relationships and host rocks identified three vein systems. The V1 system, mainly composed of smoky quartz (Qtz I) and pyrite, is extensional, crosscuts the bedding plane S 0 of BIF, and is parallel to the fold axis. The V2 system, of the same composition, is represented by veins that are parallel to the S 1-2 foliation and S 0 . This system is also characterized by silicification zones in the BIF-carbonaceous phyllite contact that has its maximum expression in the hinge zone of folds. The V3 system has milky quartz (Qtz II) veins, which result from the recrystallization of smoky quartz, located mainly in shear zones and faults; these veins form structures en echelon and vein arrays. The most common ore minerals are pyrite, As-pyrite and arsenopyrite. Fluid inclusion-FI trapped in all quartz veins present composition in the H 2 O-CO 2 ± CH 4 -NaCl system. Fluid evolution can be interpreted in two stages: i) aqueous-carbonic fluid trapped in Qtz I, of low salinity (~2% equiv. wt.% NaCl), and ii) carbonic-aqueous fluid, of moderate salinity (average 9 eq. wt.% NaCl) hosted in Qtz II. Both stages are characterized by decrepitation temperatures in the range of 200 to N 300°C, and suggest a fluid of metamorphic origin. Applying an arsenopyrite geothermometer, the calculated formation temperature for the Cabeça de Pedra orebody is 300 to 375°C. The vertical intersection of the isochors allows a minimum pressure calculation of 2.6 kbar. The composition of individual FIs of this orebody, obtained by LA-ICP-MS analyses, compared with results of FIs for the Carvoaria Velha deposit, Córrego do Sítio lineament, highlights a standard composition typical of metamorphic fluids with Na N K N Ca N Mg, which increase or decrease in concentration as a function of salinity in both deposits. Trace elements vary according to fluid-rock reactions, and are directly related to the host rock composition. The comparison of data sets of the two deposits shows that the Cabeça de Pedra FIs have a higher enrichment in Zn, while Cu, As and Sb are richer in Carvoaria Velha, suggesting influence of the host rock geochemistry. The suggested mechanisms for gold precipitation at the Cabeça de Pedra orebody, Lamego gold deposit are: i) hydrolysis of the carbonaceous matter of phyllite and BIF, affecting fO 2 , destabilizing sulfur complexes and enhancing gold precipitation; ii) replacement of BIF iron carbonates by sulfides; and iii) continuous pressure changes that lead to silica precipitation and free gold. Other than playing the long-recognized role of the carbonaceous phyllites as a fluid barrier, the data highlight their importance as a source of metals.

Ore Geology Reviews, 2015

ABSTRACT

Journal of South American Earth Sciences, 2014

ABSTRACT The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important ura... more ABSTRACT The Lagoa Real uranium district in Bahia, northeastern Brazil, is the most important uranium province in the country and presently produces this metal in an open-pit mine operated by Indústrias Nucleares do Brasil. Uranium-rich zones are associated with plagioclase (dominantly albite ± oligoclase) -rich rocks, albitites and metasomatized granitic-gneisses, distributed along NNW/SSE striking shear zones. We have used the ID-TIMS U–Pb method to date zircon and titanite grains from the São Timóteo granitoid, and albite-rich rocks from the Lagoa Real district in order to assess the age of granite emplacement, deformation/metamorphism and uranium mineralization. The isotopic data support the following sequence of events (i) 1746 ± 5 Ma – emplacement of the São Timóteo granitoid (U–Pb zircon age) in an extensional setting, coeval with the beginning of the sedimentation of the Espinhaço Supergroup; (ii) 956 ± 59 Ma hydrothermal alteration of the São Timóteo granitoid and emplacement of the uranium mineralization (U–Pb titanite age on an albite-rich sample); (iii) 480 Ma metamorphism, remobilization and Pb loss (U–Pb titanite age for the gneiss sample), during the nucleation of shear zones related to the collision between the São Francisco-Congo and Amazonia paleoplates. The 956 ± 59-Ma mineralization age is apparently associated with the evolution of the Macaúbas-Santo Onofre rift. This age bracket may bear an important exploration implication, and should be included in the diverse age scenario of uranium deposits worldwide.